VISUAL COGNITION, 0000, 00 (00), 000 /000
Explicit memory for rejected distractors during
visual search
Melissa R. Beck and Matthew S. Peterson
Psychology Department, George Mason University, Fairfax, Virginia, USA
Walter R. Boot
Beckman Institute and Department of Psychology, University of Illinois
Urbana-Champaign, Champaign, Illinois, USA
Miroslava Vomela
Psychology Department, George Mason University, Fairfax, Virginia, USA
Arthur F. Kramer
Beckman Institute and Department of Psychology, University of Illinois
Urbana-Champaign, Champaign, Illinois, USA
Although memory for the identities of examined items is not used to guide visual
search, identity memory may be acquired during visual search. In all experiments
reported here, search was occasionally terminated and a memory test was presented
for the identity of a previously examined item. Participants demonstrated memory
for the locations of the examined items by avoiding revisits to these items and
memory performance for the items’ identities was above chance but lower than
expected based on performance in intentional memory tests. Memory performance
improved when the foil was not from the search set, suggesting that explicit identity
memory is not bound to memory for location. Providing context information
during test improved memory for the most recently examined item. Memory for the
identities of previously examined items was best when the most recently examined
item was tested, contextual information was provided, and location memory was
not required.
Please address all correspondence to Melissa R. Beck, Department of Psychology, George
Mason University, MS 3F5, 4400 University Dr., Fairfax, VA 22030, USA. E-mail:
[email protected]
This work was supported by grants from the National Institute of Health to MSP (R01
MH64505) and the Office of Naval Research to AFK and WRB. Parts of this research were
presented at the 2005 Vision Sciences Society in Sarasota Florida.
http://www.psypress.com/viscog
# 2006 Psychology Press Ltd
DOI: 10.1080/13506280600574487
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BECK ET AL.
It is generally agreed that memory is involved in visual search, but that it’s
role is limited (for review see Wolfe, 2003). This limited capacity role of
memory in visual search has been demonstrated by the tendency for
fixations and attention to be biased away from previously examined items
(Klein & MacInnes, 1999; Kristjansson, 2000; McCarley, Wang, Kramer,
Irwin, & Peterson, 2003; Peterson, Kramer, Wang, Irwin, & McCarley, 2001;
Takeda, 2004; but see Horowitz & Wolfe, 1998). This memory allows people
to avoid needlessly reinspecting items that have already been identified as not
being the target of search. The form of memory used to bias attention away
from previously examined items is largely unknown (but see Boot,
McCarley, Kramer, & Peterson, 2004). It has been demonstrated that
changing the location but not the identity of a previously examined item
disrupts the ability to bias attention away from the item (Beck, Peterson, &
Vomela, in press). Consequently, it would appear that attention in visual
search is guided by memory for the locations of previously inspected items
rather than their identities. Although memory for examined item’s identities
may not be used to guide attention, here we examine the extent to which
explicit memory for the identity of rejected distractors is acquired during
visual search.
MEMORY IN VISUAL SEARCH
Researchers supporting the role of memory in visual search argue for the
necessity of keeping track of previously examined items in order to prevent
unnecessary reinspection of these items (Dodd, Castel, & Pratt, 2003; Klein
& MacInnes, 1999; Müller & von Muhlenen, 2000; Takeda & Yagi, 2000).
Peterson et al. (2001) demonstrated that the probability of revisiting an item
during search is less than chance for up to 11 or more previously examined
items. One possible explanation for the lack of revisits observed by Peterson
and colleagues is that some form of prospective memory or systematic scanpath planning was used to bias attention towards unexamined items rather
than away from examined items. For example, a simple strategy of searching
from top to bottom would prevent revisits even in the absence of some form
of retrospective memory for examined items. To prevent scan-path strategies
from contaminating memory estimates, McCarley and colleagues (2003)
used an eye movement contingent display. In their task, no more than three
items were visible at any one time: The currently fixated item and two
potential saccade targets. Items were the same size and the same spacing as
used by Peterson et al. The key was that during an eye movement, two new
potential saccade targets appeared and the remaining items were removed.
This forced participants to choose which of two items to examine. After
IDENTITY MEMORY DURING SEARCH
3
several saccades, one of the potential saccade targets was always an
unexamined item (new item) and the other was an item that had been
examined previously (old item) in the trial. If there is memory, then the new
items should always be chosen, and if there is no memory, then the new item
will be chosen 50% of the time. McCarley and colleagues found that
participants moved their eyes to the new item more frequently than
predicted by chance, and this effect lasted for roughly the last four to five
items examined. This suggests that people have retrospective memory for the
last four examined items or locations.
Retrospective memory for the last four to five items is driven by the
locations of these items rather than their identities. For example, Beck et al.
(in press) demonstrated that changing the location of an examined item
disrupted search, whereas changing the identity of an examined item did not.
In their task, search items (rotated Ts and Ls) were surrounded by large
unique coloured shapes (e.g., a red circle). While participants performed the
visual search task, the shape surrounding a previously examined item
changed identity (e.g., red circle changed into a yellow square) or a
previously examined item moved to a new location. When a previously
examined item changed identity, fixations were still biased away from the
item. However, when a previously examined item moved from one location
to another, fixations were no longer biased away. This suggests that the
memory used to bias attention away from previously examined items is
based on the items’ locations rather than their identities.
