Visual search for global/local stimulus features in humans and baboons

Fagot and Deruelle (1997) demonstrated that, when tested with identical visual stimuli, baboons exhibit an advantage in processing local features, whereas humans show the “global precedence” effect initially reported by Navon (1977). In the present experiments, we investigated the cause of this species difference. Humans and baboons performed a visual search task in which the target differed from the distractors at either the global or the local level. Humans responded more quickly to global than to local targets, whereas baboons did the opposite (Experiment 1). Human response times (RTs) were independent of display size, for both local and global processing. Baboon RTs increased linearly with display size, more so for global than for local processing. The search slope for baboons disappeared for continuous targets (Experiment 2). That effect was not due to variations in stimulus luminance (Experiment 3). Finally, variations in stimulus density affected global search slopes in baboons but not in humans (Experiment 4). Overall, results suggest that perceptual grouping operations involved during the processing of hierarchical stimuli are attention demanding for baboons, but not for humans.     

Interactions between search mechanisms in conjunction search.

We report the results of a technique designed to measure interactions between different visual search processes. We interrupted pop-out search before it produced a detection response, by adding extra distractors to the display so that a target initially defined by a single feature difference (e.g., a yellow horizontal line among yellow vertical lines) could then only be found on the basis of the conjunction of two features (a yellow horizontal line among yellow vertical lines and pink horizontal lines; difficult search). This technique has been used to measure the duration of the perceptual components of pop-out search, independent of over-all response time, for targets presented among different sets of distractors. In addition, when pop-out failed because it was interrupted, past work has shown that it nevertheless provided useful information to the processes responsible for difficult search. That is, partial pop-out assisted difficult search, when extra distractors made search difficult because the target was between the two types of distractors in the relevant feature space (Olds, Cowan, & Jolicoeur, 2000a,b,c). The present results demonstrate that partial pop-out also assists difficult search when difficult search is a conjunction search, and therefore these interactions may occur at a stage where information from different feature dimensions is combined.     

Visual search asymmetry: The influence of stimulus familiarity and low-level features

Wang, Cavanagh, and Green (1994) demonstrated a pop-out effect in searching for an unfamiliar target among familiar distractors (U—F search) and argued for the importance of a familiarity difference between the target and the distractors in determining search efficiency. In four experiments, we explored the generality of that finding. Experiment 1 compared search efficiency across a variety of target-distractor pairs. In Experiments 2, 3, and 4, we used Chinese characters and their transforms as targets and distractors and compared search performance between Chinese and non-Chinese participants. We demonstrated that search asymmetry and search efficiency in the U—F condition are influenced by the presence of low-level feature differences between the familiar and the unfamiliar stimuli. Our findings suggest that the familiarity of the distractors, rather than the familiarity difference between the target and the distractors, determines search efficiency. We also documented a counterintuitive familiarity-inferiority effect, suggesting that knowledge of search stimuli may, sometimes, be detrimental to search performance.     

Roles of salience and strategy in conjunction search

In some cases, the search for a conjunction target proceeds through the smaller group of elements in a display, whereas in others, search is limited to those elements that share a particular feature with the target. In 6 experiments, participants searched for a conjunction target among displays consisting of various proportions of 2 distractor types. Smaller-group search was more prevalent than target-feature search with denser displays and with features that were highly discriminable. Explicit instructions to limit search to a specific feature affected performance only when the discriminability of the guiding feature was much greater than the other target feature. Together, these experiments show that bottom-up factors have more influence in guiding conjunction searches than previously thought.     

Visual search asymmetry: the influence of stimulus familiarity and low-level features.

