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\def\myheading{Adaptive Object Attention}
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\markright{\myheading \hspace{1.8in} Zemel, Behrmann, Mozer, \& Bavelier}

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\title{
%%% {\large To appear in: {\em }}\\[.6in]
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%{\large Submitted to: {\em Journal of Experiment Psychology: Human Perception
%    \& Performance}}\\[.6in]
{\LARGE 
Experience-Dependent Perceptual Grouping and Object-Based Attention}}

\author{}
\date{}

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\centerline{
\begin{tabular}{ccc}
{\Large Richard S. Zemel} & \hspace*{.5in} & {\Large Marlene Behrmann}\\[3pt]
%Departments of Psychology \& Computer Science & & Department of Psychology\\
University of Toronto & & Carnegie Mellon University\\
Toronto, ON  M5S 1A4 & & Pittsburgh, PA  15213\\
{\tt zemel@cs.toronto.edu} & &{\tt behrmann+@cmu.edu} \\[18pt]
{\Large Michael C. Mozer} & & {\Large Daphne Bavelier} \\[3pt] 
%Department of Computer Science & & Georgetown Institute for Cognitive \& Computational Sciences\\
University of Colorado & & University of Rochester\\
Boulder, CO  80309-0430 & & Rochester, NY  14627\\
{\tt mozer@cs.colorado.edu} & & {\tt daphne@bcs.rochester.edu}
\end{tabular}
}

\vspace*{.3in}
%\centerline{\today}

\begin{abstract}
Numerous studies have shown that attention can be allocated to objects as well
as locations in the visual field even if the objects are partially occluded.
A fundamental question concerns the nature of the `objects' for which this
attentional benefit applies.  Current studies have shown that objects can be
defined on the basis of Gestalt grouping principles as well as on the basis of
familiarity.  Both the effects of grouping as well as familiarity can be
understood in terms of a more general hypothesis: that perceptual experience
with particular feature combinations determines whether or not two features
will be integrated as an object of attention. We present data from four
studies showing that recently experienced novel feature combinations gain the
object attentional benefit and that this effect is realized by different
feature combinations under a range of experimental conditions.  These studies
indicate that object attention is adaptive and responsive to the statistical
structure of the environment.
\end{abstract}

\vspace*{.5in}

{\small
Acknowledgements: We thank Susan Watt, Jim Nelson, and James
Christopher for help with running subjects and analysis, and Mary
Peterson for helpful comments on the manuscript.
Supported by the McDonnell Foundation grant JSMF 95-1 (RSZ),
NIMH Program Project grant MH47566-06 (MB and RSZ), 
NSF award IBN-9873492 (MCM), 
and a McDonnell Foundation grant (DB).
}

\clearpage

\section*{Introduction}

Attention can be directed toward objects as well as toward locations in the
visual field, thereby affording preferential processing for the features
of a specific object.  Evidence for this finding comes from
studies which show that it is difficult to attend to two objects
simultaneously.  For example, when judgements depend on two features in a
display, responses are more rapid when both features belong to the same
object, even when the objects are spatially superimposed (Duncan,
1984; Baylis \& Driver, 1993; Kramer \& Watson, 1995).  
A second source of evidence  
are studies showing that people find it difficult to ignore features
that belong to an attended object (Kramer \& Jacobson, 1991; Baylis \&
Driver, 1992; Yantis, 1992).

A fundamental question concerns the nature of the `objects' for which
this attentional benefit applies.  In most experiments demonstrating
object-based effects, grouping principles such as continuation (Moore,
Yantis, \& Vaughan, 1998), collinearity (Lavie \& Driver, 1996),
similarity (Kramer \& Jacobson, 1991), color (Baylis \& Driver,
1993) or motion (Driver \& Baylis, 1989; Behrmann, Zemel, \& Mozer, 1999)
are sufficient to define the objects in the display.  
Vecera and Farah (1997) have also shown that object-based attention is 
stronger for highly familiar shapes (upright letters) than for unfamiliar 
shapes that benefit from the same grouping principles (upside-down letters).

Current studies have not established whether these two means of
`defining' objects---based on generic grouping principles or long-term
familiarity---are sufficient to predict when two features will be
treated as belonging to the same object.
A hypothesis that provides a more general viewpoint, which accounts
for both of these other definitions, is that perceptual experience
with particular feature combinations determines whether or not two
features will be integrated as an object of attention.  We have
previously developed a computational model, named MAGIC, based on this
hypothesis (Mozer, Zemel, Behrmann, \& Williams, 1992).  MAGIC was
trained to group features from a set of images containing
multiple objects in which each elementary feature was labeled as to
which object it belonged.  After training, MAGIC successfully
segregated features of novel images into separate objects. 
Examination of the representations derived by MAGIC revealed that the
critical aspects were the configurations of image features with a
consistent labeling relative to one another.  For example, the model
discovered that the 
elements within one of the two perpendicular segments of a T-junction were
consistently labeled as belonging to the same object,
while elements across segments were segregated into different objects.
In this way, MAGIC embodies the hypothesis that perceptual experience
defines which features will be grouped together and which features will not.
Under this hypothesis, generic grouping principles emerge based on compiled
experience with a variety of feature and object combinations in images.

In this paper, we investigate the role of perceptual experience in
object-based attention, examining questions such as whether short-term
experience with a shape is sufficient to facilitate its being processed 
as a unitary whole;
and the extent to which this experience with a shape may override other cues
as to whether two features belong to a common object. 

In order to explore these issues, we utilize a paradigm for studying
object-based attention developed in previous work (Behrmann, Zemel, \&
Mozer, 1998).  In these experiments, subjects decided whether 
the number of bumps appearing at two of four possible ends of two
overlapping objects (or bars; see Figure~\ref{fig:bzm}) were the same
(Figure~\ref{fig:bzm}a-c) or different (Figure~\ref{fig:bzm}d-f). The
two features (sets of bumps) could appear on the ends of a single object
(Figure~\ref{fig:bzm}a and~\ref{fig:bzm}d) or on the ends of two
different objects (Figure~\ref{fig:bzm}b and~\ref{fig:bzm}e).
Consistent with the {\it object-cost} hypothesis that it is difficult
to attend to two objects simultaneously, 
subjects' responses were significantly slower to two features of two
different objects than to two features of a single object
% subjects' RTs were significantly slower on displays when the features were  
% on two different objects than on displays in which the features were within
% the same object
(cf. Duncan, 1984). 
% BZM [xx msec slower from BZM]
These object costs---significant RT differences
for responding to features of different objects versus features of a single
object---provide an 
assay to determine when features are grouped into a single object.
Instructions to the subjects carefully omitted any mention of objects,
making this probe particularly useful because it does not involve any
subjective definition of object-hood.

In our earlier experiments, we also included a third type of display in which
the bumps were on the occluded object (Figure~\ref{fig:bzm}c
and~\ref{fig:bzm}f) and, again, evaluated whether there was any cost relative
to the single object trials.  Occlusion is a particularly challenging
condition for an object-based account of selection---not only are the elements
of a single occluded object spatially distant but they are also discontinuous
(see Yantis, 1995; Moore, Yantis, \& Vaughan, 1998 for other studies of
occlusion effects on object-based attention).  Interestingly, however, there
was no object cost for the occluded trials, reflected in equivalent RTs for
the single nonoccluded and the single occluded displays, both of which
differedfrom the two object trials.  These results held up both in the
``X-displays'' in which the bars crossed to form an ``X'', and also in
``V-displays'' in which the sets of bumps were all at 90 degrees from each
other (see, e.g., Figure~\ref{fig:bzm}g-i).  The evidence from these studies
suggests that elements of a single object, even if occluded, are grouped
together and preferentially processed relative to elements of other objects in
the scene.

\begin{figure}
\centerline{
\begin{tabular}{ccc}
\psfig{figure=Figs/1a.eps,width=2in,height=2in,clip=t} &
\psfig{figure=Figs/1b.eps,width=2in,height=2in,clip=t} &
\psfig{figure=Figs/1c.eps,width=2in,height=2in,clip=t}
\\[-.1in]
(a) &
(b) &
(c)\\[.2in]
\psfig{figure=Figs/1d.eps,width=2in,height=2in,clip=t} &
\psfig{figure=Figs/1e.eps,width=2in,height=2in,clip=t} &
\psfig{figure=Figs/1f.eps,width=2in,height=2in,clip=t} 
\\[-.1in]
(d) &
(e) &
(f)\\[.2in]
\psfig{figure=Figs/1g.eps,width=2in,height=2in,clip=t} &
\psfig{figure=Figs/1h.eps,width=2in,height=2in,clip=t} &
\psfig{figure=Figs/1i.eps,width=2in,height=2in,clip=t}  
\\[-.1in]
(g) &
(h) &
(i)
\end{tabular}
}
\caption{Examples of stimuli used in Behrmann et al. (1998) and here.
Subjects had to make same/different judgements based on the number
of bumps at two different locations in each figure.  The top and
third row represent `same' judgements, while the middle row displays are
`different'.   The left-most column depicts a single occluder 
condition, in which the bumps are on one, occluding object, the middle
column shows the two object condition, and the right column shows
the single occluded condition.  The first two rows are examples of
X displays, containing two overlapping bars, while the third row
shows examples of V displays, containing overlapping V shapes.}
\label{fig:bzm}
\jypsline
\end{figure}

Our experiments established that features of an occluded object enjoy the 
same processing advantage as features of an unoccluded object relative to 
features of two different objects:
removing explicit continuity as an object-defining cue did not affect 
object-based attention.
An additional experiment also established a
boundary condition of this result.  When we changed the relation between
the two discontinuous fragments of the occluded object so that they
no longer formed a plausible single bar (Figure~\ref{fig:spread}b), the
object cost reappeared and performance was no longer equivalent to that of
a single nonoccluded object.