One key question remains: Although individuals show memory for the
locations of the last few items, why does changing the identity of an
examined item have no effect on search? One possibility is that the identity
information is remembered, but this information is not used to prevent
revisitations. Alternatively, the locations but not the identities of examined
items might be remembered, and hence changing the identity of an examined
item has no effect on search.
Although research examining eye movements during visual search
suggests that memory is limited to the location of items and not their
identities, there have been several demonstrations of memory for visual
details of previous viewed stimuli (Hollingworth & Henderson, 2002;
Standing, 1973). Similarly, explicit long-term memory (LTM) tests show
memory for specific visual details of distractors during search (Castelhano
& Henderson, 2005; Williams, Henderson, & Zacks, 2005). Williams et al.
(2005) presented participants with an incidental memory test for the targets
and the distractors from a visual search task that was completed at least 10
minutes prior to the memory test. Each distractor and target was viewed on
two separate visual search trials, and then memory was tested using a twoalternative forced choice (2AFC) test that required discriminating the target
or distractor from a nonsearch item with the same verbal label (e.g., a
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BECK ET AL.
yellow telephone) but different visual features. Memory performance for
targets was fairly high (approximately 85%) while performance for
distractors was much lower but still above chance (approximately 60%).
Castelhano and Henderson (2005) found similar results when participants
searched real-world scenes for a specific target and then were asked to
discriminate a nontarget object from another object of the same basic level
category or from the mirror image of the same object. Performance on the
incidental LTM memory test was lower than on an intentional LTM
memory test, but still significantly above chance (60 /75%). This suggests
that even without the intent to remember, some visual details are stored in
LTM.
Above chance performance on LTM tests for distractors appears in
conflict with the lack of memory disruption when an item changes identity
during search. Possibly, distractor identity information is remembered
during search, but it is not used to guide visual attention during search.
Therefore, an explicit test of memory for the identity of examined distractors
during the visual search task may reveal memory for distractor identities.
When participants are asked to complete an intentional memory task that
does not coincide with a visual search task, they can remember roughly ‘‘5.4
objects worth of information’’ (Irwin & Zelinsky, 2002). In Irwin and
Zelinsky’s study, participants were permitted to examine seven objects for 1 /
15 fixations. Then, they were asked to indicate which object was in a
particular location. Memory performance for the last three items fixated and
the item about to be fixated on the saccade when the memory test was
presented was high (/80%). Therefore, viewers have the ability to
intentionally encode the identities and locations of recently viewed items.
The current studies examine the extent to which observers use this ability
during visual search.
MEMORY FOR IDENTITY VERSUS LOCATION
Previous research suggests that memory for location may have special
importance over memory for identity. For example, research suggests an
overlap between visual search and spatial, but not object, working
memory (Oh & Kim, 2004; Woodman & Luck, 2000). In addition,
memory for the locations of distractors across visual search trials can be
used to improve the efficiency of visual search (contextual cueing; Chun
& Jiang, 1998). Therefore, memory for the locations of distractors during
visual search may be more important than memory for the identities of
distractors.
Theories of how visual information is combined across eye movements
generally assume that spatial information is the pointer to a representation,
IDENTITY MEMORY DURING SEARCH
5
which contains identity or feature information about previously attended
items (Henderson, 1994; Henderson & Anes, 1994; Henderson & Siefert,
2001; Kahneman, Treisman, & Gibbs, 1992). These theories suggest that
identity information about an object is tied to spatiotemporal information
that is determined by the relative position of the item in relation to other
items in the display. If identity and location are stored jointly, memory
performance on a test that required only identity information should be
equivalent to a test that required identity and location information.
However, it has been suggested that features can be stored individually
and that binding these features requires maintenance by attention (Wheeler
& Treisman, 2002; Wolfe, Oliva, Butcher, & Arsenio, 2002). Therefore,
performing a visual search task may use attentional resources that are
otherwise used to maintain bound features. This would predict that we
should find good memory for individual features but not for these features
when referencing them through a particular location.
Here we examined the extent to which explicit memory for previously
examined distractors is available during visual search. Participants searched
displays for one of two possible targets. Occasionally during the visual search
task, search was stopped, and memory for one of the previously examined
items in the display was tested using a 2AFC. Memory was probed by asking
participants which of two objects was in a particular location. We tested
memory for the identities of previously examined objects and for these
identities bound to specific locations by manipulating the type of foil used in
the memory probe. If the foil was another previously viewed distractor,
participants needed to remember which item was in a specific location in
order to successfully perform the memory task. If the foil was not one of the
previously viewed distractors, participants could simply remember which
object was viewed during the trial irrespective of location. These experiments
allow us to examine the extent to which explicit memory for distractor
identity is available during a visual search task and whether memory for
identity is bound to location. In addition to examining explicit memory for
distractor identity, we will replicate findings showing attentional guidance by
memory for distractor locations. Attentional guidance by memory for the
locations of previously examined distractors is demonstrated by infrequent
(lower than chance) revisits to previously examined distractors.
EXPERIMENT 1
The search task in Experiment 1 was similar to the oculomotor contingent
paradigm used by McCarley and colleagues (2003), in which only two items
from the search set (other than the fixated item) are visible at any one time
(see Figure 1). This oculomotor contingent search task was used in order to
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BECK ET AL.
