Set-size effects for spatial frequency change and discrimination in multiple targets

In visual search tasks with a near-threshold target amongst distracters, log detection thresholds rise in proportion to the log of the number of stimuli. Previous research has shown a very steep slope for this set-size effect where the target is a change in spatial frequency (SF) across an ISI, suggesting a low-level explanation for 'change blindness (Wright et al., 2000). Here, we analyse stimulus and task variables in order to determine the contributions of stimulus detection and attention processes. Stimuli consisted of two 150 ms frames each containing 1 to 4 Gabor targets, with an ISI of 250 ms. In a 2AFC detection task with uniform distracters, slopes of 0.23-0.52 were found, in line with visual search results. 2AFC SF discrimination tasks gave slopes of 0.68, 0.69 with homogeneous distracters and 0.76-0.96 with inhomogeneous distracters, consistent with averaging of stimuli within a frame. If the distracters were also made to change across ISI, averaging was impossible, and focal attention was required to solve the discrimination. This always gave set-size slopes > 1. It is concluded that, under conditions where a stimulus array can be analysed globally, change detection performance is limited by signal detection mechanisms, rather than limited capacity attention or memory mechanisms. However, where this is prevented, for example by changing more than one item, limitations due to attention or memory produce an even steeper set-size effect.     

Signal detection theory applied to three visual search tasks--identification, yes/no detection and localization

Adding distracters to a display impairs performance on visual tasks (i.e. the set-size effect). While keeping the display characteristics constant, we investigated this effect in three tasks: 2 target identification, yes-no detection with 2 targets, and 8-alternative localization. A Signal Detection Theory (SDT) model, tailored for each task, accounts for the set-size effects observed in identification and localization tasks, and slightly under-predicts the set-size effect in a detection task. Given that sensitivity varies as a function of spatial frequency (SF), we measured performance in each of these three tasks in neutral and peripheral precue conditions for each of six spatial frequencies (0.5-12 cpd). For all spatial frequencies tested, performance on the three tasks decreased as set size increased in the neutral precue condition, and the peripheral precue reduced the effect. Larger set-size effects were observed at low SFs in the identification and localization tasks. This effect can be described using the SDT model, but was not predicted by it. For each of these tasks we also established the extent to which covert attention modulates performance across a range of set sizes. A peripheral precue substantially diminished the set-size effect and improved performance, even at set size 1. These results provide support for distracter exclusion, and suggest that signal enhancement may also be a mechanism by which covert attention can impose its effect.     

Attentional Requirements on Feature Search Are Modulated by Stimulus Properties

We report a series of dual-task experiments, in which a rapid serial visual presentation (RSVP) task was combined with a visual search task. Orientation, motion, and color were used as the defining target features in the search task. Lag between target onsets was manipulated and interference between the two tasks was quantified by measuring detection scores for the search task as a function of lag. While simultaneous performance of an orientation detection task with an RSVP letter identification task resulted in a performance decrease for lags up to 320 ms, no such decrease was detected for highly salient motion- and color-defined targets. Subsequently, detectability of the motion and color feature was matched to that of the orientation-feature resulting in the reintroduction of a (smaller) performance decrease, but only during simultaneous performance (lag 0 ms). The results suggest that there are two causes for the impaired search performance occurring when a feature search task is combined with an RSVP task. The first is short-lasting interference probably due to attentional competition; the second, which plays a role only when targets for both tasks share features, is interference that may be attributed to a central processing bottleneck.     

Impaired Contingent Attentional Capture Predicts Reduced Working Memory Capacity in Schizophrenia

Although impairments in working memory (WM) are well documented in schizophrenia, the specific factors that cause these deficits are poorly understood. In this study, we hypothesized that a heightened susceptibility to attentional capture at an early stage of visual processing would result in working memory encoding problems. 30 patients with schizophrenia and 28 demographically matched healthy participants were presented with a search array and asked to report the orientation of the target stimulus. In some of the trials, a flanker stimulus preceded the search array that either matched the color of the target (relevant-flanker capture) or appeared in a different color (irrelevant-flanker capture). Working memory capacity was determined in each individual using the visual change detection paradigm. Patients needed considerably more time to find the target in the no-flanker condition. After adjusting the individual exposure time, both groups showed equivalent capture costs in the irrelevant-flanker condition. However, in the relevant-flanker condition, capture costs were increased in patients compared to controls when the stimulus onset asynchrony between the flanker and the search array was high. Moreover, the increase in relevant capture costs correlated negatively with working memory capacity. This study demonstrates preserved stimulus-driven attentional capture but impaired contingent attentional capture associated with low working memory capacity in schizophrenia. These findings suggest a selective impairment of top-down attentional control in schizophrenia, which may impair working memory encoding.     