\begin{figure}
\setlength{\unitlength}{1in}
\begin{picture}(6.5, 2.6)(0, 0)
\put(.2,0.3){\psfig{figure=Figs/1c.eps,width=1.8in,height=1.8in,clip=t}}
\put(2.5,0.1) {\psfig{figure=Figs/SZ-Amb.eps,height=2.2in,clip=t}}
\put(4.8,0.1) {\psfig{figure=Figs/SZ.eps,height=2.2in,clip=t}}
\put(0.9,0.0) {{\large (a)}}
\put(3.1,0) {{\large (b)}}
\put(5.,0) {{\large (c)}}
\end{picture}
\caption{(a) Reaction times on displays where the bumps appear on the two
fragments that correspond to the ends of an occluded bar are
equivalent to those on unoccluded bars, and significantly faster
than when the bumps appear on two different bars.  (b) When the
fragments are shifted so they no longer form a plausible occluded
bar, the RTs are equivalent to the two bar displays. (c) If perceptual
experience plays a determining role in parsing, then subjects exposed
to this shape may group the fragments in (b) into a single
object. } 
\label{fig:spread}
\jypsline
\end{figure}

A primary aim of the experiments in this paper is to determine what 
% aspects of the earlier studies are essential in defining the 
defines the
conditions under which attentional processes treat fragments as
belonging to the same object or different objects. 
One issue concerns why the non-aligned fragments in
Figure~\ref{fig:spread}b are not treated as a single occluded object.
One class of theories proposes general-purpose processes by which
image fragments are integrated into objects.  For example, 
this result is consistent with a theory that the
particular geometric relations between fragments determines whether
they will form objects (Kellman \& Shipley, 1991). 
Under this theory of {\it relatability}, spatially separated fragments
are interpolated when their edges can be connected by a smooth
monotonic curve; when the edges are no longer collinear, the fragments
are not relatable and do not belong to a single occluded object. 

A different hypothesis, consistent with MAGIC, is that these general-purpose
mechanisms could emerge from perceptual experience.  On this view,
experience plays a determining role in perceptual organization.
% over-ride general mechs; or complementary mechanism to relatability
A corollary of this view is that short-term perceptual experience
may override the general-purpose, compiled grouping mechanisms.
When short-term experience is consistent with longer term regularities
than heuristics such as relatability will apply, but in other
circumstances they will not.
Consider the situation in which subjects are exposed to
a shape that could potentially link together the
two non-aligned fragments of the occluded object into a plausible object
(see Figure~\ref{fig:spread}c).
Experience-dependent grouping would then predict that even if this
novel shape is rather convoluted and irregularly shaped,
the object advantage will apply even when only the
non-aligned fragments are visible.  We
test this prediction in Experiment 1.

To summarize our direction, experiment 1 demonstrates that short-term
experience can affect grouping, and the next logical
issue concerns the circumstances under which this occurs.
One relevant such question
is whether explicit evidence of occlusion is necessary for
object-based attention to apply to the non-contiguous fragments.
Is it essential in displays such as Figure~\ref{fig:bzm}c
and~\ref{fig:bzm}f that the occluding bar be present, or  
can the same effect be produced without it, in a display
containing only the fragments?
Removing the occluder reduces the chances that {\it amodal completion}
can be applied to the fragments.
Theories of completion (e.g., Kanisza \& Gerbino, 1982)
generally stress that amodal completion is much stronger than
any other form of completion.  
An important question is whether experience-dependent grouping is
strong enough to operate in the absence of amodal completion, and if
so, under what conditions.
We explore this question in Experiments 2-4.

\section*{Experiment 1: Amodal Completion Arising From Experience}

The aim of this experiment was to determine whether exposure to specific
displays can alter the perceptual organization of an image.  This
hypothesis predicts that the grouping of image fragments
into objects based on general-purpose principles, such as relatability,
uniform connectedness, and good continuation, can be superceded with
experience. 

In this experiment, subjects were split into two groups, Experimental and
Control. 
In the first block of trials, subjects in both
groups saw displays such as those shown in Figure~\ref{fig:sz}a,b,
and they either had to respond to the fully visible object, or to
the fragments that would 
typically not be grouped as belonging to a single object.  In the
second block of trials, Experimental subjects saw 
displays containing an object that linked these fragments up as parts of
a single object (Figure~\ref{fig:sz}c), while Control subjects saw
the fragments alone without the fully visible object.

The key prediction concerns the third block of trials, in which both
groups of subjects again saw displays like Figure~\ref{fig:spread}a,b.
The prediction is that because the
Control group subjects' experience will support
an interpretation of this display as three separate objects (the two
fragments and the central bar), so the fragments will have an object
cost, just as they did in the initial block. 
On the other hand,
the subjects who saw the linking object will
interpret the two fragments as belonging to a single occluded shape,
as evidenced by their relatively speeded responses to those fragments.

\subsection*{Method}

{\it Participants}. A total of thirty-two subjects participated in
this experiment. The data of two subjects were excluded from the
analysis because of high error rates ($>$ 10\%). Subjects were drawn
from the Carnegie Mellon University community and were paid \$5 for
their participation. Subjects ranged in age from 18 to 24 years. All had
normal or corrected to normal visual acuity. None 
of the subjects was aware of the purpose of this study.

{\it Apparatus and materials}. This experiment was conducted on a
Macintosh IIci computer. Stimuli were presented on a 14-inch color
monitor (Basic color monitor: model M1595LL/A) using Psychlab
experimental software version 1.0 (Bub \& Gum, 1991).  The displays
were presented as black-and-white line drawings on a white
background. Viewing distance was approximately 50 cm.
%% Rich -- need to check angles!
% with the result that the display subtended a visual angle of 4.8\degree
% \ horizontally and vertically.

\begin{figure}
\setlength{\unitlength}{1in}
\begin{picture}(6.5, 6)(0, 0)
\put(0.8,5.2) {{\Large \underline{Block 1}}}
\put(3,5.2) {{\Large \underline{Block 2}}}
\put(5.2,5.2) {{\Large \underline{Block 3}}}
\put(0.0,3.5) {{\Large Experimental \& Control}}
\put(4.4,3.5) {{\Large Experimental \& Control}}
\put(2.7,3.3) {{\Large Experimental}}
\put(2.8,1.6) {{\Large Control}}
\put(.4,1.9) {{\large (a)}}
\put(1.4,1.9) {{\large (b)}}
\put(2.8,3.6) {{\large (c)}}
\put(3.65,3.6) {{\large (d)}}
\put(3.65,.1) {{\large (e)}}
\put(0.,0) {\psfig{figure=Figs/design.eps,height=5.in,width=6.5in,clip=t}}
\put(0.08,2) {\psfig{figure=Figs/SZ-Amb-bar.eps,width=1.in,clip=t}}
\put(1.08,2) {\psfig{figure=Figs/SZ-Amb.eps,width=1.in,clip=t}}
\put(2.38,3.78) {\psfig{figure=Figs/SZ-bar.eps,width=1.in,clip=t}}
\put(3.48,3.78) {\psfig{figure=Figs/SZ.eps,height=1.1in,clip=t}}
\put(2.38,.28) {\psfig{figure=Figs/SZ-bar.eps,width=1.in,clip=t}}
\put(3.48,.28) {\psfig{figure=Figs/SZ-Ends.eps,height=1.1in,clip=t}}
\put(4.42,2) {\psfig{figure=Figs/SZ-Amb-bar.eps,width=1.in,clip=t}}
\put(5.44,2) {\psfig{figure=Figs/SZ-Amb.eps,width=1.in,clip=t}}
\end{picture}
\caption{Design of Experiment 1.  Both groups of subjects (Experimental and Control)
performed 3 blocks of trials.  For both groups, block 1 trials contained
Ambiguous displays in which the bumps were either on
the (a) Bar or (b) Fragments.  Half the trials in block 2 contained
displays of a single (c) Bar.  For the Experimental group, the other trials in
block contained the (d) Z shape, while Control group subjects saw the
(e) Fragments display. Block 3 contained the same stimulus set as block 1 for
both subject groups.}  
\label{fig:sz}
\jypsline
\end{figure}

There were four types of displays (see Figure~\ref{fig:sz}):
\begin{enumerate} 
\item (Figure~\ref{fig:sz}a,b) {\bf Ambiguous}. This display could either be
interpreted as a 
rectangular bar occluding a Z-shaped object, or as a rectangular
bar with two smaller rectangular ends butted up against it.  The
rectangular bar was 8.7 cm in length (10.2\degree) and 2.5 cm in width
(2.9\degree).  The two ends were created by taking the two visible
fragments of an orthogonal occluded bar of the same dimensions and
displacing them by slightly more than the width of the rectangle
(3.3\degree).  This display appeared equally often in four
different orientations, as shown in Figure~\ref{fig:szOrient}.
Furthermore, the displays fell into two conditions, based on the locations
of the features (bumps):
\begin{itemize}
\item[(a)] {\it Bar-Bumps}: The bumps appeared on the opposite end of the
single coherent bar.   
\item[(b)] {\it Fragment-Bumps}: The bumps appeared on the two fragments.  
\end{itemize} 
\item (Figure~\ref{fig:sz}c) {\bf Bar}. This display was created 
by removing all but the bar
from the Ambiguous displays.  The Bar appeared in two different
orientations, and only the Bar-Bumps condition was relevant to this
display. 
\item (Figure~\ref{fig:sz}d) {\bf Z}.  This display was created by 
removing the bar from
the Ambiguous displays and replacing it with contours
connecting the two remaining fragments.  This display also appeared
equally often in the four different orientations equivalent to those
shown in Figure~\ref{fig:szOrient}.  Only the Fragment-Bumps condition
was possible in this display.
\item (Figure~\ref{fig:sz}e) {\bf Fragments}. This display was created 
by removing the bar from
the Ambiguous displays and simply adding a single line to each of the
two remaining fragments to form separate rectangular ends.  Once
again, the Fragments appeared in four orientations, and only the
Fragment-Bumps was relevant to this display.
\end{enumerate}

\begin{figure}
\setlength{\unitlength}{1in}
\begin{picture}(6.5, 1.5)(0, 0)
\put(0.2,0) {\psfig{figure=Figs/SZ-Amb.eps,height=1.5in,clip=t}}
\put(1.8,0) {\psfig{figure=Figs/SZ-Amb4.eps,height=1.5in,clip=t}}
\put(3.3,0.2) {\psfig{figure=Figs/SZ-Amb2.eps,width=1.5in,clip=t}}
\put(4.9,0.2) {\psfig{figure=Figs/SZ-Amb3.eps,width=1.5in,clip=t}}
\end{picture}
\caption{Examples of the four orientations of Ambiguous
displays used in Experiment 1.} 
\label{fig:szOrient}
\jypsline
\end{figure}