Wang, Cavanagh, and Green (1994) demonstrated a pop-out effect in searching for an unfamiliar target among familiar distractors (U-F search) and argued for the importance of a familiarity difference between the target and the distractors in determining search efficiency. In four experiments, we explored the generality of that finding. Experiment 1 compared search efficiency across a variety of target-distractor pairs. In Experiments 2, 3, and 4, we used Chinese characters and their transforms as targets and distractors and compared search performance between Chinese and non-Chinese participants. We demonstrated that search asymmetry and search efficiency in the U-F condition are influenced by the presence of low-level feature differences between the familiar and the unfamiliar stimuli. Our findings suggest that the familiarity of the distractors, rather than the familiarity difference between the target and the distractors, determines search efficiency. We also documented a counterintuitive familiarity-inferiority effect, suggesting that knowledge of search stimuli may, sometimes, be detrimental to search performance.     

Cross-trial priming in visual search for singleton conjunction targets: role of repeated target and distractor features

Kristjánsson, Wang, and Nakayama (2002) demonstrated that visual search for conjunctively defined targets can be substantially expedited ("primed") when target and distractor features are repeated on consecutive trials. Two experiments were conducted to examine whether the search response time (RT) facilitation on target-present trials results from repetition of target-defining features, distractor features, or both. The experiments used a multiple conjunctive search paradigm (adapted from Kristjánsson et al., 2002), in which the target and distractor features were varied (i.e., repeated) independently of each other across successive trials. The RT facilitation was numerically largest when both target and distractor features were repeated, but not significantly larger than that when only distractor features were repeated. This indicates that cross-trial priming effects in conjunctive visual search result mainly from the repetition of distractor, rather than target, features.     

Perceptual learning in visual conjunction search.

Perceptual learning in colour/orientation visual conjunction search was examined in five experiments. Good transfer occurred to other conjunction arrays when only one element of the conjunction (either colour or orientation) was changed. When both elements (colour and orientation) were changed, but the same feature spaces were used (i.e. other colours and orientations) or when a new dimension was introduced to the transfer task (shapes instead of orientation), transfer was poor. The results suggest that perceptual learning of visual conjunction search is constrained mainly by stimulus parameters rather than by changes in cognitive strategies which are common to all search tasks. Contrary to other reports we found little evidence of long-term retention of learning.     

A feature-weighting account of priming in conjunction search

Previous research on the priming effect in conjunction search has shown that repeating the target and distractor features across displays speeds mean response times but does not improve search efficiency: Repetitions do not reduce the set size effect—that is, the effect of the number of distractor items—but only modulate the intercept of the search function. In the present study, we investigated whether priming modulates search efficiency when a conjunctively defined target randomly changes between red and green. The results from an eyetracking experiment show that repeating the target across trials reduced the set size effect and, thus, did enhance search efficiency. Moreover, the probability of selecting the target as the first item in the display was higher when the target—distractor displays were repeated across trials than when they changed. Finally, red distractors were selected more frequently than green distractors when the previous target had been red (and vice versa). Taken together, these results indicate that priming in conjunction search modulates processes concerned with guiding attention to the target, by assigning more attentional weight to features sharing the previous target’s color.     

Motion coherence and conjunction search: Implications for guided search theory

Feature integration theory has recently been revised with two proposals that visual conjunction search can be parallel under some circumstances—either because items with nontarget features are inhibited, or because items with target features are excited. We examined whether excitatory or inhibitory guidance controlled conjunction search for an X oscillating in one direction among Os oscillating in that direction and Xs oscillating in another. Search was affected by whether items oscillated in phase with each other, and it was exceptionally difficult when items with target motion moved out of phase with each otherand items with nontarget motion moved out of phase. The results suggest that conjunction search can be guided both by excitation of target features and by inhibition of nontarget features.     

Motion coherence and conjunction search: implications for guided search theory.

Feature integration theory has recently been revised with two proposals that visual conjunction search can be parallel under some circumstances--either because items with nontarget features are inhibited, or because items with target features are excited. We examined whether excitatory or inhibitory guidance controlled conjunction search for an X oscillating in one direction among Os oscillating in that direction and Xs oscillating in another. Search was affected by whether items oscillated in phase with each other, and it was exceptionally difficult when items with target motion moved out of phase with each other and items with nontarget motion moved out of phase. The results suggest that conjunction search can be guided both by excitation of target features and by inhibition of nontarget features.     