Neuronal correlates of the set-size effect in monkey lateral intraparietal area

It has long been known that the brain is limited in the amount of sensory information that it can process at any given time. A well-known form of capacity limitation in vision is the set-size effect, whereby the time needed to find a target increases in the presence of distractors. The set-size effect implies that inputs from multiple objects interfere with each other, but the loci and mechanisms of this interference are unknown. Here we show that the set-size effect has a neural correlate in competitive visuo-visual interactions in the lateral intraparietal area, an area related to spatial attention and eye movements. Monkeys performed a covert visual search task in which they discriminated the orientation of a visual target surrounded by distractors. Neurons encoded target location, but responses associated with both target and distractors declined as a function of distractor number (set size). Firing rates associated with the target in the receptive field correlated with reaction time both within and across set sizes. The findings suggest that competitive visuo-visual interactions in areas related to spatial attention contribute to capacity limitations in visual searches.     

Hemispheric Asymmetry in the Auditory Facilitation Effect in Dual-Stream Rapid Serial Visual Presentation Tasks

Even though auditory stimuli do not directly convey information related to visual stimuli, they often improve visual detection and identification performance. Auditory stimuli often alter visual perception depending on the reliability of the sensory input, with visual and auditory information reciprocally compensating for ambiguity in the other sensory domain. Perceptual processing is characterized by hemispheric asymmetry. While the left hemisphere is more involved in linguistic processing, the right hemisphere dominates spatial processing. In this context, we hypothesized that an auditory facilitation effect in the right visual field for the target identification task, and a similar effect would be observed in the left visual field for the target localization task. In the present study, we conducted target identification and localization tasks using a dual-stream rapid serial visual presentation. When two targets are embedded in a rapid serial visual presentation stream, the target detection or discrimination performance for the second target is generally lower than for the first target; this deficit is well known as attentional blink. Our results indicate that auditory stimuli improved target identification performance for the second target within the stream when visual stimuli were presented in the right, but not the left visual field. In contrast, auditory stimuli improved second target localization performance when visual stimuli were presented in the left visual field. An auditory facilitation effect was observed in perceptual processing, depending on the hemispheric specialization. Our results demonstrate a dissociation between the lateral visual hemifield in which a stimulus is projected and the kind of visual judgment that may benefit from the presentation of an auditory cue.     

Effects of stimulus size and spatial organization on pigeons’ conditional same-different discrimination

In two experiments, we explored the effects of varying the size and the spatial organization of the stimuli in multi-item arrays on pigeons’ same-different discrimination behavior. The birds had previously learned to discriminate a simultaneously presented array of 16 identical (Same) visual items from an array of 16 nonidentical (Different) visual items, when the correct choice was conditional on the presence of another cue: the color of the background (Castro et al., in press). In Experiment 1, we trained pigeons with 7-item arrays and then tested them with arrays containing the same item, but in a variety of sizes. In Experiment 2, we tested the birds with the items grouped in novel locations: the top, the bottom, the left, or the right portions of the display area, which generated different vertical and horizontal alignments. Accuracy scores revealed virtually perfect stimulus generalization across various item sizes and spatial organizations. Reaction times revealed that the birds perceived different sizes of a single icon as the same stimulus (Experiment 1) and that the birds processed vertical arrangements faster than horizontal arrangements (Experiment 2). These results suggest that the pigeons noticed both physical and spatial changes in the stimuli (as shown by their reaction times), but that these changes did not disrupt the birds’ discriminating the sameness or differentness of the multi-item arrays (as shown by their accuracy scores).     