In all trials of this experiment, and in every experiment described
below, subjects performed the same/different number-of-bumps decision.
On each trial, bumps appeared at the extremities of either the
Fragments or the Bar.  Each set of bumps was either in a {\it two-bump} or
{\it three-bump} configuration: the end was divided into two equal parts for
the two-bump and into three equal parts for the three-bump displays.
There was an equal number of `same' and `different' judgments in each
of the two conditions. On `same' trials, there were either two (known
as a 2-2 trial) or three bumps (known as a 3-3 trial) and there were
an equal number of 2-2 and 3-3 `same' trials. On `different' trials,
there were always two bumps at one location and three bumps on the
other and the locations of the 2 and 3 bumps were evenly
counterbalanced.

The subject's task was simply to decide whether the number of bumps at
the two locations was the same or different. Responses were indicated
with the [Z] or [/] keys with the left and right index fingers on the
standard keyboard. The assignment of keys to `same' or `different'
responses was counterbalanced across subjects.

{\it Design}.  
Subjects performed 3 experimental blocks of trials.  Blocks 1 and 3 were
equivalent for the two groups; the difference occurred in block 2. In blocks 1
and 3, all subjects made same/different judgements to Ambiguous displays.
Feature location (Bar-Bumps vs. Fragment-Bumps) was crossed with bump-number
combinations (2-2, 2-3, 3-2, and 3-3) and orientation (the 4 shown in
Figure~\ref{fig:szOrient}), yielding a base set of 32 trials which was
replicated 8 times for a total of 256 trials in block 1.  A practice block
consisting of 16 trials---a randomly-selected half of this base set---was
completed before block 1 to accustom subjects to the display and response
keys.  These data were not analyzed.

In block 2, the subjects' experience was manipulated. Both groups saw Bar
displays on half of the trials in block 2. On the other trials, Experimental
group subjects saw Z displays while Control group subjects saw Fragments
displays.  Block 2 also involved 256 trials, consisting of 8 replications of
the basic crossing of trial type (Bar or Z for Experimental group, Bar or
Fragments for Control group), bump-number combination and orientation. A break
of a few minutes was given between blocks.

The experiment contained one between-subjects variable (Experimental or
Control group) and two within-subjects variables: block (1 and 3); and feature
location (Fragment-Bumps and Bar-Bumps).  We expected that both groups would
improve in their overall reaction times in block 3 compared with block 1 due
to a general practice effect.  More importantly, we predicted that if learning
is mediated by exposure to a linking object, we would find a 3-way interaction
in the data: the Experimental group would process the Fragment-Bumps of the
Ambiguous displays much faster than the Control group, but only in block 3 and
not in block 1.  This implies in terms of object costs---RT differences for
features of different objects versus features of a single object---that the
costs in block 1 for the Ambiguous Fragment-Bumps trials disappear in block 3
for the Experimental group, but not for the Control group.  Thus the critical
comparison here is between the two groups of subjects within each of the two
conditions.

{\it Procedure}.  Each trial proceeded as follows: A fixation point
appeared for 1 second (sec) followed by a 500 millisecond (msec)
delay. Thereafter, the display appeared and remained on the screen
until a response was made. An inter-trial interval of 1 sec occurred
following the response and prior to the next trial.  The same
procedure was followed in all experiments presented in this paper.

{\it Treatment of results.} The data from the practice trials were
discarded from the analysis.  Error trials were excluded from the
reaction time (RT) analysis.  
First a global analysis 
was conducted. crossing group, bump location, block, judgement,
and orientation.  
The mean RT and errors for each
crossing of group, feature location, block, judgement,
and orientation were calculated for each
subject and were then subject to analyses of variance.  Reaction times
that exceeded two standard deviations above or below a subject's mean
(before the removal of the data) were also excluded from the analysis.  
%A similar analysis procedure
%was used in all subsequent experiments.
Further analyses were then conducted on these data, as discussed below.
% Then separate ANOVAs were conducted within each
% of the three blocks, crossing group and bump location.  Finally a
% within-group analysis was conducted, crossing block and bump location.

\subsection*{Results and Discussion}

The overall error rate for this experiment was low, comprising 2.2\%
of the total number of trials.  A further 3.5\% of the data was
excluded as exceeding the two standard deviations cutoff. 

The global error analysis of varaiance did not reveal significant effects 
within or between any of the factors, so the error data were not
subject to further analysis.

In the global analysis of variance, no difference was found in RT patterns as
a function of orientation ($F(1,28)=2.04, p=.14$).
As is typically the case in the same-different paradigm
(Nickerson, 1965), same judgements were found to be significantly faster than
different judgements ($F(1,28)=5.51, p<.05$).  However, no significant
interaction was found between these variables, judgement or
orientation, and no interaction between either of them and the other
factors.  Consequently, the RT data were pooled across judgement and
orientation for subsequent analyses.

The RT data contained a significant effect for block
($F(2,56)=17.55, p<.001$) and feature location ($F(1,28)=20.82, p<.001$).
The main effect of block corresponds to an overall practice effect as the
experiment unfolds, while the main effect of feature location corresponds
to the overall object cost.
The RT data contained no effect for group ($F(1,28)=.01, p=.93$). 
No pair-wise interactions were found between the variables.  The 
critical finding with respect to our hypothesis was the sole interaction
between the factors: a significant
three-way interaction between block, group, and feature location
($F(2,56)=3.32, p=.04$).
This three-way interaction indicates that the object cost varies
between the blocks as a function of group.

Given these interactions between block and the other variables,
we then conducted separate analyses within each block of
the experiment,
crossing one between-subjects variable (Experimental or Control group) and
one within-subjects variable (Fragment-Bumps or Bar-Bumps feature location).
Note that the second block was included for
the sake of completeness.  The critical data concerns blocks
one and three, since the stimulus sets were the same across the
groups in these blocks.
Figure~\ref{fig:szData} shows the mean RTs 
for these variables (group and feature location) in the two
critical blocks.

\begin{figure}
\setlength{\unitlength}{1in}
\begin{picture}(6.5, 3.5)(0, 0)
\put(1.1,0) {\psfig{figure=Figs/SZ1R.eps,height=3.5in,clip=t}}
\cut{
\put(1.78,2.5) {\small (3)}
\put(2.07,1.7) {\small (2)}
\put(2.73,2.78) {\small (3)}
\put(3.02,1.92) {\small (0)}
\put(3.71,2.09) {\small (2)}
\put(4.,1.23) {\small (1)}
\put(4.68,1.31) {\small (1)}
\put(4.97,1.23) {\small (1)}
}
\end{picture}
\caption{Mean reaction times and standard error bars in blocks 1 and
3 for the two subject groups as a function of feature location (on the Bar 
or on the two Fragments) for the Ambiguous displays in Experiment 1.} 
\label{fig:szData}
\jypsline
\end{figure}

The block one RT data contained a significant effect of feature location
($F(1,28)=17.81,p<.001$), but no effect of group ($F(1,28)=.43,p=.515$) and no
interaction between the two variables.  This analysis reveals a
clear object cost, suggesting that the bumps on the fragments are treated as
being on separate objects. 

In block two, the RT data again contained a significant effect of feature
location ($F(1,28)=7.47,p=.01$), but no effect of group ($F(1,28)=.06,p=.8$).
There was an interaction between the two variables ($F(1,28)=4.79,p=.04$),
which reflects the different stimuli seen by the two groups in one
of the feature location conditions.
In the Bar-Bumps condition, which in this case involved a unitary bar, 
the reaction times of the two groups were
similar (710 ms for the Experimental group; 700 ms for the Control subjects).
The Control group responded similarly on the Fragment-Bumps (713 ms), which in
this case contained the unitary Z-shaped display. 
The Experimental group was considerably slower on the Fragment-Bumps (731 ms),
revealing an object cost for the Fragment stimuli.  

The RT data from block three also contains a significant effect of feature
location ($F(1,28)=7.7,p<.01$), but no effect of group ($F(1,28)=.63,p=.43$).
Most importantly, a significant interaction again exists between these
variables in the third block ($F(1,28)=5.85,p=.02$), indicating the 
residual effect of
the differential experience of the groups in the second block.

Finally, in order to further analyze the three-way interaction between the
main variables of group, block, and feature location, 
we conducted separate ANOVAs in the two
groups with block and feature location as within-subject variables.  
We only included RT data from blocks 1 and 3 to focus the analysis
on trials involving physically identical display sets.
For the Control group,
% who saw the fragments in block 2, 
a clear object effect is evidenced by the significant effect of feature
location.  These subjects were consistently faster when the bumps appeared on
the single object (Bar-Bumps) than on two separate objects (Fragment-Bumps):
$F(1,14)=25.99, p<.001$.  There was also a general speed-up between blocks 1
and 3 (15 ms for Fragment-Bumps, 21 ms for Bar-Bumps), but this difference did
not reach significance ($F(1,14)=3.75, p=.07$).  Critically, no interaction
existed between these two variables ($F(1,14)=.15, p=.71$).  This implies that
the object cost in blocks 1 and 3 were equivalent: 30 ms for block 1, 36 ms
for block 3.

