Publications

Vivid visual mental imagery is thought to influence perceptual processing, but much of the current knowledge on this comes from one highly cited, though underpowered (N= 8) study from 2007, which found that more vivid imagery increases interference between imagined and perceptual content. However, that study has not been repeated since. We therefore conducted a conceptual (Experiment 1) and direct (Experiment 2) replication study. In Experiment 1, we recruited 185 online participants across the mental imagery spectrum, including individuals with self-reported aphantasia (impoverished or absent mental imagery) and hyperphantasia (extremely vivid imagery). In Experiment 2, we recruited 56 participants, 28 with self-reported aphantasia and 28 gender- and age-matched typical imagers. Consistent with the original 2007 study's interpretation, we predicted that those with more vivid imagery would exhibit stronger imagery-perception interference, as measured by decreased performance in a priming task when a color and word were congruent (e.g., red prime, word RED) compared to incongruent (e.g.,blue prime, word RED). We were unable to replicate this effect in either experiment. Instead, we observed performance benefits for color word congruency across the mental imagery spectrum, with no difference in the magnitude of this effect across imagery ability or vividness, even among those with extreme imagery variations (aphantasia, hyperphantasia). Interestingly, we observed a relationship between a measure of mental imagery externalism and the congruency effect, suggesting that individuals with the ability to project their mental images into the external environment (i.e., prophantasia) may exhibit stronger congruency effects. The results of this study challenge our current understanding of the role of mental imagery in perception.

OSF preregistration: The present study seeks to investigate how different levels of top-down input affect visual processing in individuals with varying levels of mental imagery ability and sensory processing sensitivity (SPS). Using a multi-paradigm approach, participants will perform tasks that include Mooney stimuli, Mueller-Lyer and Ebbinghaus illusions, and the illumination bias. These tasks respectively probe high-level contextual (cognitive) priors and low-level structural (perceptual) priors. By examining these diverse tasks, we aim to elucidate how individual differences in mental imagery and sensory sensitivity influence reliance on prior information and processing of sensory inputs. This research contributes to understanding the nuanced roles of cognitive and sensory factors in visual perception across different populations.

The current study utilizes the Ganzflicker paradigm—a flickering stimulus that induces visual hallucinations—to provide insight into the internally-generated visual experiences that correlate with individual differences in visual imagery. Here, we analyzed rich narrative descriptions of Ganzflicker hallucinations from 4,365 participants using natural language processing, sensorimotor norms, and AI visualizations. We find that overall perceptual richness and visual detail in descriptions increase with imagery vividness. Examining the specific content of these descriptions reveals that vivid imagers report more face and hand-related content than those with weaker imagery. Exploratory AI-generated visualizations of these descriptions provide additional insights, as those with weak imagery report patterns of simple visual features, like colors and geometric forms, while strong imagers' hallucinations are filled with complex real-world stimuli. These findings suggest imagery differences may lie not in early visual processing but in the integration of basic visual features into complex object- and scene-level representations.

A rapidly alternating red and black display known as Ganzflicker induces visual hallucinations that reflect the generative capacity of the visual system. Recent proposals regarding the imagery spectrum, that is, differences in the visual system of individuals with absent imagery, typical imagery, and vivid imagery, suggest these differences should impact the complexity of other internally generated visual experiences. Here, we used tools from natural language processing to analyze free-text descriptions of hallucinations from over 4,000 participants, asking whether people with different imagery phenotypes see different things in their mind's eye during Ganzflicker-induced hallucinations. Strong imagers described complex, naturalistic content, while weak imagers reported simple geometric patterns. Embeddings from vision language models better captured these differences than text-only language models, and participants with stronger imagery used language with richer sensorimotor associations. These findings may reflect individual variation in coordination between early visual areas and higher-order regions relevant for the imagery spectrum.

OSF preregistration: Here, we will shed light on the associations between perceptual extremes (in mental imagery, sensory (processing) sensitivity), and divergent perceptual experiences. Additionally, we will test the predictions made by Reeder and colleagues (2024). We will test these predictions in a large-scale cohort study overrepresenting participants at the extremes of the mental imagery ability-continuum, synaesthesia, and sensory (processing) sensitivity. We will calculate associations between concepts based on self-report questionnaires, but we will also include behavioural tasks measuring mental imagery and sensory (processing) sensitivity.