1
1
2
1
3
Event 1
Event 2
1
2
2
Event 3
4
3
Event 4
4
5
Event 5
Figure 1. The sequence of events within a search trial in Experiment 1. The dashed circle indicates
the observer’s point of fixation. Numbers are used to indicate different stimulus items. Stimuli in the
actual displays were letters of the alphabet too small to be discriminated without foveation. Note that
from Event 3 onward, the observer was forced to choose between executing a saccade from the
currently fixated item toward a new item and executing a saccade toward a decoy item that had already
been seen. See the text for additional details. Reproduced from McCarley et al. (2003).
replicate the finding that there is memory for the locations of the last three to
four examined items even when the ability to plan future saccades is
eliminated and to ensure that the memory task does not interfere or change
search behaviour. Furthermore, we added a memory probe for the last four
examined items on a subset of the trials to determine the extent to which
there was also explicit memory for the identity of previously examined items.
The target was an S or a U, and distractors were most of the remaining
letters of the alphabet. On approximately 30% of the trials, the search was
stopped, the items were removed from the screen, and a red box appeared
around an empty location. Subjects were given a 2AFC task in which one of
the items was the item that had appeared at that location and the alternative
choice (foil) was an item that had appeared somewhere else (identity bound
to location condition) or an item that had not appeared at all (identity only
condition). High accuracy in both conditions would indicate that identity
information is retained and it is bound to the location of the examined
distractor. However, it may be that identity information is retained but not
bound to location in which case we would expect high accuracy in the
identity condition but not the identity bound to location condition. Finally,
if there is no explicit memory for identity we would expect poor memory
performance in both conditions.
Method
Participants. Twenty participants from the University of Illinois were
paid US$24 for their participation in three 45-minute experimental sessions
on separate days. They were each assigned to either the identity (n/10) or
the identity bound to location condition (n/10). The average age of the
participants in the identity condition was 23 and the average age of
the participants in the identity bound to location condition was 26. All
participants demonstrated normal or corrected-to-normal visual acuity.
IDENTITY MEMORY DURING SEARCH
7
Apparatus. Stimulus presentation was controlled by a Pentium-based
computer. Stimuli were presented on a 21-inch colour SVGA monitor with a
resolution of 1280 /1024 pixels and a refresh rate of 85 Hz. Eye movements
were recorded using an Eyelink II eyetracker (500 Hz temporal resolution,
0.28 spatial resolution). An eye movement was classified as a saccade if its
distance exceeded 0.28 and either its acceleration reached 9500 deg/s2 or its
velocity reached 30 deg/s. A chinrest 71 cm from the monitor was used to
stabilize head position and the monitor subtended approximately a 27.848
visual angle.
Stimuli and procedure. Participants viewed dynamic search displays
containing small white letters (0.38 /0.38) on a black background. These
letters were too small to be identified without foveation. Participants were
asked to search for an S or a U target among distractor letters comprising of
the remaining letters of the alphabet from A to V. Participants were asked to
press one of two buttons depending on the identity of the target.
Only one, two, or three of these items were presented on the screen at any
one time. The trial started with participants fixating a white fixation point at
the centre of the screen and pressing the spacebar. The fixation point was
then removed and the first search item appeared in the display (Event 1; see
Figure 1). Participants had to saccade to this item to determine its identity.
As participants moved their eyes to this item, a second item was added to the
display (Event 2). If the first item fixated was the search target participants
had to indicate whether it was an S or a U. If this item was not the target,
participants were required to move their eyes to the newly appearing item
and determine whether it was the target. During this eye movement another
item was added to the display (Event 3). Note that at this point if the fixated
item was not the target, participants had the option of two saccade targets
with the next eye movement. Participants could either saccade back to the
first item visited (old item), or saccade to the uninspected item (new item).
These two saccade targets were equidistant from the fixated item. With each
additional saccade participants made, one old item and one new item were
presented. The old item was sampled randomly from the list of items already
presented during the trial. If the randomly chosen item was the item just
previously fixated, this item remained on screen. If the randomly chosen
item was not the item just previously fixated, the just previously fixated item
was removed from the display and the randomly chosen old item was
presented in the same location and with the same identity as it had
previously. Stimuli were constrained such that the two items presented with
each saccade were at least 5.4 degrees from one another and from the about
to be fixated item. If no stimuli could be found to satisfy these conditions,
the trial terminated. Over the series of events within a trial, participants were
8
BECK ET AL.
presented with the choice to saccade to an old item or to saccade to a new
item. The old item could be the item fixated one (lag/1) or more fixations
ago.
Attentional guidance by memory for previously inspected locations is
demonstrated in this paradigm by greater than chance selection of the new
item, since participants have the opportunity to saccade to either an old item
or a new item. Targets occurred on 25% of nonprobe trials. On most trials
(67%), the trial continued until the target was found or 11 saccades were
made. On the remaining trials (33%), the screen was erased during the
saccade following the sixth fixation on an item, and a red box (0.8 /0.8)
appeared around one location. Within this box, a question mark appeared,
and one letter appeared to the left of this box and one letter appeared to the
right of this box. One letter was always the letter that appeared at that
location, and this letter could occur equally often to the left or right of the
probe box. The letter opposite the letter that was at that location was a foil
letter. In the identity only condition, the foil was an item that never appeared
during the trial. In the identity bound to location condition, the foil was a
letter that participants had fixated during the trial, but occurred at a
location other than the probe location. The foil was randomly chosen from
the list of old items. In each condition, the probed letter could be the letter
fixated one, two, three, or four fixations back (lag). Participants were asked
to choose which item was at the probed location by pressing one of two
buttons on the controller.