For the Experimental group, there was a much larger drop in
RTs from block 1 to 3 for Fragment-Bumps than Bar-Bumps (63 ms vs. 27 ms).
In this group, the 2-way interaction between feature location and block is 
significant: $F(1,14)=6.7, p=.02$.
Considered individually, feature location was not significant ($F(1,14)=4.1,
p=.06$); block, however, was highly significant ($F(1,14)=17.2, p<.001$).
All of these effects (the significant 2-way interaction and effect of
block, but lack of feature-location effect) can be traced primarily to
the same cause -- the speed-up for the Fragment-Bump condition between
blocks 1 and 3.  This speed-up wiped out the object cost (effect of
feature-location) in block 3 that was present in block 1 (38 ms vs 2 ms). 

The effect of experience can be clearly seen by analyzing the data across 
blocks. For each group, the difference in the Bar-Bumps RTs
between blocks 1 and 3 describes a baseline practice effect.  For the
Control group, this mean difference was 21 msec, while for the Experimental
group it was 27 msec.  

If the second block of trials does not
differentially affect the two feature-location conditions, then one would
predict that the speed-ups for the Fragment-Bump displays
would be approximately the same as these values.  This is the case for
the Control group, where the difference in Fragment-Bump RTs between
blocks 1 and 3 was 15 msec.
However, the Fragment-Bumps speed-up for the Experimental group was
significantly greater: 63 msec. 

Two main conclusions may be drawn from these results.  First, the
results replicate the finding in Behrmann et al. (1998) that displaced
fragments are not treated as an occluded object in the Ambiguous
displays.  This is manifested in the significant effect of feature location
in block 1, where subjects are faster on Bar-Bumps than
Fragment-Bumps, suggesting that the fragments are not being perceived
as a single object.  This finding is consistent with the principle of
relatability (Kellman \& Shipley, 1992), which predicts that the
contours of the two fragments will not be interpolated because of the
misaligned geometric relationship between them.

The second conclusion is the more interesting one: exposure to a novel
object changed the processing of the ambiguous visual input.  The
speed-up from blocks 1 to 3 in the Control group subjects is similar
in the two feature-location conditions, indicating that viewing the 
displays in block 2 (Bar and Fragments) had a similar effect on the
processing of these two conditions.  For the Experimental group, however,
viewing the block 2 displays (Bar and Z object) had a differential effect on
the Fragment-Bumps and Bar-Bumps conditions. 
For this group of subjects, the RT means are almost identical for
these two feature locations in block 3 (723 vs. 721 msec), indicating an
object-based effect in the Ambiguous displays that is as strong for the
Fragments as for the fully visible Bar.

\section*{Experiment 2: V $\rightarrow$ Ends Transfer}

The results of Experiment 1 demonstrate that exposure to a novel shape
that links together feature fragments affects the later processing of
displays in which fragments may be interpreted as part of a single
object.  This suggests that subjects perform a form of
amodal completion given knowledge of an object that can link the
fragments in a display.  A natural next question concerns the necessity
of occlusion: Does this completion require the presence of an occluding 
object?  

Consider for example Bregman's well-known B displays, where one recognizes a
smattering of edge fragments as a set of block-letter Bs once the occluding
blobs are added to the image (see Figure~\ref{fig:bregman}).  The question is
whether conditions exist under which the fragments of the Bs may be sufficient
to allow for completion without the presence of the occluding blobs.  A
completion mechanism largely driven by perceptual experience would predict
that repeated experience with B shapes could influence perceptual organization
such that the effect of the object can be detected in an attention task, even
without the occluding blobs.  Experiment 2 was designed to address this
prediction.

\begin{figure}
\centerline{
\begin{tabular}{cc}
\psfig{figure=Figs/bregNew1.eps,height=3in,clip=t} &
\psfig{figure=Figs/bregNew2.eps,height=3in,clip=t} 
\end{tabular}
}
\caption{Evidence of occlusion facilitates completion of the occluded shapes
on the right, while they are more difficult to perceive on the left
(after Bregman, 1981).}
\label{fig:bregman}
\end{figure}

\cut{
A second aim of this experiment was to explore whether the learning effect
on object attention obtained in Experiment 1 using a between-subjects
design could be obtained using a within-subjects design.  
A stringent test of the hypothesis that subjects' perceptual
experience alters subsequent visual processing, and that completion
does not require the presence of an occluder, is to examine whether
the very same subjects will show a shift in their interpretation
of the ambiguous displays after exposure to a disambiguating stimulus.
}

To examine these issues, we devised a series of experiments using the same
methodology as Experiment 1, and similar stimuli to our earlier
experiments.  As in Experiment 1, subjects first saw a block of trials with an
ambiguous display, then a block using a full display that favored a particular
interpretation of the ambiguous display, followed by another block of the
ambiguous display.  In Experiment 2, the stimuli were derived from the V
displays (Figure~\ref{fig:bzm}g-i), for which we found an object-based
attention effect (Behrmann et al., 1998).  The central prediction in the
new experiment was that exposure to V displays would affect the processing of
displays in which there was no occlusion information in the image, and the
fragments could be consistent with many different shape configurations.  These
{\it Ends} displays (see Figure~\ref{fig:VEnds}) were similar to the Fragments
displays in Experiment 1, except that the fragments here corresponded to the
ends of two overlapping V shapes (the V display).   The V display thus acted
like the Z display in Experiment 1, and we used the Ends displays
to examine the influence of experience on subjects' performance.
% (analogous to the use of the Ambiguous displays in Experiment 1).

\subsection*{Method}

{\it Participants}. Eighteen subjects, nine male and nine female, 
between 18 and 23 years of age were recruited from the undergraduate
subject pool at the University of Arizona. All subjects had normal or
corrected visual acuity by self report, and were unaware of the
purpose of the experiment.

{\it Apparatus and materials}. The same apparatus used in Experiment 1
was used here, except that we used a 13-inch color
monitor. 

The Ends displays contained four rectangles of 2.5 cm x 1.5 cm, oriented at
45 degrees (see Figure~\ref{fig:VEnds}). The Ends were made by
removing the center of a display which contained two V's lying atop
one another (see Figure~\ref{fig:bzm}g-i).
\cut{
each other in the center to form an 'X' (see Figure~\ref{fig:bzm}a-f).
Note that the dimensions of the V displays were
identical to those of the Ends (and X) displays, as the V displays
were constructed from the X displays by simply changing the location
of one inner line segment to transform the occluder from a bar into a
sideways V.  
Consequently the Ends displays are also equivalent to the V displays with the
central part removed.
This relationship will be important in Experiment 3.
}
The diagonal extent of this display matched the dimensions of
%Both bars had the same dimension as 
the bar in the displays of
Experiment 1 (8.7 cm long by 2.5 cm wide). The horizontal line drawn
from the midpoint of one rectangular End to the midpoint of the 
horizontally-aligned other End was 6.5 cm.  On each trial, the features
(bumps) appeared on two of the four Ends, in either a two or three-bump
configuration.  The bump configurations were the same as in the previous
experiment. 

The displays used in this experiment fell into two conditions, based
on feature location:
\begin{enumerate}
\item Diagonal-Bumps (Figure~\ref{fig:VEnds}a-b): the bumps appeared
on the diagonally opposite ends.  For the V displays, this
configuration corresponds to the bumps lying on two different objects.
For the X displays, it corresponds to them lying on a single object
(either the occluding or occluded bar).
\item Vertical-Bumps (Figure~\ref{fig:VEnds}c-d): the bumps appeared
either on the end pairs on the right or left hand side of the display.
Here the object relationship is reversed.  For the V displays, this
configuration corresponds to a single object, while for the X
displays, it corresponds to the two-object condition.
\end{enumerate}

There was an equal number of `same' and `different' judgments in each
of the two conditions, as in the previous experiment, and the
locations of the bumps were evenly counterbalanced (diagonal-left or
right for Diagonal-Bumps, vertical-left or right for Vertical-Bumps).
The V displays had one other degree of freedom, orientation: whether the left
or right-facing V was on top.  This variable was also counter-balanced.  The
total number of displays for the Ends was 16; this number was doubled to
equal the number of V displays.

As in Experiment 1, the subject's task was simply to decide whether
the number of bumps on the two ends was the same or
different. Responses were indicated with the [Z] or [/] keys with the
left and right index fingers on the standard keyboard. The assignment
of keys to `same' or `different' responses was counterbalanced across
subjects. Reaction times (RT) to make the decision was recorded in
milliseconds and accuracy was noted.

{\it Design}.  The experiment was run in 3 blocks.  In the first block,
the subjects saw 
only the Ends.  In the next block, the subjects saw only V displays.
These two blocks constitute the {\it Initial} epoch for these two displays
respectively.  In the final block (the {\it Test} epoch), Ends and V
displays were randomly intermixed.  
Note that subjects did not see
any examples of the V displays before block 2.
The design was entirely within-subject, with the relevant
independent variables being feature location (Diagonal-Bumps,
Vertical-Bumps), display (Vs or Ends), and epoch (Initial, Test). 
Judgement (same, different) was another independent variable,
as was orientation for the V displays.
The design is summarized in Figure~\ref{fig:VEnds}.