Characterizing visual performance fields: effects of transient covert attention, spatial frequency, eccentricity, task and set size

We investigated whether transient covert attention would differentially affect 'performance fields' (shape depicted by percent correct performance at particular locations in the visual field) for orientation discrimination, detection and localization tasks, while manipulating a number of visual factors. We found that although attention improved overall performance, it did not affect performance fields. Two patterns were observed regardless of the presence of a local post-mask, the stimulus orientation, or the task. A horizontal-vertical anisotropy (HVA) became more pronounced as spatial frequency, eccentricity and set size increased. A vertical meridian asymmetry (VMA) became more pronounced as spatial frequency and eccentricity increased. We conclude that performance fields are determined by visual, rather than by transient attentional, constraints.     

Effects of set-size and lateral masking in visual search

In the present research, the roles of lateral masking and central processing limitations in visual search were studied. Two search conditions were used: (1) target differed from distractors by presence/absence of a simple feature; (2) target differed by relative position of the same components only. The number of displayed stimuli (set-size) and the distance between neighbouring stimuli were varied as independently as possible in order to measure the effect of both. The effect of distance between stimuli (lateral masking) was found to be similar in both conditions. The effect of set-size was much larger for relative position stimuli. The results support the view that perception of relative position of stimulus components is limited mainly by the capacity of central processing.     

Training Top-Down Attention Improves Performance on a Triple-Conjunction Search Task

Training has been shown to improve perceptual performance on limited sets of stimuli. However, whether training can generally improve top-down biasing of visual search in a target-nonspecific manner remains unknown. We trained subjects over ten days on a visual search task, challenging them with a novel target (top-down goal) on every trial, while bottom-up uncertainty (distribution of distractors) remained constant. We analyzed the changes in saccade statistics and visual behavior over the course of training by recording eye movements as subjects performed the task. Subjects became experts at this task, with twofold increased performance, decreased fixation duration, and stronger tendency to guide gaze toward items with color and spatial frequency (but not necessarily orientation) that resembled the target, suggesting improved general top-down biasing of search.     

Attentional weighting: a possible account of visual field asymmetries in visual search?

Several previous visual search studies measuring reaction times have demonstrated scanning biases across the visual field (i.e. a tendency to begin a serial search in a particular region of space). In the present study, we measured visual discrimination thresholds for a target presented amongst distractors using displays that were short enough to greatly reduce the potential for serial (i.e. scanning) search. For both a motion and orientation task, subjects' performance was significantly better when the target appeared in the inferior, as compared to the superior, visual field (no differences were observed between left and right visual fields). These findings suggest that subjects may divide attention unevenly across the visual field when searching for a target amongst distractors, a phenomenon we refer to as 'attentional weighting'. To rule out the possibility that these visual field asymmetries were sensory in nature, thresholds were also measured for conditions in which subjects' attention was directed to the location of the target stimulus, either because it was presented alone in the display or because a spatial cue directed subjects' attention to the location of that target presented amongst distractors. Under these conditions, visual field asymmetries were smaller (or non-existent), suggesting that sensory factors (such as crowding) are unlikely to account for our results. In addition, analyses of set-size effects (obtained by comparing thresholds for a single target vs. the target presented amongst distractors) could be accounted for by an unlimited capacity model, suggesting that multiple stimuli can be processed simultaneously without any limitations at an early stage of sensory processing. Taken together, these findings suggest the possible existence of biases in attentional weighting at a late stage of processing. The bias appears to favor the inferior visual field, which may arise from the fact that there is more ecologically-relevant information in this region of space.     

Visual performance fields: frames of reference

Performance in most visual discrimination tasks is better along the horizontal than the vertical meridian (Horizontal-Vertical Anisotropy, HVA), and along the lower than the upper vertical meridian (Vertical Meridian Asymmetry, VMA), with intermediate performance at intercardinal locations. As these inhomogeneities are prevalent throughout visual tasks, it is important to understand the perceptual consequences of dissociating spatial reference frames. In all studies of performance fields so far, allocentric environmental references and egocentric observer reference frames were aligned. Here we quantified the effects of manipulating head-centric and retinotopic coordinates on the shape of visual performance fields. When observers viewed briefly presented radial arrays of Gabors and discriminated the tilt of a target relative to homogeneously oriented distractors, performance fields shifted with head tilt (Experiment 1), and fixation (Experiment 2). These results show that performance fields shift in-line with egocentric referents, corresponding to the retinal location of the stimulus.     