OSF preregistration: The impact of aphantasia on emotions and memory remains poorly understood. There have only been two studies (Wicken et al., 2021; Keogh et al., 2023), with modest sample sizes, reliance on hypothetical scenarios (e.g. plane crash), and a focus on a single emotion: fear. The current study will expand this limited knowledge by investigating short- and long-term memory for positive and negative emotional scenarios using a large, online sample of individuals with and without aphantasia. Specifically, emotional stimuli will be either perceived (short videos) or imagined (short stories). Participants will rate each stimulus on valence, arousal, and emotional impact. Additionally, they will be asked to freely recall details from the stimuli in sessions that occur immediately after exposure, and again after one-week and one-year. This design will allow us to assess the role of mental imagery in short- and long-term memory of imagined and perceived emotional content.

Hallucinations and delusions can be symptoms of psychiatric illness, but more often -though less commonly known - are actually part of a healthy range of experiences found throughout the general population. The studies in this Special Collection paint a picture of the wide range of hallucinatory and delusional experiences across diverse populations, as well as comparative perspectives between clinical and non-clinical samples. In this editorial, I make three related points that are exemplified in the articles published here. First, that hallucinations and delusions are part of a normal distribution of human diversity; their mere presence does not indicate psychosis or psychiatric illness. Second, that the ubiquity of hallucinatory and delusional experiences across clinical and non-clinical populations suggests common cognitive and neural mechanisms. Finally, despite these commonalities, it is important to understand the difference between psychiatric symptoms and healthy experience. In summary, I conclude that it is important to investigate both common mechanisms and distinguishing factors to comprehensively elucidate these oft-misunderstood experiences. This Special Collection provides a showcase of the cutting-edge research that encompasses these objectives.

Predictive processing theories state that our subjective experience of reality is shaped by a balance of expectations based on previous knowledge about the world (i.e. priors) and confidence in sensory input from the environment. Divergent experiences (e.g. hallucinations and synaesthesia) are likely to occur when there is an imbalance between one's reliance on priors and sensory input. In a novel theoretical model, inspired by both predictive processing and psychological principles, we propose that predictable divergent experiences are associated with natural or environmentally induced prior/sensory imbalances: inappropriately strong or inflexible (i.e. maladaptive) high-level priors (beliefs) combined with low sensory confidence can result in reality discrimination issues, a characteristic of psychosis; maladaptive low-level priors (sensory expectations) combined with high sensory confidence can result in atypical sensory sensitivities and persistent divergent percepts, a characteristic of synaesthesia. Crucially, we propose that whether different divergent experiences manifest with dominantly sensory (e.g. hallucinations) or nonsensory characteristics (e.g. delusions) depends on mental imagery ability, which is a spectrum from aphantasia (absent or weak imagery) to hyperphantasia (extremely vivid imagery). We theorize that imagery is critically involved in shaping the sensory richness of divergent perceptual experience. In sum, to predict a range of divergent perceptual experiences in both clinical and general populations, three factors must be accounted for: a maladaptive use of priors, individual level of confidence in sensory input, and mental imagery ability. These ideas can be expressed formally using nonparametric regression modeling. We provide evidence for our theory from previous work and deliver predictions for future research.