The three experimental blocks each consisted of 192 trials, with a few
minutes break between each block. Trials were randomized within a
block.  Prior to starting the experiment, subjects were given 32
practice trials, two of each of the Ends trials.  Timing and response
measurements were the same as in the previous experiment.

\begin{figure}
{\it Block 1}: Ends displays

\vspace{.1in}
\centerline{
\begin{tabular}{cccc}
\psfig{figure=Figs/Ends-Diag-Same1.eps,height=1.2in,clip=t} &
\hspace*{.3in}
\psfig{figure=Figs/Ends-Diag-Diff.eps,height=1.2in,clip=t} &
\hspace*{.3in}
\psfig{figure=Figs/Ends-Vert-Same.eps,height=1.2in,clip=t} &
\hspace*{.3in}
\psfig{figure=Figs/Ends-Vert-Diff.eps,height=1.2in,clip=t}\\[-.1in]
(a) &
\hspace*{.3in}
(b) &
\hspace*{.3in}
(c) &
\hspace*{.3in}
(d) 
\end{tabular}
}

\vspace{.1in}
{\it Block 2}: V displays

\vspace{.1in}
\centerline{
\begin{tabular}{cccc}
\psfig{figure=Figs/1h.eps,height=1.2in,clip=t} &
\hspace*{.3in}
\psfig{figure=Figs/V2.eps,height=1.2in,clip=t} &
\hspace*{.3in}
\psfig{figure=Figs/1g.eps,height=1.2in,clip=t} &
\hspace*{.3in}
\psfig{figure=Figs/V4.eps,height=1.2in,clip=t}\\[-.1in]
(e) &
\hspace*{.3in}
(f) &
\hspace*{.3in}
(g) &
\hspace*{.3in}
(h) 
\end{tabular}
}
\vspace{.1in}
{\it Block 3}: Ends and Vs
\vspace{.1in}
\caption{Design and stimuli used in Experiment 2.
In block 1, subjects performed same/different judgements
to Ends displays, examples of which are shown here (a-d).
These same Ends displays were also used in Experiment 3.
In block 2, subjects performed the same task on V displays
(e-h).
Displays a, b, e and f are examples of Diagonal-Bumps displays, while
c, d, g, and h are Vertical-Bumps;
a, c, e, and g are same judgements, b, d, f, and h are different.
In block 3, both types of displays, Ends and Vs, were then mixed.
}
\label{fig:VEnds} 
\jypsline 
\end{figure}

\subsection*{Results and discussion}

The errors constitute a small proportion (2.3\%) of the
trials.  As in Experiment 1, trials
exceeding the two standard deviation cutoff were removed, resulting
the removal of an additional 2.8\% of the trials.

A global analysis of variance with
correct RTs as the dependent measure and epoch (Initial, Test);
feature location (Diagonal-Bumps, Vertical-Bumps);
display (Ends, V); and judgement (Same; Different) was conducted.
Initial epoch refers to block 1 for Ends and block 2 for Vs, and Test refers
to block 3 in which Ends and Vs are shown.
Same judgements were again significantly faster than Different
($F(1,17)=4.7, p<.05$), but no significant interactions were found
between this and the other variables.
As in Experiment 1,
a similar ANOVA on error rates revealed no significant factors.
A separate analyses of variance conducted
crossing orientation with epoch and feature location on the V displays
(since the Ends displays have only one orientation)
revealed no significant interactions (all $F < 1$).
Thus we pooled the data across
orientation and judgement for subsequent analyses. 
The mean RT data for the three primary factors are shown in
Figure~\ref{fig:VendsData}. 

\begin{figure}
\centerline{
\psfig{figure=Figs/Vends1.eps,height=3.5in,width=5in,clip=t}
}
\caption{Mean RTs as a function of display (Ends, Vs), epoch (Initial, Test),
and feature location (Diagonal-Bumps, Vertical-Bumps) in Experiment 2.  The first
panel of the display shows the mean RTs to Ends and Vs during blocks 1 and 2
respectively.  The second panel shows the mean RTs to these same displays in
the third block, which consisted of both display types.  Note that Diagonal
corresponds to the bumps being on separate objects, while Vertical corresponds
to them being on a single object.}
\label{fig:VendsData}
\jypsline
\end{figure}

The global ANOVA revealed that
feature location ($F(1,17)=10.66, p<.01$) and epoch ($F(1,17)=7.76, p=.01$)
were significant, 
but display was not ($F(1,17)=.45, p=.5$).
No pairwise interactions were significant, but the three-way interaction
between the variables was ($F(1,17)=6.03, p=.02$).
Consequently, we conducted separate ANOVAs within each block of the
experiment.

The first result from the study is that,
for Ends displays in the Initial epoch, RTs for Diagonal-Bumps and
Vertical-Bumps were no different: $F(1,17)=.07,  p=.8$.  This indicates
that for the Ends displays, subjects do not treat the ends separated
vertically differently from those separated diagonally.  Given that
these are discontinuous, spatially separated features, the virtually
equivalent mean RTs (difference of 4 msec) is not surprising.

Secondly, for V displays in the Initial epoch, RTs for the Vertical
(single-object) condition were significantly faster than the Diagonal
(two-object) condition: $F(1,17)=7.6, p=.01$.  The RT difference was
35 ms.  
This result replicates the basic
object effect for the Vs found in Behrmann et al. (1998).

We conducted another ANOVA to further examine the 
replication of the object effect for the Vs found in Behrmann et al. (1998).
In that paper, it was found that when the features were on a single V
object, they were processed equally whether they were part of the
occluding or occluded object.
A separate ANOVA on the V displays crossing orientation
(whether the upright or upside-down V is in front of the other V)
with height (whether the bumps lie vertically across the top or bottom of the display)
revealed no significant effects of either variable
($F(1,17)=.43, p=.52$; $F(1,17)=2.13, p=.16$)
and no interaction ($F(1,17)=1.8, p=.2$).
This shows that our replication of the earlier results extends to the
occluder/occluded distinction. 

Most notable is the analysis of the Test epoch.
The ANOVA on this epoch revealed a highly significant effect of
feature location ($F(1,17)=16.36, p=.001$), no effect of
display ($F(1,17)=0.06, p=.8$), and no interaction between them
($F(1,17)=0.01, p=.9$).  Thus the object effect is found for both
displays in this epoch.  For the Vs, the object cost is approximately
equivalent in the two epochs: a difference of 37 ms in mean RTs
in the Test epoch versus 35 ms in the Initial one. 

The critical result with respect to our hypothesis is the object effect
for the Ends displays in the Test epoch.
Whereas feature location was not significant for the Ends
in the Initial epoch---a 4 ms difference in the wrong direction---mean
reaction times for same-object features were 38 ms faster than
different-object in the Test epoch.  

This last result, considered alongside the significant three-way interaction
between display, feature location, and epoch,
clearly shows that exposure to the V displays in block 2 induces
an object effect in the Ends displays in the following block, {\it within a
single group of subjects within one testing session}.  The equivalence of the
diagonal and vertical feature locations for these subjects in the first block
is undone by their perceptual experience during block 2. Thus, even though the
Ends are ambiguous in and of themselves, they are treated as equivalent to the
single-object and two-object conditions of the V displays by virtue of
subjects' experience with the V displays.

This finding is analogous to the disambiguating effect that the Z displays had
on the Experimental group subjects in Experiment 1.  In this case, however,
the effect 
of the novel object on the grouping of fragments is oberved in the attention
task even without the presence of an occluding shape. This result is
surprising, because experience with the V displays induced ends to be grouped
together even though all perceptual information in the display indicates that
they are {\it not} related, i.e., the four Ends are each closed rectangles.

One possible account for this finding is that the short-term effect of
experience with the object is so strong that it can over-ride evidence that
the Ends are closed self-contained objects, and produce a percept of the
linking V shape.  This account would suggest that the Bs in
Figure~\ref{fig:bregman}a would be perceived following some exposure to the B
shapes.   An alternative explanation for the results of this experiment is
that the object attention process is imprecise, and can be tricked into
linking the Ends even when the image evidence is not consistent with the
interpretation of the two fragments as belonging to a single object. 
Whereas the first account is driven by a whole-object matching process,
this second account relies on a limited analysis of local features.
We return to this issue in the General Discussion.

In any case,
the finding provides strong evidence that subjects' perceptual experience
alters subsequent visual processing: the effect of experience is sufficiently
robust so that even though information in the image might be interpreted to
the contrary, that the bumps are not part of a larger object by virtue of the
closing bar, they are still grouped together.

\section*{Experiment 3: X $\rightarrow$ Ends Transfer}

The previous experiment confirmed the hypothesis that image fragments
may become treated or interpreted in a particular way depending on the
specific experience of the subject.  Exposing a subject to images that
link particular fragments into objects affects their subsequent
organization of these fragments, both in the presence of and absence of
occlusion.  In the occlusion-present Ambiguous displays of Experiment 1,
the grouping of features on the fragments was neutral, potentially
interpretable as belonging to a single occluded object or two separate
objects.   Following
experience with the Z displays, subjects grouped the Fragment-Bumps
together. 
In the occlusion-absent Ends displays of
Experiment 2, prior to any exposure to the V displays, subjects did
not show any RT differences to Vertical-Bumps or Diagonal-Bumps.
Here 
However, after experience with the V displays, cues that the Ends were
unrelated were overcome, and the vertically
aligned features of the Ends display were responded to faster than the
features that were aligned diagonally.

One important issue concerns whether subjects are somehow predisposed to 
grouping vertically aligned features, and the slightest experience with
shapes in which features group vertically is sufficient to induce the
vertical bias in the neutral Ends displays.  A stronger demonstration
of the effect of experience would utilize a different object display,
in which the features are grouped diagonally instead of vertically,
and show that this reverses the grouping of features in the same neutral 
Ends displays.
We addressed this issue in Experiment 3 by using the
X displays as the disambiguating displays instead of the V
displays.  This leads to the directly opposite prediction: vertically
aligned bumps now belong to two different objects while diagonally
aligned bumps belong to the same object, so subjects should now show
an object advantage for Diagonal-Bumps on the ends as opposed to the
advantage obtained for Vertical-Bump ends in Experiment 2.