Participants were instructed that the search task was the primary task
and that the majority of trials would be search trials. Participants
completed three blocks of 240 trials. Each block was completed on separate
days.
Results
As was done in McCarley et al. (2003), the first block of trials was
considered practice and was not included in the reported analysis because we
wanted to be sure that participants were familiar with the task. Excluding
this first block did not change the results in any meaningful way.
Search performance. If participants failed to respond prior to the 11th
fixation on an item, the trial was ended. Participants could fail to respond
because no target was presented (after 11 fixations) or because they saw the
target but failed to report its presence. Participants failed to respond to the
target on 20% of the target present trials in the identity only condition and
IDENTITY MEMORY DURING SEARCH
9
0.8
Identity only
Proportion of revisits
0.7
Identity bound to location
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
Lag
Figure 2. Proportion of revisits to old items in Experiment 1. Chance is .5 (dotted line) because there
was always one new item and one old item to choose from.
18% of the target present trials in the identity bound to location condition.1
When participants did respond during a trial, they were 95% accurate in the
identity only condition and 97% accurate in the identity bound to location
condition.
Eye movements. Figure 2 presents the mean proportion of fixations
when the old item was chosen as the saccade target over the new item for the
nonprobe trials in the identity only and the identity bound to location
conditions. Data are plotted as a function of the number of gazes since the
old item was last examined (revisit lag). Because there were always two items
to choose from (one old item and one new item), the probability of randomly
choosing an old item was always .50. One-sample t-tests revealed that old
1
Particularly high miss rates in this paradigm may be attributed to the dynamic nature of the
search display or the low probability of a target occurring. Because the item that was just
examined would often be removed from the screen after the eyes moved to a new item,
participants were often unable to reexamine an item just after it was examined. Therefore, if
participants fixated the target without responding, they rarely had a second chance to view the
target. This is unlike traditional search paradigms in which participants might fixate the target,
move their eyes without recognizing it, and move their eyes back to the target location later
during the trial. As noted by Peterson et al. (2001), refixations during a static search display
were often directed toward the target after the eyes had fixated too briefly the first time. Another
potential explanation might be the low incidence of target occurrence in this paradigm (only
25% of the nonprobe trials). This may have caused participants to adopt a stringent criterion for
detecting the target.
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BECK ET AL.
items examined one, two, or three fixations prior to the current fixation were
reexamined at rates lower than .50, smallest t(9) / /4.44 at revisit lag 3 for
the identity only condition, all p sB/.002, for both the identity only and the
identity bound to location conditions. Therefore, there was memory for item
location for the past three items examined. Furthermore, there were no
differences in the rates of revisits between the identity and identity bound
conditions for any of the revisit lags.
Explicit memory performance. One-sample t -tests revealed that memory
performance (proportion correct) on the probe trials in the identity only
condition was significantly higher than chance (.50) at all lags, t(9) / /6.24,
p B/.001 for lag 2 and p/.001 for lags 1, 3, and 4 (see Figure 3). For the
identity bound to location condition, only lags 1 and 4 were significantly
higher than chance (both p-values B/.01 and lowest t-value for lag 4), t(9) /
/3.37, p /.008. Lags 2 and 3 were not significantly different from chance
(p s/.09). Memory performance was entered into an ANOVA with lag (1, 2,
3, 4) as a within-subject factor and condition (identity only, identity bound
to location) as a between-subjects factor. Performance was higher in the
identity only condition (M /70%) than the identity bound to location
condition (M /59%), F (1, 18) /7.16, MS /0.24, p /.02. Lag had no effect
1
Proportion correct
0.9
0.8
0.7
0.6
0.5
0.4
0
1
2
3
4
Lag
Identity only
Identity bound to location
Figure 3.
Memory performance in Experiment 1.
5
IDENTITY MEMORY DURING SEARCH
11
on memory performance, F (3, 54) / 1.95, MS / 0.02, p / .13, and the
interaction between lag and condition was nonsignificant, F (3, 54) / .19,
MS / 0.001, p / .91.
Discussion
Replicating previous research, attentional guidance by memory for the
locations occurred for at least the last three items examined, demonstrated
by low refixation rates for old items during visual search. When asked to
explicitly identify these items, participants were 70% accurate when
distinguishing an item they had looked at just one fixation prior to the
memory test from an item that was never viewed during the trial, and about
60% accurate at distinguishing the last examined item from other previously
examined items. Better memory performance when location information was
not needed to accurately choose the identity of the probed item (identity only
condition) suggests that identity and location information about a previously examined item are not necessarily linked in memory. Furthermore, in
both conditions, memory performance did not change across lag, suggesting
that more recently inspected items are remembered equally well as items
examined several fixations prior to the memory test.
EXPERIMENT 2a
Although explicit memory performance in Experiment 1 was above chance,
it is possible that the nature of the gaze contingent search paradigm used in
Experiment 1 may have impaired memory performance. The removal of
items from the screen during saccades may disrupt memory representations
associated with the items (Henderson, 1994; Henderson & Anes, 1994;
Henderson & Siefert, 2001; Kahneman et al., 1992). In Experiments 2a and
2b we examine explicit memory for the identities of previously examined
items during search using a more traditional search procedure. In this search
task all items remain on screen until a response is made. If memory was poor
in Experiment 1 because removing items from the screen disrupted explicit
access to their representations, then we would expect to find improved
memory performance when all items remain on screen. In Experiment 2a the
foil was always another previously examined item in the display and in
Experiment 2b the foil was an item that was not presented in the display.