Approximately 4% of the population has aphantasia, which is defined as impoverished, or absent, sensory mental imagery. Previous research suggests that people with aphantasia (aphants) may have a higher prevalence of mental health conditions and neurodivergence compared to the general population, but aphantasia presents a special challenge for diagnosis and treatment. Many mental health conditions are currently characterized by imagery-related symptomology (such as sensory flashbacks in post-traumatic stress disorder or negative body image in eating disorders), and the dominant therapeutic treatments rely heavily on imagery techniques. Thus far, little is known about how this impacts mental healthcare experiences in individuals with aphantasia. In the current study, we used a mixed-methods (questionnaire, interview) approach to comprehensively investigate the effects of aphantasia on seeking diagnoses and treatments for mental illness. Quantitative analyses on the questionnaire data revealed that virtually all psychiatric disorders manifest with a lack of imagery-related symptomology in aphantasia compared to typical imagery controls; aphants report 'lack of awareness or understanding of aphantasia' as a common factor in missed- or misdiagnosis by mental health professionals, although the prevalence of missed and misdiagnoses are no different from typical imagery controls; and aphants are very likely to report that therapies involving mental imagery, especially visual imagery in CBT, are ineffective in their mental health treatment compared to controls. Two main themes were generated following qualitative analysis of interviews: Quest for Identity and Mental Health Journey. Feelings of being different, memory challenges, and self-discovery based on help-seeking contributed to the first theme. Aphants found different levels of success in their mental health journeys depending on whether they experienced anxiety and/or depression, neurodiversity conditions, or trauma and/or complex mental health conditions, with the latter group critically relying on professional empathy and understanding for positive outcomes. Together, these results point to a widespread impact of aphantasia on diverse aspects of mental healthcare.

Visual working memory content is commonly thought to be composed of a precise visual representation of stimulus information (e.g., color, shape). Nevertheless, previous research has shown that individuals represent this visual information in different formats, historically dichotomized into verbal and visual formats. With growing popular knowledge of aphantasia, or the absence of sensory mental imagery, recent studies have demonstrated that individuals with aphantasia perform similarly to individuals with typical imagery on visual working memory tasks. This suggest that the use of non-visual strategies may be sufficient to perform visual working memory tasks, which were previously thought to be strictly visual. To investigate the effects of different strategies on performance in a visual working memory task, we recruited individuals across the visual imagery spectrum and tested their ability to identify relatively small (3deg), medium (6deg), or large (10deg) changes in the degree of orientation of gratings held in working memory. Subsequently, participants indicated the extent to which they used five different strategies: visual, spatial, verbal, semantic, and sensorimotor. Results revealed that individuals with aphantasia and typical imagery performed similarly to each other across all task difficulty levels. Individuals with typical imagery dominantly used visuospatial strategies, but surprisingly, individuals with aphantasia overwhelmingly preferred the use of non-visual spatial and sensorimotor strategies over verbal strategies. These results suggest that non-visual spatial and sensorimotor strategies can be adopted in visual working memory tasks and these strategies are equally effective as visuospatial strategies. This calls for a rethinking of the visual versus verbal dichotomy, and provides evidence for the use of other non-visual mental representations in working memory tasks.

Rhythmic visual flicker is known to elicit pseudo-hallucinations, making it an up-and-coming method to investigate anomalous perceptual experiences without pharmaceutical intervention. Ganzflicker is a full-screen visual flicker that can be experienced online. In the first exploratory Ganzflicker paper (N = 204), we investigated whether people with different self-reported visual mental imagery abilities report different visual experiences in the Ganzflicker. Results showed that people with no-to-low imagery (aphantasia distribution) were much less likely to experience complex and vivid pseudo-hallucinations compared to people with moderate-to-vivid imagery (imagery distribution). In this follow-up, I collected data from 6664 individuals from around the world, replicated the main results of the previous study, and additionally found that people from the imagery distribution report more frequent pseudo-hallucinations for a longer duration than people from the aphantasia distribution. I also conducted new analyses across individual imagery vividness ratings. This revealed a dramatic increase in susceptibility to pseudo-hallucinations from reports of 'no imagery' to 'low imagery vividness' within the aphantasia distribution. There is a positive linear relationship between imagery vividness and pseudo-hallucination vividness, whereas the relationship between imagery vividness and pseudo-hallucination complexity is categorical, as indicated by a jump in the likelihood to experience complex pseudo-hallucinations from the aphantasia distribution to the imagery distribution with no evidence for within-distribution variations. Finally, word cloud analyses of written descriptions of Ganzflicker experiences revealed unique language used by individuals from each distribution. In sum, Ganzflicker is an accessible, efficient, and effective method of investigating multiple aspects of anomalous perceptual experiences in people with different mental imagery abilities.