A common computational process in cueing and conjunction search tasks

The question of whether Posner’s beam is the same as Treisman’s glue is addressed to construct a computational model that integrates both target and cue information. The cueing and conjunction search tasks are conducted to analyze a common process that may be underlying the tasks. The dynamic interaction between target and cue information produces attentional benefit- and cost-based in the cueing task. Furthermore, the search order for target candidates in a conjunction search task is determined through the integration of target and cue information, which is basically the same as in the cueing task. Our simulations suggest that consistency (or validity) is considered as a computational process that may be commonly involved in the both tasks.     

Distractor ratio and grouping processes in visual conjunction search.

According to feature integration theory, conjunction search is conducted via a serial self-terminating search. However, effects attributed to search processes operating on the entire display may actually reflect search restricted to elements defined by a single feature. In experiment 1 this question is addressed in a reaction-time (RT) paradigm by varying distractor ratios within an array of fixed size. For trials in which the target was present in the array, RT functions were roughly symmetric, the shortest RTs being for extreme distractor ratios, and the longest RTs being for arrays in which there were an equal number of each distractor type. This result is superficially consistent with Zohary and Hochstein's interpretation that subjects search for only one distractor type and are able to switch search strategy from trial to trial. However, negative-trial data from experiment 1 case doubt on this interpretation. In experiment 2 the possible role of 'pop out' and of distractor grouping in visual conjunction search is investigated. Results of experiment 2 suggest that grouping may play a more important role than does distractor ratio, and point to the importance of the spatial layout of the target and of the distractor elements in visual conjunction search. Results of experiment 2 also provide clear evidence that groups of spatially adjacent homogeneous elements may be processed as a unit.     

A salient distractor does not disrupt conjunction search

Leading theories of attention posit that bottom-up and top-down factors simultaneously affect attentional priority in visual search. Recent evidence, however, suggests that subjects may rely exclusively on top-down guidance when searching for a target defined by a specific known feature (Bacon & Egeth, 1994). In the present experiment, we addressed this issue in a conjunction search task. We investigated how searching for a green O among green Ts and red Os is affected by the presence of a distractor with a unique shape (green X), color (blue O), or both (blue X). We showed that the salient distractor does not disrupt performance on target-present trials, but produces a large interference on target-absent trials. We conclude that salience-based and top-down processes are alternative modes of guidance, rather than joint contributors in the allocation of attentional priority.     

Judgement under uncertainty and conjunction fallacy inhibition training

Intuitive predictions and judgements under uncertainty are often mediated by judgemental heuristics that sometimes lead to biases. Our micro-developmental study suggests that a presumption of rationality is justified for adult subjects, in so far as their systematic judgemental biases appear to be due to a specific executive-inhibition failure in working memory, and not necessarily to a lack of understanding of the fundamental principles of probability. This hypothesis was tested using an experimental procedure in which 60 adult subjects were trained to inhibit the classical conjunction bias on a frequency judgement task derived from Tversky and Kahneman's work. Pre- and post-test performance was assessed via a probability judgement task. The data indicated a training effect, suggesting that subjects traditionally labelled as “irrational” with respect to the classical rules of inductive reasoning are in fact “inefficient inhibitors”. These findings are discussed in terms of a polymorphous view of rationality.     

