Which particular selective pressures guided the evolution of zebra stripes? This question, one of biology's most celebrated conundrums, is also of potential interest to readers of Perception, as, on most accounts, stripes' benefits to zebras reflect perception of stripes by their predators, parasites, or conspecifics. Although stripes do not seem to camouflage zebras or warn off predators, various accounts implicate perception, including proposals that stripes disrupt predator perception, aid intra-species recognition, or deter biting flies. Currently, only the last of these enjoys strong empirical support: narrow stripes on zebras' heads, necks, shoulders, limbs, and flanks are known to deter biting flies, and variation in patterns of those stripes is associated with parasite burden, not predators, across species. Rump stripes, however, are different. Typically broad and horizontal, there is some evidence that they vary with hyaena threat rather than parasites, consistent with an antipredator role. Here, viewing images of zebras, filtered to simulate lion and spotted-hyaena vision at distance or in motion, human observers typically judged rump stripes to be the most attention-capturing regions of the images. Computational models implicated visual salience in this effect, and pursuit simulations showed that by driving predators' attention to a zebra's rear, rump stripes could minimise the probability of capture.

New computed tomography and image enhancement technologies are increasingly used in cabin baggage screening at airports. The current work sought to establish whether these technological advancements had pushed beyond human psychological capability by examining the impact of different image enhancements on threat detection. Sixty-one naïve adults participated in an online study and received targetless search training before being tested in a simulated baggage screening task - in addition to three-dimensional images this task incorporated two-dimensional slice views, material stripping and zoom functionality. The two-dimensional slice view improved threat detection sensitivity and lowered response criterion relative to a standard three-dimensional image, as screeners found dangerous items they had initially missed, likely due to improved ability to resolve superposition or identify edges. In contrast, material stripping and zoom did not impact sensitivity, criterion or screeners’ confidence in their responses, suggesting that they contributed no additional informational value.

Conventional visual search tasks do not address attention directly and their core manipulation of ‘set size’ – the number of displayed items – introduces stimulus confounds that hinder interpretation. However, alternative approaches have not been widely adopted, perhaps reflecting their complexity, assumptions, or indirect attention-sampling. Here, a new procedure, the ATtention Location And Size (‘ATLAS’) task used probe displays to track attention’s location, breadth, and guidance during search. Though most probe displays comprised six items, participants reported only the single item they judged themselves to have perceived most clearly – indexing the attention ‘peak’. By sampling peaks across variable ‘choice sets’, the size and position of the attention window during search was profiled. These indices appeared to distinguish narrow- from broad attention, signalled attention to pairs of items where it arose and tracked evolving attention-guidance over time. ATLAS is designed to discriminate five key search modes: serial-unguided, sequential-guided, unguided attention to ‘clumps’ with local guidance, and broad parallel-attention with or without guidance. This initial investigation used only an example set of highly regular stimuli, but its broader potential should be investigated.

Computed Tomography (CT) is increasingly used in screening of cabin baggage in airports. The current study aimed to establish whether screening with CT confers a detection advantage over dual-view (DV) X-ray when resolution is controlled. We also evaluated whether a 'targetless' search strategy - in which screeners identify and reject safe items - improved detection relative to target-based methods. In an online study, 104 novice screeners were trained with either CT or DV, and either a targetless or a target-based search strategy. Screeners were then tested in a simulated cabin baggage screening task. CT screeners performed with greater sensitivity than DV screeners. Search strategy did not affect sensitivity, although the target-based strategy resulted in a more liberal criterion. We conclude that CT imaging confers a benefit to screening performance over DV when image resolution is controlled. This is likely due to the ability to rotate the image to resolve occlusions.

Trying to ignore an object can bias attention towards it - a phenomenon referred to as the 'attentional white bear' (AWB) effect. The mechanisms behind this effect remain unclear. On one hand, the AWB may reflect reactive, 'search and destroy' distractor suppression, which directs attention toward irrelevant objects in order to suppress further attention to them. However, another possibility is that the AWB results from failed proactive distractor suppression - attempting to suppress attention to an irrelevant object from the outset may inadvertently result in an attentional shift towards it. To distinguish these two possibilities, we developed a categorical visual search task that addresses limitations present in prior studies. In five experiments (Ntotal = 96), participants searched displays of naturalistic stimuli cued only with distractor categories (targets were unknown and unpredictable). We observed an AWB and later attenuated it by presenting a pre-search stimulus, likely disrupting guidance from distractor templates in working memory. We conclude that the AWB resulted from a failure of proactive suppression rather than a search and destroy process.