Method
Participants. Eleven students (8 females and 3 males) from George
Mason University participated in this study for course credit. The average
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BECK ET AL.
age of the participants was 26 years. All participants demonstrated normal
or corrected-to-normal visual acuity.
Apparatus. A Power Macintosh G4 (Dual 1 GHz), equipped with a 20inch (viewable) ViewSonic P225fb with a resolution of 640 /480 pixels and
capable of running at 120 Hz, running custom software was used to present
the stimuli, control the timing of experimental events, and record participants’ response times. This computer was networked to a Dell Pentium 4
machine that collects eye-tracking data in conjunction with an Eyelink II
system (same system as used in Experiment 1). The chinrest was located 61
cm from the monitor which subtended approximately a 30.28 visual angle.
Stimuli and procedure. Participants viewed static search displays containing small white letters (0.668 /0.568) on a black background. Participants were asked to search for an S or a U target among distractor letters
comprising of the remaining letters of the alphabet from A to V. Participants
were asked to press one of two buttons depending on the identity of the
target.
The trial started with participants fixating a white fixation point at the
centre of the screen and pressing the spacebar. The fixation point was then
removed and a search array of 12 items appeared on the screen (11
distractors and 1 target). When the items were not fixated, white circular
placeholders (Gaussian function, SD/0.38) covered each item. When the
direction and speed of a saccade indicated that a specific item was about to
be fixated the placeholder was removed to reveal the item. Therefore,
participants had to fixate each item to determine its identity.
On most trials (67%), the trial continued until the target was found. On
the remaining trials (33%), the trial was stopped after the fifth fixation on an
item; a red circle (diameter /2.48) appeared around one of the locations. All
the items remained on screen covered by placeholders, regardless of fixation
position. One letter appeared at the top left of the circle and one letter
appeared at the top right of the circle. One letter was always the letter that
appeared at that location, and this letter could occur equally often to the left
or right of the probe circle. The other letter was a foil letter, a letter that
participants had fixated during the trial, but occurred at a location other
than the probe location. The foil was randomly chosen from the list of items
fixated previously. In each condition, the probed letter could be the letter
fixated one, two, three, or four fixations back (lag). Participants were asked
to choose which item was at the probed location by pressing one of two
buttons on the keyboard.
Participants were instructed that the search task was the primary task and
that the majority of trials would be search trials. Participants completed two
blocks of 160 trials separated by a self-timed break.
IDENTITY MEMORY DURING SEARCH
13
Results and discussion
Search performance. Participants with accuracy lower than 90% were
excluded from analysis (n /1). Overall search accuracy for the remaining 10
subjects was 97%.
Eye movements. In order to examine memory for the location of
previously examined letters, we compared the rate of refixations on
previously examined items in the nonprobe trials to the rates that would
be expected in a memoryless search. Similarly to Peterson et al. (2001), a
Monte Carlo simulation of memoryless search was conducted (i.e., the
probability of randomly revisiting an item on each fixation was calculated
based on the number of items available for revisiting). Eleven ‘‘subjects’’
were run through a simulation of 143 trials (the average number of nonprobe
trials) at a set size of 12 with the following constraints. Items were randomly
selected for each fixation excluding the most recently fixated item. If a
selected item had been previously selected during the trial (a revisit), the
number of items between this selection and the previous was recorded (revisit
lag). The probability of revisiting an item was calculated by dividing the
number of revisits at each revisit lag by the number of gazes in the trial.
The probability of revisiting items in the observed data of the nonprobe
trials was similarly calculated by dividing the number of revisits at each
revisit lag by the number of gazes in a trial. Observed data was compared to
predicted data (memoryless model) using multiple t-tests. The observed
probability of refixations was significantly lower than that of the memoryless
model for revisit lags 1 /4, smallest t(11) /6.58 at revisit lag 1, all ps B/.001
(see Figure 4). These data suggest that there is memory for the locations of at
least the last four examined items.
Explicit memory performance. One-sample t-tests revealed that memory
performance was significantly higher than chance (50%) at lags 1, 2, and 3,
t(9) /5.785, p B/ .01, t(9) /3.55, p B/.01, and t (9) /2.79, p/.02, for lags 1,
2, and 3, respectively, but memory performance at lag 4 was not significantly
different from chance, t (9) /1.49, p /.17. Memory performance on the
probe trials (percentage correct) were entered into an ANOVA with lag (1, 2,
3, 4) as a within-subjects factor. Memory performance varied across lag, F (3,
27) /5.34, MS /0.05, p /.005. A trend analysis used to further examine the
main effect of lag on memory performance, revealed a significant linear
trend, F (1, 9) /14.79, MS /0.12, p/.004, and accounted for 62% of the
within-subjects variation for memory performance. As can be seen in Figure
5 this trend was characterized by a decrease in memory performance as lag
increased.
14
BECK ET AL.
0.08
Memoryless
Observed
Proportion of revists
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
1
2
3
4
5
Lag
Figure 4. Proportion of revisits to old items in Experiment 2a (observed) and the proportion
predicted by the memoryless search model (memoryless).
