Identical Distractors Research Articles

Swapping or dropping? Electrophysiological measures of difficulty during multiple object tracking

In the multiple object tracking task, participants are asked to keep targets separate from identical distractors as all items move randomly. It is well known that simple manipulations such as object speed and number of distractors dramatically alter the number of targets that are successfully tracked, but very little is known about what causes this variation in performance. One possibility is that participants tend to lose track of objects (dropping) more frequently under these conditions. Another is that the tendency to confuse a target with a distractor increases (swapping). These two mechanisms have very different implications for the attentional architecture underlying tracking. However, behavioral data alone cannot differentiate between these possibilities. In the current study, we used an electrophysiological marker of the number of items being actively tracked to assess which type of errors tended to occur during speed and distractor load manipulations. Our neural measures suggest that increased distractor load led to an increased likelihood of confusing targets with distractors while increased speed led to an increased chance of a target item being dropped. Behavioral experiments designed to test this novel prediction support this assertion.

Learning of an oddity rule by pigeons in a four-choice touch-screen procedure

Six pigeons were trained to peck at a target (odd stimulus) that was presented on a touch-screen together with three identical distractors (non-odd stimuli). The target could be either a square or a circle that was either blue or green, and the distractors in each trial were always of the opposite form and color to the target. Thus, the birds could solve the task by attending to color, form, or both. Transfer tests showed that performance was not disrupted by novel forms, stimulus sizes, distractor numbers, and display configurations, but broke down with novel stimulus types (textured stimuli, clip art images, and photographs). Transfer to novel colors was, for the most part, restricted to trials in which only one component-target or distractors, but not both-had a novel color. This suggested that the pigeons used a couple of if-then rules rather than an oddity concept to solve the task, and that color differences between target and distractors were the only cue upon which responding was based. A control experiment with the order of color and form tests being reversed excluded the possibility of the prevalence of color being an artifact of task order and reinforcement contingencies.

It is time to integrate: The temporal dynamics of object motion and texture motion integration in multiple object tracking

In multiple-object tracking, participants can track several moving objects among identical distractors. It has recently been shown that the human visual system uses motion information in order to keep track of targets (St. Clair et al., Journal of Vision, 10(4), 1–13). Texture on the surface of an object that moved in the opposite direction to the object itself impaired tracking performance. In this study, we examined the temporal interval at which texture motion and object motion is integrated in dynamic scenes. In two multiple-object tracking experiments, we manipulated the texture motion on the objects: The texture either moved in the same direction as the objects, in the opposite direction, or alternated between the same and opposite direction at varying intervals. In Experiment 1, we show that the integration of object motion and texture motion can take place at intervals as short as 100ms. In Experiment 2, we show that there is a linear relationship between the proportion of opposite texture motion and tracking performance. We suggest that texture motion might cause shifts in perceived object locations, thus influencing tracking performance.

Inhibition of return in a visual foraging task in non-human subjects

Inhibition of return is thought to help guide visual search by inhibiting the orienting of attention to previously attended locations. We have previously shown that, in a foraging visual search task, the neural responses to objects in parietal cortex are reduced after they have been examined. Here we ask whether the animals’ reaction times (RTs) in the same task show a psychophysical correlate of inhibition of return: a slowing of reaction time in response to a probe placed at a previously fixated location. We trained three animals to perform an RT version of the visual foraging task. In the foraging task, subjects visually searched through an array of five identical distractors and five identical potential targets; one of which had a reward linked to it. In the RT variant of the task, subjects had to rapidly respond to a probe if it appeared. We found that RTs were slower for probes presented at locations that contained previously fixated objects, faster to potential targets and between the two for behaviorally irrelevant distractors that had not been fixated. These data show behavioral inhibitory tagging of previously fixated objects and suggest that the suppression of activity seen previously in the same task in parietal cortex could be a neural correlate of this mechanism.