When searching for a known target, mental representations of target features, or templates, guide attention towards matching objects and facilitate recognition. When only distractor features are known, distractor templates allow irrelevant objects to be recognised and attention to be shifted away. This is particularly true in X-ray baggage search, a challenging real-world visual search task with implications for public safety, where targets may be unknown, difficult to predict and concealed by an adversary, but distractors are typically benign and easier to identify. In the present study, we draw on basic principles of distractor suppression and rejection to investigate a counterintuitive 'targetless' approach to training baggage search. In a simulated X-ray baggage search task, we observed significant benefits to target detection sensitivity (d') for targetless relative to target-based training, but no effects of performance-contingent rewards or the inclusion of superordinate semantic categories during training. The benefits of targetless search training were most apparent for stimuli involving less spatial overlap (occlusion), which likely represents the difficulty and greater individual variation involved in searching more visually complex images. Together, these results demonstrate the effectiveness of a counterintuitive targetless approach to training target detection in X-ray baggage search, based on basic principles of distractor suppression and rejection, with potential for use as a real-world training tool.

SummaryZebras’ stripes cannot protect them from predators, Darwin concluded, and current consensus tends to support his view1,2. In principle, stripes could support crypsis or aposematism, could dazzle, confuse or disrupt predators’ perception3–8, yet no such effects are manifest in predator-prey interactions9–11. Instead, narrow stripes covering zebras’ head, neck, limbs and flanks are an effective deterrent to tabanids12, vectors for equine disease13,14. Accordingly, while other potential benefits, e.g., thermoregulation15,16and intraspecific communication17, cannot be excluded, zebra stripes likely evolved primarily to deter parasites18–20. Rump stripes, however, do not fit this, or any extant view. Typically horizontal and broader in sub-species with width variation, they are ill-suited to crypsis or parasite-deterrence12and vary with hyaena threat18, perhaps shaped by an additional selective pressure. We observed that rump (and rear-flank) stripes remain highly conspicuous when viewed in motion or at distance, while other stripes do not. To study this striking effect, we filtered images of zebra to simulate acuity limitations in lion and hyaena photopic and mesopic vision. For mountain zebra and plains zebra without shadow striping, rump stripes were the most conspicuous image regions according to computational salience models, corroborated by human observers’ judgements of maximally attention-capturing image locations, which were strongly biased toward the rear. By hijacking exogenous attention mechanisms to force predator attention to the rear, salient rump stripes confer benefits to zebra, estimated here in pursuit simulations. Benefits of rump stripe salience may counteract anti-parasite benefits and costs of conspicuity to shape rump and shadow stripe variation.1HighlightsZebra stripes likely evolved to deter biting flies, but rump stripes are ill-suited to this.Rump-stripes remain highly conspicuous when viewed at distance or in motion.Computational models and human observers’ judge rump stripes are most salient stripes.Salient rump stripes drive predator attention to rear, hindering capture by predators.Observe this striking effect in moving zebra at: viscog.psychol.cam.ac.uk/resources-and-downloads

Another person’s gaze direction is a rich source of social information, especially eyes gazing toward prominent or relevant objects. To guide attention to these important stimuli, visual search mechanisms may incorporate sophisticated coding of eye-gaze and its spatial relationship to other objects. Alternatively, any guidance might reflect the action of simple perceptual ‘templates’ tuned to visual features of socially relevant objects, or intrinsic salience of direct-gazing eyes for human vision. Previous findings that direct gaze (toward oneself) is prioritised over averted gaze do not distinguish between these accounts. To resolve this issue, we compared search for eyes gazing toward a prominent object versus gazing away , finding more efficient search for eyes ‘gazing toward’ the object. This effect was most clearly seen in target-present trials when gaze was task-relevant. Visual search mechanisms appear to specify gazer-object relations, a computational building-block of theory of mind.