As in Experiment 1, participants used memory of the locations of
previously examined items to bias fixations toward new items, and explicit
memory performance for the identities of the items was above chance for the
three most recently examined items. Furthermore, the linear trend for
memory performance across lag suggested that memory for an examined
1
Proportion correct
0.9
0.8
0.7
0.6
0.5
0.4
0
1
2
3
Lag
Figure 5. Memory performance in Experiment 2a.
4
5
IDENTITY MEMORY DURING SEARCH
15
item’s identity rapidly fades as more items are examined. In Experiment 2b
we examine memory for previously examined items not bound to location.
The foil is always an item that did not appear during the trial. Improved
memory performance would suggest that memory for the items is available
but it is not bound to the locations of the items.
EXPERIMENT 2b
Method
Participants. Twenty-four students (23 females and 1 male) from George
Mason University participated in this study for course credit. The average
age of the participants was 20 years. All participants demonstrated normal
or corrected-to-normal visual acuity.
Stimulus and procedures. All stimuli and procedures were the same as in
Experiment 2a except on memory probe trials; the foil was a letter that had
not been viewed during the trial.
Results and discussion
Search performance. Participants with accuracy lower than 90% were
excluded from analysis (n /1). Overall search accuracy for the remaining 23
participants was 97%.
Eye movements. A Monte Carlo simulation of memoryless search was
conducted for 24 ‘‘subjects’’, 143 trials (the average number of nonprobe
trails), and a set size of 12 with the same constraints reported in Experiment
2a. Observed data was compared to predicted data (memoryless model)
using multiple t-tests. The observed probability of refixations was significantly lower than that of the memoryless model for revisit lags 1/4, smallest
t(44) / /15.17 at revisit lag 1, all ps B/.001 (see Figure 6). These data
suggest that there is memory for the locations of at least the last four
examined items.
Explicit memory performance. One-sample t-tests revealed that memory
performance was significantly higher than chance (50%) at all lags, smallest
t(22) /9.5 at lag 4, p B/.001. Memory performance on the probe trials
(percentage correct) were entered into an ANOVA with lag (1, 2, 3, 4) as a
within-subject factor. Memory performance varied across lag, F (3, 66) /
11.563, MS /0.08, p B/.001. A trend analysis used to further examine the
main effect of lag on memory performance, revealed a significant linear
trend, F (1, 22) /19.88, MS/0.19, p B/.001, and accounted for 48% of the
16
BECK ET AL.
0.08
Memoryless
Observed
Proportion of revisits
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
1
2
3
4
5
Lag
Figure 6. Proportion of revisits to old items in Experiment 2b (observed) and the proportion
predicted by the memoryless search model (memoryless).
within-subjects variation for memory performance. There was also a
significant quadratic trend, F(1, 22) / 7.67, MS / 0.03, p / .01, accounted
for 26% of the within-subjects variation for memory performance. As can
be seen in Figure 7, these trends are characterized by an initial strong
decrease in memory performance as lag increased that then flattens out for
the longer lags.
Fixations were again biased away from previously examined items
demonstrating the memory for the locations of previously examined items.
Explicit memory performance for previously examined item’s identities was
significantly above chance for all lags. As in Experiment 1, when memory is
tested for identities not bound to location, performance is better than when
it is bound to location (Experiment 2a). In both Experiment 2a and 2b, there
was a linear trend for memory performance across lag. Memory performance was best for the items most recently examined, suggesting that
memory for the identity of previously examined items fades as more items
are examined.
EXPERIMENT 2c
Memory performance using a more traditional search task improved
memory performance on average from 59% (identity bound to location
condition of Experiment 1) to 63% (Experiment 2a) when location memory
was required. When location memory was not required memory perfor-
IDENTITY MEMORY DURING SEARCH
17
1
Proportion correct
0.9
0.8
0.7
0.6
0.5
0.4
0
1
2
3
4
5
Lag
Figure 7. Memory data for Experiment 2b.
mance improved from 70% (identity only condition from Experiment 1) to
77% (Experiment 2b) on average. In addition, there was a lag effect for
previously examined items in the traditional search paradigm that was not
present in Experiment 1. The lag effect appears to be driven mainly by better
performance for the most recently examined item. This may have occurred
because, in the traditional search paradigm (Experiments 2a and 2b),
contextual information is available during the memory test. In Experiment 1,
all items were removed from the screen during test and in Experiments 2a
and 2b, each item was covered by a circular blob so that identity information
was not available, but spatial contextual information was available. Therefore, spatial context may be helpful in accessing identity memories for
recently examined items, thereby causing the lag effect in Experiments 2a
and 2b. Experiment 2c tests this possibility by removing the spatial
contextual cues during the memory test.
Method
Participants. Twenty-five students (22 females and 3 males) from George
Mason University participated in this study for course credit. The average
age of the participants was 20 years. All participants demonstrated normal
or corrected-to-normal visual acuity.
Stimulus and procedures. All stimuli and procedures were the same as in
Experiment 2b except on memory probe trials all items were removed from
18
BECK ET AL.
the screen for the memory test. The location of the memory probe item was
circled and the probe and foil were presented on an otherwise blank screen.
Results
Search performance. All subjects had search accuracy above 90% and
were included in the analysis. Overall search accuracy was 97%.
Eye movements. Two subjects were excluded for missing eye movement
data due to experimenter error. A Monte Carlo simulation of memoryless
search was conducted for 25 ‘‘subjects’’, 143 trials, and a set size of 12 with
the same constraints reported in Experiment 2a. Observed data was
compared to predicted data (memoryless model) using multiple t-tests.