Attentional capture by emotional faces is contingent on attentional control settings

Attentional capture by schematic emotional faces was investigated in two experiments using the flanker task devised by Eriksen and Eriksen (1974). In Experiment 1, participants were presented with a central target (a schematic face that was either positive or negative) flanked by two identical distractors, one on either side (schematic faces that were positive, negative, or neutral). The objective was to identify the central target as quickly as possible. The impact of the flankers depended on their emotional expression. Consistent with a threat advantage hypothesis (negative faces are processed faster and attract more processing resources), responses to positive faces were slower when these were flanked by (response incompatible) negative faces as compared with positive or neutral faces, whereas responses to negative faces were unaffected by the identity of the flankers. Experiment 2 was a standard flanker task with letter stimuli except that the task-neutral flankers were schematic faces that were either positive, negative, or emotionally neutral. In this case, in which faces and emotional expressions were to be ignored, performance seemed entirely unaffected by the faces. This result suggests that attentional capture by emotional faces is contingent on attentional control settings.

A processing advantage associated with analytic perceptual tendencies: European Americans outperform Asians on multiple object tracking

Analytic visual processing and holistic visual processing have been conceptualized in terms of attention to focal objects vs. the background. We expand the study of perceptual biases associated with these attentional patterns using the multiple object tracking task, which measures people's ability to track multiple moving target objects amidst otherwise identical distractors. We test two competing hypotheses: (1) Asians' more frequent eye saccades will enable them to quickly cycle through the multiple target objects before the objects move too far away, giving them another perceptual advantage; and (2) European Americans' tendency to focus attention on the focal objects while inhibiting attention to less important objects might facilitate tracking of multiple moving objects. We find that European Americans significantly outperform Asians on multiple object tracking. The research expands the conceptualization of analytic processing and holistic processing to include selective attention as a key component, a facet that has not been previously identified.

Perception of emergent configurations in humans (Homo sapiens) and chimpanzees (Pan troglodytes).

We examined the perceptions of emergent configurations in humans and chimpanzees using a target-localization task. The stimulus display consisted of a target placed among multiple identical distractors. The target and distractors were presented either solely, within congruent contexts in which salient configurations emerge, or within incongruent contexts in which salient configurations do not emerge. We found that congruent contexts had similar facilitative effects on target localization by humans and chimpanzees, whereas similar disruptive effects emerged when the stimuli were presented within incongruent contexts. When display size was manipulated, targets under the congruent-context condition were localized in a parallel manner, but those under the no-context and incongruent-context conditions were localized in a serial manner by both species. These results suggest that both humans and chimpanzees perceive emergent configurations when targets and distractors are presented within certain congruent contexts and that they process such emergent configurations preattentively.

Delineating the Neural Signatures of Tracking Spatial Position and Working Memory during Attentive Tracking

In the attentive tracking task, observers track multiple objects as they move independently and unpredictably among visually identical distractors. Although a number of models of attentive tracking implicate visual working memory as the mechanism responsible for representing target locations, no study has ever directly compared the neural mechanisms of the two tasks. In the current set of experiments, we used electrophysiological recordings to delineate similarities and differences between the neural processing involved in working memory and attentive tracking. We found that the contralateral electrophysiological response to the two tasks was similarly sensitive to the number of items attended in both tasks but that there was also a unique contralateral negativity related to the process of monitoring target position during tracking. This signal was absent for periods of time during tracking tasks when objects briefly stopped moving. These results provide evidence that, during attentive tracking, the process of tracking target locations elicits an electrophysiological response that is distinct and dissociable from neural measures of the number of items being attended.

Deciding where to attend: Priming of pop-out drives target selection.

With attention and eye-movements humans orient to targets of interest. This orienting occurs faster when the same target repeats: priming of pop-out (PoP). While reaction times (RTs) can be important, PoP's real function could be to steer where to orient, a possibility underexposed in many current paradigms, as these predesignate a target to which to orient. In a novel procedure we intermixed pop-out trials (one oddball target, two identical distractors) with choice trials (one item of each kind) where observers freely chose an item to attend to. Pop-out trials strongly drove subsequent choice: observers typically chose the preceding target. Conversely, choice trials affected subsequent pop-out RTs. Conventional PoP measures correlated positively with our choice measures among observers, suggesting common mechanisms. Our results support PoP accounts centered on altered target priority, and underscore PoP's importance for visual exploration.