Direct gaze - someone looking at you - is an important and subjectively-salient stimulus. Its processing is thought to be enhanced by the brain's internalised predictions - priors - that effectively specify it as the most likely gaze direction. Current consensus holds that, befitting its presumed importance, direct gaze attracts attention more powerfully than other gazes. Conversely, some Predictive Coding (PC) models, in which exogenous attention is drawn to stimuli that violate predictions, may be construed as making the opposite claim - i.e., exogenous attention should be biased away from direct gaze (which conforms to internal predictions), toward averted gaze (which does not). Here, searching displays with salient, 'odd-one-out' gazes, we observed attentional bias (in rapid, initial saccades) toward averted gaze, as would be expected by PC models. However, this pattern obtained only when conditions highlighted gaze-uniqueness. We speculate that, in our experiments, task requirements determined how prediction influenced perception.

Direct gaze-someone gazing at you-is an important social cue that might be expected to capture visual attention, even in the presence of other faces. Consistent with this, direct gazing eyes are often detected more rapidly in arrays of averted gazing eyes, than vice versa; a search asymmetry termed the stare-in-the-crowd effect (SITCE). Here, we examine top-down influences on the SITCE by manipulating observers' knowledge of the target's gaze prior to the search display. Our findings revealed two dissociable components of the SITCE. The first, which scaled with set size but was unaffected by prior knowledge, was attributed to noisy, parallel gaze processing that guides attention toward direct gaze (Process 1). The second, an overall response time advantage for direct versus averted gaze targets, irrespective of set size, was attributed to criteria for determining target presence versus absence (Process 2). Prior knowledge of the target's gaze direction increased the direct gaze advantage, rather than speeding up responses for both target types (typically expected for 100% valid cues). This unusual pattern suggests that top-down gaze-related influences may comprise an obligatory bias toward direct gaze. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Advances in neuroscience offer the exciting prospect of understanding 'free' choices - the subject of the free will debate in philosophy. However, while physiological techniques and analysis have progressed rapidly to meet this challenge, task design has not. The challenge is now to develop laboratory tasks that adequately capture 'free' picking or choosing. To isolate 'internally' generated intentions from those impelled by external stimulus, observers are asked to 'choose freely' or to wait for a felt 'urge'. However, no previous work has explicitly distinguished between instructions that refer to 'urges' versus to 'choosing'. The philosopher Alfred Mele (e.g., 2009; 2014) has argued that the distinction is of crucial conceptual importance, but the two have not yet been empirically distinguished. Here, we show that conscious and unconscious, task-irrelevant primes, bias observers' binary choices when they are instructed to 'choose freely', not when they 'wait for an urge', underscoring the practical importance of Mele's conceptual distinction. Neuroscience must incorporate this distinction if we are to understand processes underpinning free choice.

Top-down search templates specify targets' properties, either to guide attention toward the target or, independently, to accelerate the recognition of individual search items. Some previous studies have concluded that target templates can specify semantic categories to guide attention, though dissociating the effects of semantic versus visual features has proven difficult. In the present experiments, we examined the roles of target templates in search performance, by measuring the "two-template costs" incurred when observers did not know which of two types of targets would be presented. For target templates, these costs only varied with set size when a template could specify a target's features. Any semantic influences did not affect the guidance of attention, only the recognition of individual items. In contrast, templates for rejection-specifying the properties of irrelevant nontargets-do appear to specify semantic properties to guide attention away from those items, without affecting recognition. These qualitative differences between the two types of templates suggest that the processes of seeking and ignoring are fundamentally different.