The observed probability of refixations was significantly lower than that of
the memoryless model for revisit lags 1/4, smallest t(38) / /9.18 at revisit
lag 1, all ps B/.001 (see Figure 8). These data suggest that there is memory for
the locations of at least the last four examined items.
Explicit memory performance. One-sample t -tests revealed that memory
performance was significantly higher than chance (50%) at all lags, smallest
t (24) /6.43, p B/.001. Memory performance on the probe trials (percentage
correct) were entered into an ANOVA with lag (1, 2, 3, 4) as a within-subject
factor. Memory performance varied across lag, F (3, 73) /5.28, MS/0.04,
p /.002. A trend analysis used to further examine the main effect of lag on
0.08
Memoryless
Observed
Proportion of revisits
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
1
2
3
4
5
Lag
Figure 8. Proportion of revisits to old items in Experiment 2c (observed) and the proportion
predicted by the memoryless search model (memoryless).
IDENTITY MEMORY DURING SEARCH
19
1
Proportion correct
0.9
0.8
0.7
0.6
0.5
0.4
0
1
2
3
4
5
Lag
Figure 9. Memory performance for Experiment 2c.
memory performance, revealed a significant linear trend, F (1, 24) /15.72,
MS /0.10, p /.001, and accounted for 39.6% of the within-subjects
variation for memory performance. As can be seen in Figure 9 these trends
are characterized by a decrease in memory from lags 1 and 2 to lags 3 and 4.
Discussion
Removing the context during the memory test did not eliminate the linear
effect of lag. However, this linear effect in Experiment 2c was not driven by a
drop in performance from the most recently examined item versus the other
items. Rather the trend was driven by a drop in performance from lags 1 and
2 relative to lags 3 and 4. Furthermore, memory performance at lag 1 in
Experiment 2c (M /77%) was significantly lower than performance at lag 1
in Experiment 2b (M /85%), t(46) /2.38, p/.02, but performance did not
differ between experiments at any of the other lags, largest t(46) /1.27, p /
.21 at lag 3. Therefore, it appears that contextual information available at
test was at least partially responsible for better memory performance for the
most recently examined item in Experiment 2b.
GENERAL DISCUSSION
The probability of maintaining the identities of the distractors during visual
search in memory is above chance, and the identities of distractors do not
20
BECK ET AL.
appear to be necessarily bound to memory for the distractors’ locations.
Across different types of visual search tasks, explicit memory performance
for the identities of previously examined distractors was on average around
66%. When memory was tested for only the identity of the previously
examined distractors, performance was reliably above chance. However,
when the memory test required knowledge of not only the distractors’
identities but also their locations, memory performance tended to be above
chance for only the most recently examined item. Furthermore, providing
contextual information during the testing phase improved memory performance for the most recently examined item. Memory for the identities of
previously examined items was best when the most recently examined item
was tested, contextual information was provided, and location memory was
not required (Experiment 2b; M /85%).
It is surprising that memory performance for the distractors’ identities is
not higher given that the memory tests occurred during the search trials and
that the participants were aware that the memory tests would occur. The
memory performance levels in the current experiments for the identities of
previously examined items are similar to those found by Williams et al.
(2005). Participants in our experiments were aware that a memory test could
occur while they were searching the stimuli, but participants in Williams
et al.’s experiments were unaware until after completing all of the visual
search trials that a memory test would be administered. Therefore, knowing
that memory may be tested does not appear to improve memory
performance, and the amount of memory available during the search task
appears to be similar to the amount available in LTM. However, these
possibilities need to be tested directly, because several other differences
between our experiments and those used by Williams et al. may be
responsible for the unexpected similarities in performance.2
Does visual search interfere with identity memory?
Although explicit memory performance for distractor identities was
generally above chance, it was still much lower than would be expected
given that the items were examined within a few seconds before the test and
2
Williams et al. (2005) used a different type of search task than was used in the experiments
reported here. Instead of asking participants to search for the presence of a target, they were
asked to report how many targets were present (0, 1, 2, or 3). The search target was different
than that given in the experiments reported here; participants were told to search for a specific
object of a specific category (e.g., ‘‘yellow phone’’). Furthermore, distractors could be from the
same specific category as the target (e.g., green phones). Any of these differences could have lead
to better performance on the LTM test than would have been found using a search task like what
was used in the experiments reported here.
IDENTITY MEMORY DURING SEARCH
21
that the number of items tested was within the capacities of working memory
(4 /5 objects: Cowan, 2001; Irwin, 1992; Irwin & Andrews, 1996; Luck &
Vogel, 1997; Pashler, 1988; Phillips, 1974). When participants are given the
sole task of memorizing the identity and locations of an array of objects,
they are able to remember about ‘‘5 objects worth of information’’ (Irwin &
Zelinsky, 2002). We used the same method of calculating the number of
objects that can be remembered (capacity) for each of the experiments
reported here.3 In Experiment 1, 3.0 objects’ worth of information was
remembered when identity bound to location memory was needed and 3.8
objects’ worth when only identity memory was needed. When a more
traditional search task was used, participants remembered 3.6 objects’ worth
of information when only identity memory was needed (Experiment 2a) and
2.7 objects’ worth of information when identity bound to location memory
was need (Experiment 2b). Similar capacity levels were found in Experiment
2c when contextual information was not provided at test and only identity
memory was needed (3.3 objects’ worth of information). Across the
experiments reported here roughly 3 objects’ worth of information was
retained during the visual search tasks. Thus, although the memory task
used in this experiment was also an intentional task, presenting it secondary
to the visual search task resulted in the loss of 2 objects’ worth of identity
memory. The intentional memory task used here may have inflated this
estimate. It is possible that during typical visual search tasks (when identity
memory is not explicitly probed), the amount of explicit identity memory is
even lower than we found here.