Visual sensitivity in search tasks depends on the response requirement

This paper reports a comparison between two tasks of visual search. Two observers carried out, in separate blocks, a saccade-to-target task and a manual-target-detection task. The displays, which were identical for the two tasks, consisted of a ring of eight equally spaced Gabor patches. The target could be defined by a difference from the distractors along four possible dimensions: orientation, spatial frequency, contrast or size. These four dimensions were used as variables in separate experiments. In each experiment, performance was measured over an extensive range of values of the particular dimension. Thresholds were thus obtained for the saccade and the manual response tasks. The nature of the response was found to modify the relative visual sensitivity. For orientation differences, manual response performance was better than saccade-to-target performance. The reverse was true for spatial frequency and contrast differences, where saccade-to-target performance was better than manual response performance. We conclude that saccade-selection in a search task draws on different visual information from that used for manual responding in the equivalent task. The two tasks thus differ in more than the different response systems used: the results suggest the action of different underlying neural visual mechanisms as well as different neural motor mechanisms.     

Learning strengthens the response of primary visual cortex to simple patterns

Training can significantly improve performance on even the most basic visual tasks, such as detecting a faint patch of light or determining the orientation of a bar (for reviews, see ). The neural mechanisms of visual learning, however, remain controversial. One simple way to improve behavior is to increase the overall neural response to the trained stimulus by increasing the number or gain of responsive neurons. Learning of this type has been observed in other sensory modalities, where training increases the number of receptive fields that cover the trained stimulus. Here, we show that visual learning can selectively increase the overall response to trained stimuli in primary visual cortex (V1). We used functional magnetic resonance imaging (fMRI) to measure neural signals before and after one month of practice at detecting very low-contrast oriented patterns. Training increased V1 response for practiced orientations relative to control orientations by an average of 39%, and the magnitude of the change in V1 correlated moderately well with the magnitude of changes in detection performance. The elevation of V1 activity by training likely results from an increase in the number of neurons responding to the trained stimulus or an increase in response gain.     

Modelling eye movements in a categorical search task

We introduce a model of eye movements during categorical search, the task of finding and recognizing categorically defined targets. It extends a previous model of eye movements during search (target acquisition model, TAM) by using distances from an support vector machine classification boundary to create probability maps indicating pixel-by-pixel evidence for the target category in search images. Other additions include functionality enabling target-absent searches, and a fixation-based blurring of the search images now based on a mapping between visual and collicular space. We tested this model on images from a previously conducted variable set-size (6/13/20) present/absent search experiment where participants searched for categorically defined teddy bear targets among random category distractors. The model not only captured target-present/absent set-size effects, but also accurately predicted for all conditions the numbers of fixations made prior to search judgements. It also predicted the percentages of first eye movements during search landing on targets, a conservative measure of search guidance. Effects of set size on false negative and false positive errors were also captured, but error rates in general were overestimated. We conclude that visual features discriminating a target category from non-targets can be learned and used to guide eye movements during categorical search.     

Visual ‘cognitive’ evoked potentials in the behaving monkey

Using a visual ‘oddball’ paradigm we studied ERPs in monkeys trained in a ‘go’ ‘no-go’ discrimination task. The stimuli were 2.5 cpd sinusoidal gratings differing only in orientation (0° or 25°). Monkeys released a lever during 1 of 2 response windows (RW), 480–1762 or 740–1672 msec, following target stimulus onset. Target stimulus presentation probabilities were 1.0, 0.5 and 0.3. The primary evoked potentials recorded to either the target or non-target stimulus were similar in all monkeys. P₃ signals progressively emerged in the monkeys only to the target stimulus. P₃ recorded at Cz, P3, and P4 had similar mean latencies and amplitudes. Eye movements showed no relationship to P₃ potentials. Neither the primary visual potentials nor P₃ changed significantly as a function of RW. P₃ amplitude was inversely related to target probability. When the target stimulus was presented 100% of the time (P = 1.0) P₃ disappeared over 4–5 blocks of trials, while the primary evoked potentials remained consistent.     