In contrast to standard search templates that specify a target object's expected features, templates for rejection (TFR) may specify features of nontargets, biasing attention away from irrelevant objects. Little is known about TFR, and virtually nothing is known about their role in guiding search across natural scenes. In such scenes, targets and nontargets may not be easily distinguished on the basis of their visual features; it has been claimed that standard search templates may therefore specify target objects' semantic features to guide attention. Here, we ask whether TFR can do so. Noting a limitation of previous procedures used to study standard search templates, we trialed an alternative method to examine semantic templates for nontarget exclusion in natural scene search. We found that when nontargets belonged unpredictably to either of two physically distinct categories, search was less efficient than when targets belonged to one known category. This two-category cost, attributed to inefficient application of search templates, was absent for two physically dissimilar but semantically related categories. Adding a training phase to highlight semantic distinctiveness of two object categories reinstated the two-category cost, precluding stimulus-based accounts of the effect. These patterns were not observed for one-image displays or when observers searched for object categories rather than ignoring them, demonstrating their specificity to TFR, the inadequacy of search-and-destroy models to account for them, and likely basis in attentional guidance. TFR can specify semantic information to guide attention away from nontargets.

'Heuristic' theories of autism postulate that a single mechanism or process underpins the diverse psychological features of autism spectrum disorder. Although no such theory can offer a comprehensive account, the parsimonious descriptions they provide are powerful catalysts to autism research. One recent proposal holds that 'noisy' neuronal signalling explains not only some deficits in autism spectrum disorder, but also some superior abilities, due to 'stochastic resonance'. Here, we discuss three distinct actions of noise in neural networks, arguing in each case that autism spectrum disorder symptoms reflect too little, rather than too much, neural noise. Such reduced noise, perhaps a function of atypical brainstem activation, would enhance detection and discrimination in autism spectrum disorder but at significant cost, foregoing the widespread benefits of noise in neural networks.

Evidence suggests that subliminal stimuli can influence ostensibly volitional, executive processes but it is unclear whether this is highly task-specific. To address this we used a set-switching task. Volunteers saw a word pair and reported either if both words had the same number of syllables or if both were concrete. Task selection was random and instructed by a hexagon/triangle preceding the words. A subliminally-presented square or diamond reliably preceded each of these consciously perceived instruction-shapes. Significant congruency effects were observed in a subsequent Test Phase in which primes no longer reliably predicted the task (and in which high/low tones now served as conscious instructions). The Generalization Phase required novel phonological (rhyme) or semantic (category) judgments. Remarkably, unconscious priming congruency effects carried over in those participants who had shown priming in the Test Phase, the degree correlating across the two conditions. In a final phase of the study, participants were asked to discriminate between the two originally presented prime shapes. Those participants whose discriminations were more accurate showed reduced priming relative to participants with less accurate discriminations. The results suggest that, rather than being highly task specific, priming can operate at the level of a generalizable process and that greater awareness of primes may lessen their impact on behavior.

Acknowledgments . This work was supported by the Nancy Lurie-Marks Family Foundation and the Medical Research Council (MRC). Bonnie Auyeung was supported by a scholarship from Trinity College, Cambridge. We are grateful to the families who have taken part in this longitudinal study over many years and to Melissa Hines, Rosa Hoekstra, Bhismadev Chakrabarti, Malcolm Bang, Svetlana Lutchmaya, Greg Davis, and Ieuan Hughes for valuable discussions. Abstract Studies of amniotic testosterone in humans suggest that foetal testosterone (fT) is related to specific (but not all) sexually dimorphic aspects of cognition and behaviour. It has also been suggested that autism may be an extreme manifestation of some male-typical traits, both in terms of cognition and neuroanatomy. In this paper, we examine the possibility of a link between autistic traits and fT levels measured in amniotic fluid during routine amniocentesis. Two instruments measuring number of autistic traits (the Childhood Autism Spectrum Test (CAST) and the Child Autism Spectrum Quotient (AQ-Child)) were completed by these women about their children ( n =235), ages 6-10 yrs. IQ was measured in a subset of these children ( n= 74). fT levels were positively associated with higher scores on the CAST and AQ-Child. This relationship was seen within sex as well as when the sexes were combined, suggesting this is an effect of fT rather than of sex per se. No relationships were found between overall IQ and the predictor variables, or between IQ and CAST or AQ-Child. These findings are consistent with the hypothesis that prenatal androgen exposure is related to children exhibiting more autistic traits. These results need to be followed up in a much larger sample to test if clinical cases of ASC have elevated fT.