Neural measures of interhemispheric information transfer during attentive tracking

People are generally able to track 4-5 objects as they move amongst visually identical distractors. However, Alvarez & Cavanagh (2005) found that if tracked objects are lateralized to one visual hemifield, tracking capacity is drastically reduced relative to bilateral tracking trials. These data suggest that tracking for each hemifield is carried out independently by the contralateral hemisphere. If so, what happens when an object moves from one hemifield to another? If the right hemisphere is tracking an object that moves to the right visual field, does the left hemisphere pick up the object representation the moment that it crosses the midline, does it preemptively start tracking the object before it crosses the midline, or does it wait until some point after the midline has been crossed? When does the right hemisphere stop tracking the object? We studied these questions using a sustained contralateral negativity that indexes tracking activity during lateralized versions of the attentive tracking task (Drew & Vogel, 2009). We measured contralateral and ipsilateral activity while a tracked object moved horizontally across the midline. As predicted, activity with respect to the hemifield where the object originated initially exhibited a strong contralateral negativity that then flipped to an ipsilateral negativity as the object moved to the opposite hemifield. We found that ipsilateral activity prospectively increased prior to the moment when the object crossed the midline whereas contralateral activity did not decrease until several hundred milliseconds after the object crossed the midline. This suggests that the two hemispheres were both tracking the object for several hundred milliseconds. Furthermore, we were able to influence the timing of this interaction by manipulating the predictability of object motion. When the object movement was less predictable, the duration of interhemispheric information sharing decreased.

Multiple object tracking through temporal gaps created by the fading of objects

In three experiments, we examined whether the encoding of object location is used in Multiple Object Tracking. Observers were asked to track four target discs among eight identical distractors on a display where the same random-dot texture was used for object surfaces and display background. Stereoscopic glasses were used to create two display conditions: 3D (where objects appeared to float in front of the background texture) and 2D (where objects appeared on the background texture). In the 2D displays, disks were only visible while they moved and became indistinguishable from the background when they stopped. In 75% of the trials, the objects halted movement mid-trial for one, two, or four seconds. Experiment 1 used textured discs with no borders. During the pauses, the discs would appear to dissolve into the background in the 2D condition but remained distinct in the 3D condition. This produced significantly lower tracking performance only in the 2D trials with the longest pause; no decline was observed in the 3D condition. Experiment 2 was identical to Experiment 1, except the discs had a white border during the entire trial, allowing the discs to remain distinct during the pauses. In this case there was no effect of pause duration. Experiment 3 used the same 2D display as Experiment 1, except that in half of the trials object borders flashed “on” before halting. Here, there was an effect of pause duration in both flash and non-flash conditions (decreased performance with longer pauses). These experiments found that objects that disappear without an abrupt offset are more difficult to track, indicating that object locations are not encoded and used to continue tracking after a gap in visibility. This suggests that the tracking mechanism does not encode location information unless cued by abrupt changes in the visual scene.

How does attention select and track spatially extended objects?: New effects of attentional concentration and amplification

Much recent research has demonstrated that attention can be allocated to discrete objects in addition to spatial locations, but relatively little research has explored the allocation of attention within individual uniform objects. While it may be that attention spreads uniformly through relatively small objects, real-world situations (e.g. driving) often involve attending to spatially extended objects, often under conditions of motion and high processing load. Here we explore how attention is used to select and track spatially extended objects in a multiple object tracking (MOT) task. Instead of the punctate objects used in most previous MOT studies, observers had to track of a number of long moving overlapped line segments in a field of identical distractors. At the same time, observers had to respond to sporadic probes, and their probe detection performance is used as a measure of the distribution of attention across the lines. In four experiments we discovered two novel phenomena: First, attention seems to be concentrated at the centers of the lines during tracking, despite their uniformity: probe detection was much more accurate at the centers of the lines than near their endpoints. Second, this ‘center advantage’ grew as the lines became longer: not only did observers get relatively worse near the endpoints, but they became better at the lines' centers — as if attention became more concentrated as the objects became more extended. Both of these effects were unusually large and robust. Additional results suggest that these effects reflect automatic visual processing rather than higher-level strategies. Beyond demonstrating that objects can serve as units of attention, these results begin to show *how* attention is actively allocated to extended objects over time in complex dynamic displays.