Attention and visual search

Selective Tuning (ST) presents a framework for modeling attention and in this work we show how it performs in covert visual search tasks by comparing its performance to human performance. Two implementations of ST have been developed. The Object Recognition Model recognizes and attends to simple objects formed by the conjunction of various features and the Motion Model recognizes and attends to motion patterns. The validity of the Object Recognition Model was first tested by successfully duplicating the results of Nagy and Sanchez. A second experiment was aimed at an evaluation of the model's performance against the observed continuum of search slopes for feature-conjunction searches of varying difficulty. The Motion Model was tested against two experiments dealing with searches in the visual motion domain. A simple odd-man-out search for counter-clockwise rotating octagons among identical clockwise rotating octagons produced linear increase in search time with the increase of set size. The second experiment was similar to one described by Thorton and Gilden. The results from both implementations agreed with the psychophysical data from the simulated experiments. We conclude that ST provides a valid explanatory mechanism for human covert visual search performance, an explanation going far beyond the conventional saliency map based explanations.     

Transcranial magnetic stimulation reveals attentional feedback to area V1 during serial visual search

Visual search tasks have been used to understand how, where and when attention influences visual processing. Current theories suggest the involvement of a high-level "saliency map" that selects a candidate location to focus attentional resources. For a parallel (or "pop-out") task, the first chosen location is systematically the target, but for a serial (or "difficult") task, the system may cycle on a few distractors before finally focusing on the target. This implies that attentional effects upon early visual areas, involving feedback from higher areas, should be visible at longer latencies during serial search. A previous study from Juan & Walsh (2003) had used Transcranial Magnetic Stimulation (TMS) to support this conclusion; however, only a few post-stimulus delays were compared, and no control TMS location was used. Here we applied TMS double-pulses (sub-threshold) to induce a transient inhibition of area V1 at every post-stimulus delay between 100 ms and 500 ms (50 ms steps). The search array was presented either at the location affected by the TMS pulses (previously identified by applying several pulses at supra-threshold intensity to induce phosphene perception), or in the opposite hemifield, which served as a retinotopically-defined control location. Two search tasks were used: a parallel (+ among Ls) and a serial one (T among Ls). TMS specifically impaired the serial, but not the parallel search. We highlight an involvement of V1 in serial search 300 ms after the onset; conversely, V1 did not contribute to parallel search at delays beyond 100 ms. This study supports the idea that serial search differs from parallel search by the presence of additional cycles of a select-and-focus iterative loop between V1 and higher-level areas.     

Attention facilitates multiple stimulus features in parallel in human visual cortex

Successfully locating a dangerous or desirable object within a cluttered visual scene is a commonplace yet highly adaptive skill. In the laboratory, this ability is modeled by visual search experiments in which subjects try to find a target item surrounded by an array of distracting stimuli. Under certain conditions, targets that are distinguishable from distractors by virtue of having a particular combination of shared sensory features (e.g., a particular color and orientation) can be found rapidly regardless of the number of distractors. To explain this highly efficient localization of feature-conjunction targets, "guided search" theories propose that attention is directed in parallel to the individual features that define the target, which then stands out from the distractors because of additive facilitation of its feature signals. Here we recorded frequency-tagged potentials evoked in human visual cortex and found that color and orientation features of target stimuli are indeed facilitated by attention in a parallel and additive manner. This additive feature-enhancement mechanism, reported here for the first time, not only enables rapid guided search but also plays a broader role in directing and sustaining attention to multi-feature objects and keeping them perceptually distinct from background clutter.     

Fast noninertial shifts of attention

It was suggested that some discrimination tasks (e.g. discrimination between the letters T and L) require serial search by scrutinizing each letter (target) with a small aperture of focal attention. Here we examine the effect of intertarget distance on discrimination performance, using two targets. We find reduction in performance at short distances, in agreement with masking studies, but constant performance independent of distance outside this masking region. This constant performance is still lower than expected from masking effects and might reflect attentive process. Sequential presentation of the targets with delays up to 30-40 ms, while reducing available processing time, does not cause reduction in performance, thus supporting the suggestion that discrimination of the two targets is a serial process. The independence of performance on distance suggests fast noninertial shifts of attention.