Given that we know from Experiment 2 that initially there is no difference
between Vertical-Bumps and Diagonal-Bumps in the Ends displays, we started
this experiment by exposing subjects to the Xs directly, without probing the
Ends alone first.   

\subsection*{Method}

{\it Participants}. Twenty-four subjects, half male and half female,
between 18 and 25 years of age were recruited from the undergraduate
subject pool at the University of Toronto. All subjects were
right-handed and had normal or corrected visual acuity by self report. 

{\it Apparatus and materials}. The same apparatus as used in
Experiment 1 was used here. The displays included the Ends used in
Experiment 2 as well as the full X displays, shown in
Figure~\ref{fig:bzm}a-f.  The dimensions of the X displays were
identical to those of the Vs and Ends display as these displays were
constructed from the full X displays.
As in the previous experiment, the displays fall into two feature
conditions, depending on whether the two sets of bumps fall
on the diagonal (Diagonal-Bumps) or on the vertical left or right
(Vertical-Bumps).  Note that here as opposed to the previous experiment,
Diagonal-Bumps corresponds to a single object (in the full X display),
while Vertical-Bumps corresponds to two different objects.
The task and response measures were the same as in the previous experiments.

{\it Procedure}. The experiment was run in 4 blocks with a few minutes
break between each block.  The first two blocks contained X displays,
and the next two contained Ends displays.  Each block consisted of 128
trials: four replications of the full set of X displays, or eight
replications of the Ends displays.  Trials were randomized within a
block. At the beginning of the experiment, subjects were shown
printouts containing examples of the X displays and were instructed to
make same/different judgements on the number of bumps.  Each trial
proceeded as in the previous two experiments.  Prior to starting the
experiment, subjects were given 32 practice trials, one of each of the
X displays.

{\it Design}. The design was entirely within-subject, with the
statistically independent variables being display type (Xs, Ends), 
feature location (Diagonal-Bumps, Vertical-Bumps), judgement (same,
different).  Orientation was another independent variable for the 
X displays.

\subsection*{Results and discussion}

As in the two previous experiments, separate analyses of variance conducted
crossing judgement (and orientation in the case of the Xs) with the
main factors---display type and feature location---revealed no significant
interactions (all $F < 1$), so the data were pooled across orientations and
judgements for subsequent analyses.  

An analysis of variance with mean correct RTs as the dependent measure
and display (Ends, X) and feature location (Diagonal, Vertical) was
conducted. The RT data are illustrated in Figure~\ref{fig:XendsData}.

\begin{figure}
\centerline{
\psfig{figure=Figs/Xends1.eps,height=3.5in,clip=t}
}
\caption{Mean reaction times as a function of display (Xs or Ends) and
feature location (Diagonal or Vertical feature locations) for
Experiment 3.  Subjects in Experiment 3 performed the same/different
number-of-bumps task on two blocks of the
X displays followed by two blocks of Ends displays.  Note that here
the Diagonal-Bumps lie on a single object, while the Vertical-Bumps
lie on different objects.  
In Experiment 2, for subjects who had
not yet seen either X or V displays, mean RTs on the Ends displays were
778 and 782 msec for Diagonal-Bumps and Vertical-Bumps, respectively.
Thus the object effect apparent here for the Ends
display emerges after exposure to the Xs.
% [INCLUDE DATA IN FIGURE - note that subjects overall were faster
% on this epxeriment]
} 
\label{fig:XendsData}
\jypsline
\end{figure}

The primary result from this study is the highly significant effect of
feature location, $F(1,23)=19.34, p<.001$.  This finding, together with the
lack of any significant interaction between feature location and display type,
$F(1,23)=.233, p=.64$, indicates that the object effect holds
identically for both the X displays and the Ends displays: the
Diagonal-Bumps (lying on a single bar) are processed more quickly than
the Vertical-Bumps (lying on separate bars).

To summarize, the results of this experiment are straightforward.
The difference between the single object and two object condition in
the X display is replicated.
In addition, this difference also applies to the Ends
displays: after being exposed to X displays, subjects respond relatively
quickly to the Ends displays 
which correspond to the single object in the full X displays and less
quickly to the Ends displays which correspond to the two object
condition in the full X displays.  We know from Experiment 2 that
naive subjects who do not have experience with full X displays do not
treat the Diagonal or Vertical bumped Ends differentially.  Thus, even
though the Ends conditions are ambiguous in and of themselves, they
come to be treated as equivalent to the single and two conditions of the X
displays by virtue of subjects' experience with these X displays.

Taken together, Experiments 2 and 3 provide complementary results
illustrating the ability of perceptual experience with an object to
affect an attention task where the displays contain only fragments.
In both cases, the fragments were closed shapes, so the images
not only lacked information about occlusion but contained contradictory
evidence against an occlusion interpretation, yet the results
demonstrate that they were treated as parts of objects due to experience.

\section*{Experiment 4: Location Specificity of Perceptual Experience}

The experiments presented above show that exposure to displays
containing an object can induce an object effect in ambiguous displays
that in and of themselves do not contain any evidence of an occlusion
relationship. 
Subjects exposed to X displays respond faster to diagonal ends than to
vertical ones in the Ends displays, corresponding to the object effect
in the X displays. 
In contrast, if a subject is exposed to the V displays, the vertically
aligned ends are responded to faster than the diagonal ends. The Ends
display, then, is initially ambiguous but is later parsed according to
the subject's recent perceptual experience.  

An important question then concerns the nature of the representations
that are activated as a function of experience with a particular
display.  A number of possibilities exist:

\begin{enumerate}

\item One possibility is that particular pairs of locations {\it on
the screen} obtain a processing advantage due to experience with a
specific shape.  In our experiments, the displays were a consistent
size and location on the screen.  Specific objects then could induce
groupings of screen locations where features of that object appear.
Clearly, at least pairs of positions must be primed by the object,
because the X and V displays involved the same set of individual
positions, and only differed with respect to which pairs of positions
belonged to the same objects.
On this view, the Ends displays do not need to activate any object
representations in order to produce the object effects observed in
Experiments 2 and 3.

\item Another possibility is that the perceptual organization process is
mediated by shape representations, but these representations are
highly viewpoint specific, i.e., tied to particular locations on the
screen.  This view is consistent with recent theories about the
relationship between object-based and spatial attention
(e.g, Goldsmith, 1998; Mozer et al., 1992; Vecera \& Farah, 1994; see General
Discussion), in that the objects 
activate particular spatial locations, and attention is then allocated
to those spatial positions.  On this view, exposure to the V (or X)
displays primes particular groupings of spatial locations that
correspond to the objects, and this priming is then apparent in
the Ends displays.

\item A third possibility is that the grouping process utilizes
shape representations that are not viewpoint specific.
%location specific?
On this view, experience with a particular shape does not only effectively
prime the grouping of its features in the specific spatial locations
in which they appear.  In addition, the experience may also prime
grouping of features in other locations that correspond to that same
object in a different position, scale, or orientation than the
original one.  Or it may prime some more abstract representation of
the object, one not tied to any particular exemplar.

\end{enumerate}

The aim of this experiment was to test the hypothesis contained in the
third view described above: that the
experience-induced object effect is not viewpoint specific, but rather
generalizes to instantiations of an object that are never actually
presented in any full-object displays.

We tested this by manipulating the relative positions within the Ends
displays, while not changing the X displays.  
We modified the design used in
Experiment 2---in which the subjects saw a block of Ends displays, then 
a block of V displays, followed by another block of Ends---in two ways: (1)
The second block contained X rather than V displays. 
Therefore any transfer to the Ends based on perceptual experience with
the X leads to the same prediction as in Experiment 3: the
Ends-Diagonal will be faster in block 3 than Ends-Vertical, since the
diagonal bumps correspond to a single object in the disambiguating
full object display.
(2) The features (bumps) in the Ends displays were in different locations than
the X displays. 
There were two types of Ends displays, one in which the distance between the
features was larger and the other smaller
than the X displays, so the feature locations did not match between
the Ends and X.
The critical question here is whether subjects still obtain the benefit of
block 2 such that Ends-Diagonal will be faster in block 3 than Ends-Vertical
even when the feature locations in the X and Ends displays do not correspond. 

Note that the manipulation of the feature locations in the Ends displays
does not explore the range of possible viewpoint variations,
including location, orientation, and size.  Without explicitly considering
manipulations along each of these dimensions, this approach still allows an
exploration of the basic issue of whether the object benefit from the X
displays will apply to the Ends displays even when the feature locations do
not match. 

\subsection*{Method}

{\it Participants}. Twenty-four subjects, ten male and fourteen female, 
between 18 and 23 years of age were recruited from the undergraduate
subject pool at the University of Arizona. All subjects had normal or
corrected visual acuity by self report, and were unaware of the
purpose of the experiment.