Effect of object discriminability on multiple object tracking

In the Multiple Object Tracking (MOT) paradigm observers track a designated set of (typically 4) objects that move independently and unpredictably among an equal number of identical distractors. It has been shown (Scholl, Pylyshyn & Franconeri, 1999) that, consistent with Pylyshyn's Visual Indexing Theory, a change in the color or shape of a tracked objects is not encoded during MOT. Even if properties are not encoded, it might still be easier to track targets if some properties distinguished targets from nontargets. However, if targets were consistently different from nontargets, selecting the targets at the end of a trial could be accomplished without actually tracking. Consequently, we developed a paradigm for investigating whether a distinctive property of objects (e.g., their color) would improve tracking even when the property could not be used directly to pick out targets without tracking them. In this paradigm the color of each object was always distinct from the color of every other object, but it did not remain fixed, and therefore targets could not be tracked merely by recalling their color. The method involved selecting 8 equispaced points on a color circle and rotating all 8 selections around the circle in a continuous manner. Three conditions were compared: (1) Colors of objects were always distinct but continuously changing, (2) colors of all objects were always the same but changed continuously in synchrony, and (3) colors of objects were identical and fixed. Results showed that that performance in these three conditions were significantly different, with (1) being best and (3) being worst. The improvement in tracking arising from maintaining distinct colors is discussed in terms of two possible explanations: Reduced confusability of target-nontarget pairs that came close to one another during a trial, and use of color differences in an “error recovery” stage.

Flanker negative priming from spatially unpredictable primes: An ERP study

In a typical flanker task, a to-be-selected central target is flanked by two to-be ignored, identical distractors. The flanker negative priming (NP) effect denotes increased reaction time and error percent when the distractor of a first display serves as the target in the next. Most theories of NP are consistent with the idea that during processing of the first display, the identity of the distractors is inhibited. If the target of the subsequent display has the same identity, NP occurs because of persisting or retrieved inhibition. However, in the standard flanker task stimuli appear at the same screen locations for all trials, allowing for anticipatory spatial selection. No strong additional inhibition of stimulus identities may then be required. Therefore, besides the standard flanker task we employed a modified task in which the location of the stimulus triplet slightly differed across trials, thus disabling spatial pre-selection. Event-related potentials (ERPs) were recorded to identify brain correlates of NP in the two tasks. Behavioral NP was present in the modified task but absent in the standard task. An ERP correlate specific to NP in the modified task concerned larger amplitude of a left-posterior processing negativity. Results support the idea that stronger inhibition of distractor identities contributes to NP in the flanker task when spatial pre-selection is disabled.

Conflicting motion information impairs multiple object tracking

People can keep track of target objects as they move among identical distractors using only spatiotemporal information. We investigated whether or not participants use motion information during the moment-to-moment tracking of objects by adding motion to the texture of moving objects. The texture either remained static or moved relative to the object's direction of motion, either in the same direction, the opposite direction, or orthogonal to each object's trajectory. Results showed that, compared to the static texture condition, tracking performance was worse when the texture moved in the opposite direction of the object and better when the texture moved in the same direction as the object. Our results support the conclusion that motion information is used during the moment-to-moment tracking of objects. Motion information may either affect a representation of position or be used to periodically predict the future location of targets.

Visual field asymmetry in attentional capture

The present study examined the spatial distribution of involuntary attentional capture over the two visual hemi-fields. A new experiment, and an analysis of three previous experiments showed that distractors in the left visual field that matched a sought-for target in color produced a much larger capture effect than identical distractors in the right visual field, revealing a visual field asymmetry in color-based contingent capture. On the other hand, abrupt onsets in the two hemi-fields did not differ in the magnitude of their capture effect, indicating a symmetric distribution of onset capture. The different spatial patterns for contingent capture and onset capture reveal differences between the two types of attentional capture, possibly indicating differences in the underlying brain mechanisms.