Age differences in what is viewed and remembered in complex conjunction search

Older and younger adults searched arrays of 12 unique real-world photographs for a specified object (e.g., a yellow drill) among distractors (e.g., yellow telephone, red drill, and green door). Eye-tracking data from 24 of 48 participants in each age group showed generally similar search patterns for the younger and older adults but there were some interesting differences. Older adults processed all the items in the arrays more slowly than the younger adults (e.g., they had longer fixation durations, gaze durations, and total times), but this difference was exaggerated for target items. We also found that older and younger adults differed in the sequence in which objects were searched, with younger adults fixating the target objects earlier in the trial than older adults. Despite the relatively longer fixation times on the targets (in comparison to the distractors) for older adults, a surprise visual recognition test revealed a sizeable age deficit for target memory but, importantly, no age differences for distractor memory.     

Age differences in what is viewed and remembered in complex conjunction search

Older and younger adults searched arrays of 12 unique real-world photographs for a specified object (e.g., a yellow drill) among distractors (e.g., yellow telephone, red drill, and green door). Eye-tracking data from 24 of 48 participants in each age group showed generally similar search patterns for the younger and older adults but there were some interesting differences. Older adults processed all the items in the arrays more slowly than the younger adults (e.g., they had longer fixation durations, gaze durations, and total times), but this difference was exaggerated for target items. We also found that older and younger adults differed in the sequence in which objects were searched, with younger adults fixating the target objects earlier in the trial than older adults. Despite the relatively longer fixation times on the targets (in comparison to the distractors) for older adults, a surprise visual recognition test revealed a sizeable age deficit for target memory but, importantly, no age differences for distractor memory.     

The effects of stimulus rotation and familiarity in visual search

Rauschenberger and Yantis (2006) observed that an intersecting circle-line combination enjoyed significantly greater search efficiency when it was oriented to resemble a "Q" than when it was oriented so that the intersecting line was vertical (cf. Treisman and Souther, 1985). Although a control experiment made it unlikely that the obliqueness of the line was responsible for the observed benefit, there was no direct evidence that this benefit was attributable to the "Q-ness" of the stimulus. In the present study, a subset of Rauschenberger and Yantis's experiments was repeated with Chinese subjects, who had never been exposed to the Latin alphabet. For these subjects, there was no benefit for the"Q"-like stimulus, in contrast to the results of Rauschenberger and Yantis's study. These results show that a simple 45 degrees rotation of a stimulus can affect search efficiency significantly--but only when this rotation bestows meaning, or familiarity, to that stimulus.     

Top-down inhibition of search distractors in parallel visual search

In three experiments, we examined distractor inhibition in parallel ("pop-out") visual search. Distractor inhibition was measured in terms of reaction time (RT) to a simple luminance increment probe presented, after the search task response, at display locations that either contained a search distractor (on-probe) or were blank (off-probe). When the search stimuli remained in view, the on-probe (relative to off-probe) RT cost was larger than in a baseline condition in which observers had only to passively view, rather than search, the display. This differential on-probe RT cost, which discounts effects of masking, was interpreted as a measure of distractor inhibition associated with target selection in parallel visual search. Taken together, the results argue that the distractor inhibition is an object-based and local phenomenon that affects all distractors (of a particular type) in an equal manner.     

The visual search analogue of latent inhibition: Implications for theories of irrelevant stimulus processing in normal and schizophrenic groups

Latent inhibition (LI) is a robust phenomenon that is demonstrated when a previously inconsequential stimulus is less effective in a new learning situation than a novel stimulus. Despite LI’s simplicity, there is considerable disagreement as to its theoretical basis. Attentional theories claim that unattended stimulus preexposures reduce stimulus associability. Alternatively, it has been asserted that associability is unaffected and that LI is a result of competition/retrieval processes. The present article reviews a series of visual search studies, some with normal subjects, both undifferentiated and divided into low and high schizotypals, and others with pathologies that entail dysfunctional attention, such as schizophrenia, Parkinson’s disease, and anxiety. The visual search conditions were designed to model those of traditional LI experiments, while tapping attentional processes independently of the learning scores that index LI. A variety of evidence from these and other studies is used to support the involvement of attentionaland retrieval processes in LI. A model of the mechanism of action of these processes in LI is presented, together with its application to schizophrenia.