Visual search tasks could lower the amount of distractor identity
information retained in memory because search tasks generally encourage
memory for the locations of the nontarget items rather than their identities.
First, the efficient strategy of biasing fixations away from old items may
contribute to reduced memory for distractor identities. Second, visual search
tasks may not require remembering the identities of the distractors because
only task relevant identities are remembered. Finally, visual search tasks may
result in memory representations of distractor identities that are not always
retrievable through explicit memory tests. Each of these possibilities is
discussed below.
3
The number of objects’ worth of information stored in memory during visual search was
calculated similarly to how it was calculated in Irwin and Zelinsky (2002). We corrected for
guessing (Busey & Loftus, 1994) by applying the formula p / (x / g)/(1 / g), where x is the
percentage correct averaged across all lags, g is the probability of guessing (g / 1/n, were n
equals the number of objects examined prior to the memory test, six in Experiment 1 and five in
Experiments 2a, b, and c), and p is the percentage correct after correcting for guessing. P was
then multiplied by the total number of objects examined (n) for an estimate of the number of
objects remembered during visual search (Sperling, 1960).
22
BECK ET AL.
Infrequent reinspections of old items during search may prevent the build
up of strong memories for distractor identities. In all of the experiments
presented here, fixations were biased away from old distractors. Therefore,
the processing time allotted to each distractor is generally limited to the
duration of one fixation (less than 300 ms). Distractors that are fixated more
frequently and receive longer total fixation durations are more likely to be
remembered (Williams et al., 2005). Specifically, as the number of fixations
on a distractor increased, LTM memory performance increased as well.
Therefore, the memory for locations used to prevent distractors from being
reexamined during visual search may be hindering the ability to build up
explicit identity memory for the distractors by limiting the number of times
distractors are examined.
Explicit memory for the identity of the distractors may be lower than
found in nondual task intentional memory tests because the distractors are
not directly relevant to the task of responding to the target’s identity or
presence. Targets are special because they are more important to the task
and are therefore likely to be processed more deeply and for longer periods
of time (Oulasvirta, 2004). In addition, participants are able to search for the
presence of more than one target with high accuracy suggesting they are able
to remember spatial and temporal information about multiple targets
(Gibson, Li, Skow, Brown, & Cooke, 2000; Thornton & Horowitz, 2004).
Williams et al. (2005) found LTM performance of about 85% for targets
versus about 50/60% for distractors. This suggests that memory for
distractors in traditional search may be lower than memory for distractors
because only information that is directly related to the task is remembered.
The combination of lower memory for identity than found in nondual
task intentional memory tests and results from previous research demonstrating that changing the identities of distractors during search does not
disrupt search performance (Beck et al., in press) suggests that the identities
of the distractors are not always encoded during visual search. Alternatively,
it is possible that memory for the distractors’ identities is encoded, but that
the memory is implicit and not used to guide attention during the visual
search task. This predicts that our explicit measure of memory would be
unable to retrieve the memory, and because the implicit memory is not used
to guide attention, changing the identity would not disrupt the search
process. Even when there is no declarative memory for an item, the item may
be processed such that when the item is encountered later, processing of the
item is facilitated (priming; Levy, Stark, & Squire, 2004). However, even
implicit memory for distractor identity may be low during visual search
because implicit memory can also be affected by task relevance (Hayhoe,
2000; Hayhoe, Bensinger, & Ballard, 1998; Holbrook, Bost, & Cave, 2003;
Land, Mennie, & Rusted, 1998; Triesch, Ballard, Hayhoe, & Sullivan, 2003).
IDENTITY MEMORY DURING SEARCH
23
Memory for identity versus location
The lack of revisits to previously examined items reported here supports
earlier findings suggesting that memory in visual search is based on the
locations of already examined items (Chun & Jiang, 1997; Oh & Kim, 2004;
Peterson et al., 2001; Woodman & Luck, 2000). In comparison to the
estimates reported here of roughly three items’ worth of identity information
remembered during visual search, estimates of the number of items for which
location information is remembered is about five items (after factoring out
contributions of scan-path strategies; McCarley et al., 2003; Peterson, Beck,
& Vomela, 2004). However, the research reported here suggests that this
information is not necessarily retrieved together. Visual search tasks may use
the attentional resources necessary for binding for other processes immediately relevant to the search task, and therefore the memories are stored
separately and not bound together (Wheeler & Treisman, 2002). Alternatively, the retrieval of location information about an item may inhibit the
retrieval of identity information (Anderson, Bjork, & Bjork, 1994).
In conclusion, memory for the identities of distractors during visual
search is above chance but lower than memory during nondual task
intentional memory tests (three objects’ worth of information versus five
objects’ worth respectively). This indicates that some identity information is
retained, but the amount retained is less than would be expected by the
limits of working memory. This may occur because the location of
distractors is relevant to the visual search task but their identities are not.
Furthermore, the retrieval of location information is not necessarily linked
to the retrieval of identity information during visual search.
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Manuscript received June 2005
Manuscript accepted January 2006
PrEview proof published online month/year