Effects of Onset- and Rhyme-Related Distractors on Phonological Processing in Children With Specific Language Impairment

This study used the cross-modal picture-word interference task of P. J. Brooks and B. MacWhinney (2000) to compare effects of phonologically related words on lexical access in children with specific language impairment (SLI). Children (7;1 [years;months]-11;2) named pictures while ignoring auditory distractors. Three stimulus asynchrony conditions varied the timing of distractors relative to the pictures. Experiment 1 presented onset-related (bell-bed), unrelated (clown-bed), neutral (go-bed), and identical (bed-bed) distractors. Experiment 2 presented rhyme-related instead of onset-related distractors (clock-sock). Children with SLI produced longer reaction times (RTs) and more errors than their typical language development (TLD) peers. For children with SLI, onset-related distractors led to slower RTs than unrelated distractors (inhibition) when presented before the picture, and faster RTs (facilitation) when presented after the picture. Children with TLD showed facilitation from onset-related distractors when presented after the picture but no inhibition when presented before the picture. Both groups failed to show facilitation from rhyme-related distractors. The priming effects from onset-related distractors and lack of effects from rhyme-related distractors in SLI supports "just-in-time" incremental processing, similar to children with TLD. However, children with SLI experience phonological interference from members of a lexical cohort while accessing words. Results are discussed with respect to observed word finding and word learning difficulties in SLI.

Grouping and Trajectory Storage in Multiple Object Tracking: Impairments Due to Common Item Motions

In our natural viewing, we notice that objects change their locations across space and time. However, there has been relatively little consideration of the role of motion information in the construction and maintenance of object representations. We investigated this question in the context of the multiple object tracking (MOT) paradigm, wherein observers must keep track of target objects as they move randomly amid featurally identical distractors. In three experiments, we observed impairments in tracking ability when the motions of the target and distractor items shared particular properties. Specifically, we observed impairments when the target and distractor items were in a chasing relationship or moved in a uniform direction. Surprisingly, tracking ability was impaired by these manipulations even when observers failed to notice them. Our results suggest that differentiable trajectory information is an important factor in successful performance of MOT tasks. More generally, these results suggest that various types of common motion can serve as cues to form more global object representations even in the absence of other grouping cues.

Rhythmic Masking Release: Effects of Asynchrony, Temporal Overlap, Harmonic Relations, and Source Separation on Cross-Spectral Grouping.

The rhythm created by spacing a series of brief tones in a regular pattern can be disguised by interleaving identical distractors at irregular intervals. The disguised rhythm can be unmasked if the distractors are allocated to a separate stream from the rhythm by integration with temporally overlapping captors. Listeners identified which of 2 rhythms was presented, and the accuracy and rated clarity of their judgment was used to estimate the fusion of the distractors and captors. The extent of fusion depended primarily on onset asynchrony and degree of temporal overlap. Harmonic relations had some influence, but only an extreme difference in spatial location was effective (dichotic presentation). Both preattentive and attentionally driven processes governed performance.

When do irrelevant visual stimuli impair processing of identical targets?

Three experiments investigated whether the repeated-letter inferiority effect (RLIE) and repetition blindness (RB) are identical phenomena or not and how the RLIE can be reconciled with the flanker compatibility effect (FCE). Participants reported a masked target and ignored an unmasked distractor. We manipulated the type of distractor (identical, alternative target, or neutral), the order of presenting distractor and target, and the predictability of target location. When distractors preceded the targets, distractors identical to the target always caused deficits in target processing (i.e., RB). With simultaneous presentation, identical distractors caused deficits (i.e., an RLIE) for unpredictable target locations only. Yet the RLIE was significantly smaller than RB. This result suggests that simultaneously presented stimuli are identified serially, in random order, if target position is unpredictable. As a result, RB arises only in the 50% of all trials with identical distractors in which the distractor is identified before the target. With simultaneous presentation and predictable target location, however, parallel processing of targets and distractors was possible that gave rise to an FCE in recognition accuracy. Analyses of false alarm rates revealed no evidence of significant response biases.