{\it Apparatus and materials}. The same apparatus used in Experiment 2
was used here. The displays included the X displays used in
Experiment 3 as well as modified versions of the Ends displays used in
Experiments 2 and 3.  Two variations of the original
displays were created by shifting the locations of the Ends: in the
Ends-Small set, the Ends were all moved towards the center of the
display by 1 cm (1.2\degree); in the Ends-Large set, the Ends
were moved away from the display center by the same amount (see
Figure~\ref{fig:endsNew}). 
As a result of this manipulation, the bumps in the Ends displays are
not in the same locations as in the full, disambiguating displays,
unlike the previous experiments.

\begin{figure}
\centerline{
\psfig{figure=Figs/Ends-BS.eps,height=3in,clip=t}
}
\centerline{
\begin{tabular}{cc}
(a) &
\hspace*{3.in}
(b) 
\end{tabular}
} 
\caption{Examples of modified Ends displays used in Experiments 4.
(a) Ends-Small. (b) Ends-Large.  The dashed lines in both displays
depict the original Ends stimuli (these are the same size in
(a) and (b)).}
\label{fig:endsNew} 
\jypsline
\end{figure}

{\it Procedure}. At the beginning of the experiment, subjects were
shown printouts containing examples of the Large and Small Ends displays and
were instructed to make same/different judgements on the number of
bumps.  Each trial proceeded as in the previous experiments.
As before, the subject's task was simply to decide whether the
number of bumps on the two ends was the same or different.   The
experiment was run in 3 blocks with a few minutes break between each
block. Trials were randomized within a block. Prior to starting the
experiment, subjects were given 32 practice trials, one of each of the
full set of Ends displays.

{\it Design}. The design was entirely within-subject.  The important
independent variables are exactly as in Experiment 2, except that there
is an additional variable (Large, Small) for the Ends displays.
As in Experiment 2, the experiment was conducted in a series of three
blocks.  In the first block, the subjects saw only
the Ends, with Large and Small randomly inter-mixed.  In the next
block, the subjects saw only X displays.   The X displays were
intermediate in position, and matched neither the Large nor Small displays.
The final block consisted
solely of Ends trials, again with Large and Small intermixed.

\subsection*{Results and discussion}

The mean correct RT data are illustrated in Figure~\ref{fig:SizeData}.
An analysis of variance with correct RT as the dependent measure and
feature location (Diagonal, Vertical), size (Small, Large), epoch (Initial,
Test), and judgement (Same, Different) was conducted on the data
from the first and third block (Initial and Test epoch) of the experiment,
since these blocks utilized the same stimulus sets.
There was no significant effect of judgement in this experiment,
($F(1,23)=.94,p=.23$), and no interactions with any other factor,
so the data was pooled across this variable. 
A similar ANOVA
conducted with percent error as the dependent measure revealed no
significant effects.

\begin{figure}
\centerline{
\psfig{figure=Figs/Size1.eps,height=4in,width=6in,clip=t}
}
\caption{Mean RTs as a function of display (Ends-Large,
Ends-Small, X), Block (1, 2, 3), and feature location (Diagonal, Vertical) in
Experiment 4.  Note that Diagonal corresponds to a single object
while Vertical corresponds to the two object condition.}
\label{fig:SizeData}
\jypsline
\end{figure}

Overall, all three factors considered individually were highly significant:
feature location ($F(1,23)=18.3,p<.001$),
epoch ($F(1,23)=40.04,p<.001$), and size ($F(1,23)=29.1,p<.001$).
The only interaction between these variables is a pairwise interaction
between feature location and epoch ($F(1,23)=12.6,p=.001$).
This interaction is critical to our hypothesis, as it reflects the
object cost induced by the intervening block of X displays.

Given this interaction, we conducted separate analyses of variance on correct
RTs crossing feature location (Diagonal, Vertical) and size (Large, Small) for
the first and third block of the experiment, and another ANOVA 
crossing feature location and orientation for the second block.

The first result of these within-block studies is the lack of a significant
difference for feature location in block 1: $F(1,23)=0.01, p=0.92$.  RTs were
faster for Small displays than Large ones, and this difference was
significant: $F(1,23)=39.14, p<.001$.  There was no interaction between the
variables: $F(1,23)=1.16, p=0.29$.  The mean RT difference between the
Diagonal and Vertical feature locations was 8 msec for the Large, and 9 msec
for the Small, in opposite directions.  This result replicates and extends the
results of the first block of Experiment 2, showing that prior to exposure to
a disambiguating display, the different feature locations of any Ends display
(regular, large, or small), are treated equally.

In block 2, we replicated the object-cost for the X displays
(Behrmann et al., 1998), as Diagonal-Bumps were processed significantly
faster than Vertical-Bumps: $F(1,23)=8.44, p=0.008$.  The mean RT
difference between these two conditions was 30 msec.  Orientation was
not significant for these displays, and there was no interaction
between orientation and feature location.

The crucial data involved the third block.  These results revealed a
significant degree of generalization of the object cost to the
different Ends displays.  As in block 1, Ends-Small displays were
processed significantly faster than Ends-Large (24 msec):
$F(1,23)=23.4, p<0.001$. 
More importantly, for both Ends displays,
diagonal-bumps were processed faster than vertical-bumps.  In the
third block, the
mean RT difference between Diagonal and Vertical was 14 and 22 msec
for the Large and Small displays, respectively.
Overall,
the effect of feature location was marginally significant
($F(1,23)=3.8, p=0.064$).  When the data for the two displays
were considered separately, 
the difference for Ends-Large approached significance
($F(1,23)=3.2, p=0.077$),
while the difference for Ends-Small was significant
($F(1,23)=4.3, p=0.05$).  

This study demonstrates that the effects of perceptual learning in
this task generalize to some degree to other screen locations.
Exposure to a block of X displays led to faster processing of the diagonal 
Ends---the Ends pairs consistent with the objects in the X displays---in
the subsequent block even though the exact feature locations did not match
in the two blocks.

This finding argues against the first two of the three alternatives presented
above, that the learning is screen location specific or tied to
location-specific shape representations.  Instead, it is more consistent with
the idea that the learning 
is affecting a more abstract form of representation.  We return to this
issue below.

\section*{General Discussion}

Numerous studies have shown that the parsing of a visual scene is an important
factor affecting the distribution of attention.  Spatial locations clearly
play an important role in this parsing process (Posner, Snyder, \& Davidson,
1980; Tsal \& Lavie, 1988).  Many other studies have shown that attention can
be directed to objects rather than to locations per se (Duncan, 1984; Egly,
Driver, \& Rafal, 1994; Kramer \& Jacobson, 1991).

A central issue is what determines the object or grouping of elements for
attentional selection.  Several studies have reinforced the pivotal role of
perceptual organization.  This role has been demonstrated in 
distractor studies, which show that it is difficult to ignore information
that belongs to the same object or group as task-relevant information.  For
example, by virtue of common fate, identification of a central target was more
affected by distant distractors that moved in the same direction as the target
than by nearby static distractors (Driver \& Baylis, 1989). Similarly, the
response-compatibility effect (enhancement from similar, and
inhibition from dissimilar distractors) was reduced when the target and
distractors were embedded in or grouped with different objects compared to
when they were grouped on the same object (Kramer \& Jacobson, 1991); for
other examples, see (Baylis \& Driver, 1992; Bundesen, 1990).

However, generic grouping principles do not suffice to define the objects
of attention, as past experience or familiarity appears to play a role.  For
example, when subjects decided whether two "x"s appear on the same or on two
different superimposed letters or non-letters, performance was superior on
letters than on nonletters (Vecera, 1993), and subjects were faster on upright
letters than upside-down letters (Vecera \& Farah, 1997).  These studies
demonstrate that attention can also be allocated preferentially to highly
familiar shapes. 

The results presented in this paper extend these effects to apply to recently
viewed novel shapes.  The primary finding in these experiments is that object
attention benefits are obtained for newly-learned objects, and for fragments
of such objects to which standard completion heuristics, e.g., relatability,
do not apply.  Relatively brief exposure to a novel, odd-shaped linking object
suffice to induce object-based attention to fragments that can be interpreted
as the visible parts of that object under occlusion.  A second primary finding
is that these experience-dependent object benefits can apply to fragments even
without any evidence of occlusion.  The effects of experience were strong
enough to overcome evidence that the fragments were separate objects (i.e.,
the presence of terminators in each End).  Also, the results of this study
show that uniform connectedness (Palmer \& Rock, 1994) is not necessary for
object attention.  Whereas Kramer and Watson (1995) found no object effects
when an object's uniform connectedness was disrupted by a new region of
different color or texture, the findings here demonstrate a robust object
effect.

\subsection*{Predictions based on subjective organization}

Considered in conjunction with the earlier results on object-based attention,
these findings highlight the role of perceptual organization in the allocation
of attention.  In all of the experiments presented here, we contrasted
subjects' responses to the same stimulus pattern as a function of their
perceptual experience.  In every study, the recent experience had a
significant influence on their organization of the displays, as assayed by
their responses.  Short-term shape familiarity, as well as long-term
familiarity and generic grouping principles, affect the scene organization
and attentional allocation.

A logical extension of this finding is that other methods of altering
subjective organization should be able to induce an object benefit.  For
example, task instructions could be used to suggest a particular parsing of
the scene.  Yantis (1992) asked subjects to track five out of ten randomly
moving dots, and to indicate, after all the dots had stopped moving, whether
or not a particular dot was a member of the target set.  He found that
subjects who were encouraged to group the target dots as a higher order form
or "object" performed better in the early phases of the experiment than those
who saw the same stimuli but did not receive such encouragement.  In a more
directly relevant study, Chen (1998) found an object effect when subjects were
instructed to view a display as two separate objects, but no effect when the
instructions suggested a single-object interpretation, for the identical
stimulus configuration.  Baylis and Driver (1992) also used task instructions
to get subjects to interpret the same displays in different ways.
These results indicate that the object effects
induced by perceptual experience may be achieved by simply modifying the
task instructions.

The hypothesis that subjective organization is a determining factor in object
attention predicts that other manipulations will also induce an object effect.
Even shorter-term familiarity may suffice; the object effect observed in the
studies presented here may be obtained in a priming study, in which the
disambiguating stimulus is used as a prime.
% MARLENE: Sekuler and Palmer , Kimchi 1998 
On the other hand, no familiarity at all may be necessary;
entirely novel objects that adhere to standard grouping principles
should also benefit from object attention.

\subsection*{Underlying mechanisms}

What mechanisms underlie the results described here, and the growing body of
object and spatial attention findings?  For some time, space and object
attention had been considered to be mutually exclusive alternatives (Kanwisher
\& Driver, 1992). More recently, attempts have been made to reconcile the two
forms of attention.  Egly and colleagues (Egly et al., 1994) have argued that
both processes co-exist. For example, they reported that a cost in RT and
accuracy is incurred when attention is shifted between a cue and a target both
when the target appeared at a second location in the cued object
(within-object, object attention) or at an equidistant location but in a
different object (between-object, spatial attention).  Data favoring the
simultaneous operation of space- and object-based processes also come from a
study by Umilta et al. (1995) who cued a vertex of a cube which either
remained stationary or rotated. Subjects not only showed facilitation when the
target appeared in the same spatial or retinal location as the cue
(stationary) but also when the target appeared in a different retinal location
but in the equivalent object-defined location as the cue (rotated
condition). Similar findings are revealed in studies on inhibition of return
in both location- and object-based coordinates (e.g., Gibson \& Egeth, 1994;
Tipper \& Weaver, 1996).

One standard account of object- and space-based attention is that low-level
visual routines identify regions of salience or coherence in the visual field
pre-attentively and in parallel. These regions are then subjected to further
analysis by focal attention processing for later object identification
(Julesz, 1981; Koch \& Ullman, 1985; Neisser, 1967; Treisman, 1982; 1988).
This view of a {\em two-stage feedforward} model in which spatial attention
follows object-based attention has been proposed to account
for numerous findings in the visual search literature as well as findings in
which grouping, based on feature similarity or proximity, occurs early, in
parallel and independent of spatial attention (Driver, Baylis, \& Rafal, 1992;
Marshall \& Halligan, 1994; Moore \& Egeth, 1997). This sequential,
hierarchical model, however, has been increasingly challenged.  
% Several studies have shown that very little perceptual processing occurs
% without attention (Mack et al., 1992; Benav et al., 1992).  
Under this model, the objects of object-based attention are defined by
low-level visual routines.  Results demonstrating the effects of familiar
specific objects on object-based attention (Vecera \& Farah, 1997), and
results showing that specific objects influence image segregation (Peterson,
1994) call this account into question.  In addition, object-based attention
findings in which the different objects share a common region in space (e.g.,
Duncan, 1984; Lavie \& Driver, 1996; Behrmann et al., 1998; and the results
reported here) are also difficult to reconcile with this feedforward approach.

An alternative to the simple feedforward scheme is one in which object- and
space-based processes operate in parallel and mutually influence each other
(Farah, 1990; Humphreys \& Riddoch, 1991; Humphreys et al., 1996). The
interaction occurs through a topographically organized {\em grouped array},
which represents the currently active bottom-up input from the environment as
well as top-down activation from matching higher-level descriptions.  Through
this explicit array, spatiotopic information and grouping information are both
present and simultaneously influence visual processing. Vecera and Farah
(1994) claimed that such an array-like representation must exist; using the
Egly et al. (1994) paradigm, they showed not only that there is a cost
associated with shifting attention within and between objects but also that
the cost of shifting attention between objects increased as the spatial
distance between the objects increased.  Similarly, as is usually the case in
the distractor paradigm, Kramer and Jacobson (1991) showed that the response
compatibility effects were diminished when the spatial distance between the
grouped elements was increased. Taken together, these findings suggest that
both space- and object- selection are operative and, as such, are more
compatible with a parallel account rather than with a serial two-stage
mechanism.

Within this parallel account of attention, the issue of how the grouped array
operates is still open.  The original proposal was that a combination of
generic grouping principles and shape-specific information act to label the
array locations (Vecera \& Farah, 1994; Kramer et al., 1997).
Geometric properties of the display, such as the relatability of
fragments (Kellman \& Shipley, 1992), would be fundamental elements of the
grouping component. 
% MARLENE: but none of this tells us about the spatial component of the
% grouped array just about the object piece.  Haimson?

With respect to the influence of familiar shapes, the standard conception is
that this involves representations of whole objects.  Within this view,
several possibilities exist.  An object could be: (a) exact exemplar, specific
to particular spatial locations and orientations; (b) fuzzy exemplar,
specifying a particular shape, but less specific in its spatial instantiation;
or (c) spatially invariant object representation.  The results of the
experiments presented here do not bear on the spatially invariant
representation hypothesis.  Limited evidence exists for this view: the results
of Vecera and Farah's (1994) study implicated spatially-invariant object
representations, but other studies have not found evidence for them (for
further discussion, see Kramer et al, 1997).  Experiment 4 in this paper
provides evidence that the object effect can transfer to different feature
locations, which makes the exact exemplar representation unlikely.  Instead,
these results are consistent with the fuzzy exemplar representation, as there
was some spatial overlap between the learned feature locations and the
generalized locations.  In addition, the fact that the degree of transfer was
greater to the Small-Ends is also consistent with the fuzzy exemplar, under
the assumption that the object attention benefit extends to all locations
encompassed by the viewed exemplar.

An alternative conception of the shape-specific component of the grouped array
involves learned configurations of local features rather than whole objects. 
This mechanism is consistent with the computational model, MAGIC (Mozer et
al., 1992).  Under this interpretation, the disambiguating displays primarily
serve to facilitate the grouping of particular pairs of Ends in the displays;
and this grouping then applies to the ambiguous displays.
This account is supported by fact that the grouping of the Ends appears to
operate even in the presence of terminators, which provide evidence that the
Ends are complete objects themselves.  This feature-based representation can
also account for the results of Experiment 4, assuming a feature-based analog
of the fuzzy exemplar model proposed for the whole-object representation.

Finally, we note that all of these different mechanisms can be learned
from statistical structure in the environment.  
Whole objects, or particular local feature configurations, can both
be extracted based on experience with various feature combinations in images.
The evidence for experience-dependence provided in this paper further
indicates that such higher-order statistical regularities play a critical role
in visual perception.

\subsection*{Issues for further study}

The studies presented here lead to many questions requiring further
research.  One important issue concerns the number of exposures to
the disambiguating stimulus that are required to obtain the object
advantage in the ambiguous displays.  In all of the experiments
described here, one block (consisting of 32-128 trials with the
relevant stimulus) was sufficient to obtain the effect.  It may
be possible that many fewer exposures are required.  A related question is
whether object effects can be induced simply by instructions.  For
example, suggesting an interpretation of the Ends displays as two
diagonal crossed bars where the central portion of the bars have been
woven through the fabric of a white screen, might reduce the required number
of exposures to obtain the object advantage.

A second important issue concerns the duration of the effects of perceptual
learning shown here.  An interesting study would test subjects at different
time intervals after exposure to the disambiguating stimuli to determine
how long this experience exerted an effect on the processing of ambiguous
stimuli (e.g., Treisman \& de Schepper, 1996).

A third issue for further study considers how the task that the
subjects perform may influence the degree of effect of perceptual experience.
In the experiments presented here, the task did not require
any interpretation of the display in terms of objects.  Instead, the subjects
simply had to find and compare the number of bumps; the objects in the display
were irrelevant to the task.  Other studies have also found effects of
experience on object attention even when the experience is not task-relevant
(e.g., Goldsmith, 1998).  This incidental form of learning is in contrast to
most studies of the effect of experience with novel objects on future
processing (Edelman \& B\"ulthoff, 1992; Gauthier \& Tarr, 1997), where the
task (e.g., familiarity judgements, identification) explicitly required object
identification. Similarly, 
studies of perceptual learning have demonstrated stronger learning when
the stimuli are task-relevant (e.g., Ahissar \& Hochstein, 1993; Shiu \&
Pashler, 1992), while Chun and Jiang (1998)
have shown that a consistent configuration of distractors can speed visual
search when it is task-relevant (indicative of target location).
Based on these studies, one would predict that making
the objects relevant to the task 
object identification would lead to stronger, and perhaps longer-lasting
effects of experience.  

\nocite{Duncan84,BaylisDriver93,KramerWatson95,KramerJacobson91}
\nocite{BaylisDriver92,Yantis92,Chen98}
\nocite{MooreEtal98,LavieDriver96,DriverBaylis89,BZM99,VeceraFarah97}
\nocite{MozerEtal92,Yantis95,ShipleyKellman92,KellmanShipley91,KaniszaGerbino82}
\nocite{BubGum91,Nickerson65,KellmanShipley92,Bregman81,Goldsmith98,TsalLavie88}
\nocite{EglyEtal94,Bundesen90,Vecera93,PalmerRock94,KanwisherDriver92}
\nocite{UmiltaEtal95,GibsonEgeth94,TipperWeaver96}
\nocite{MackEtal92,BenavEtal92,MarshallHalligan94,MooreEgeth97,DriverEtal92}
\nocite{Julesz81,KochUllman85,Neisser67,Treisman82,Treisman88}
\nocite{Farah90,HumphreysRiddoch91,HumphreysEtal96,VeceraFarah94,KramerEtal97}
\nocite{TreismanDeSchepper96,AhissarHochstein93,ShiuPashler92,ChunJiang98}
\nocite{GauthierTarr97,EdelmanBulthoff92,Peterson94}

\bibliography{plearn}

\end{document}

Kramer-Watson versus our's?

possible additions
  block effects/time-course
  MSE
  errors in figs
  *** for significant effects

daphne -- obj cost/advantage; time-course in visual learning?

mary --
  bregman's Bs - why not seen without occluder?
  role of orientation
  instructions (one group Ends as objs, others as parts)
  block effects in Expt 3?  
  visual angles of objects (particularly Ends-Large/Small)
  statistical structure
  focus - familiarity gained withiin within experimental setting
     sufficient for affecting perception, induce object effect
  viewpoint in Expt 4 - DONE
  overlap in positions Large/Small - DISCUSSION