With the recent identification of aphantasia (no mind's eye) and hyperphantasia (extremely vivid mind's eye), mental imagery 'extremes' research has become increasingly popular. One major methodological issue is there is no defined criteria for typical and atypical self-reports of imagery, and while the Vividness of Visual Imagery Questionnaire (VVIQ) is widely used, it was not developed to classify imagery extremes. In the current study, we established a short interview method to categorize individuals by visual imagery ability, and compared this to VVIQ cut-offs (=75: hyperphantasia). Interestingly, many individuals who thought they had aphantasia (based on VVIQ scores) experienced imagery. The interview method revealed 4 distinct imagery categories: aphantasia, hypophantasia (low imagery), typical imagery, and hyperphantasia. We recruited 15 participants from each category (internet volunteers and students) to perform a test battery measuring imagery projection ability (prophantasia), visual working memory (VWM) precision and strategies, imagery priming, pseudo-hallucination prevalence and features (Ganzflicker), and imagery interference with memory. Only prophantasia differences, VWM strategy differences, and pseudo-hallucination complexity differences were diagnostic of extreme imagery. 78% of hyperphantasics and 33% of those with typical imagery have prophantasia, whereas aphantasics and hypophantasics do not experience prophantasia. Differences in dominant VWM strategy (visual versus spatial) are driven by individuals with hyperphantasia (visual) and aphantasia (spatial), but not typical imagery or hypophantasia (who use visuospatial strategies). Delving into Ganzflicker, aphantasics do not experience complex pseudo-hallucinations, whereas >50% of hyperphantasics do, and those with typical imagery and hypophantasia share a similar (low) likelihood of having complex experiences. If individuals are grouped based on VVIQ cut-off convention, these nuances in results are overlooked. Bayesian correlations and contingency tables tests corroborated these differences. It is therefore important to recruit a wide range of imagery abilities for extreme imagery research, and distinguish low imagery (hypophantasia) from no imagery (aphantasia).

An imbalance between top-down and bottom-up processing on perception (specifically, over-reliance on top-down processing) can lead to anomalous perception, such as illusions. One factor that may be involved in anomalous perception is visual mental imagery, which is the experience of 'seeing' with the mind's eye. There are vast individual differences in self-reported imagery vividness, and more vivid imagery is linked to a more sensory-like experience. We, therefore, hypothesized that susceptibility to anomalous perception is linked to individual imagery vividness. To investigate this, we adopted a paradigm that is known to elicit the perception of faces in pure visual noise (pareidolia). In four experiments, we explored how imagery vividness contributes to this experience under different response instructions and environments. We found strong evidence that people with more vivid imagery were more likely to see faces in the noise, although removing suggestive instructions weakened this relationship. Analyses from the first two experiments led us to explore confidence as another factor in pareidolia proneness. We, therefore, modulated environment noise and added a confidence rating in a novel design. We found strong evidence that pareidolia proneness is correlated with uncertainty about real percepts. Decreasing perceptual ambiguity abolished the relationship between pareidolia proneness and both imagery vividness and confidence. The results cannot be explained by incidental face-like patterns in the noise, individual variations in response bias, perceptual sensitivity, subjective perceptual thresholds, viewing distance, testing environments, motivation, gender, or prosopagnosia. This indicates a critical role of mental imagery vividness and perceptual uncertainty in anomalous perceptual experience.

There are considerable individual differences in visual mental imagery ability across the general population, including a 'blind mind's eye', or aphantasia. Recent studies have shown that imagery is linked to differences in perception in the healthy population, and clinical work has found a connection between imagery and hallucinatory experiences in neurological disorders. However, whether imagery ability is associated with anomalous perception - including hallucinations - in the general population remains unclear. In the current study, we explored the relationship between imagery ability and the anomalous perception of pseudo-hallucinations (PH) using rhythmic flicker stimulation ('Ganzflicker'). Specifically, we investigated whether the ability to generate voluntary imagery is associated with susceptibility to flicker-induced PH. We additionally explored individual differences in observed features of PH. We recruited a sample of people with aphantasia (aphants) and imagery (imagers) to view a constant red-and-black flicker for approximately 10min. We found that imagers were more susceptible to PH, and saw more complex and vivid PH, compared to aphants. This study provides the first evidence that the ability to generate visual imagery increases the likelihood of experiencing complex and vivid anomalous percepts.

Visual search can be aided by a search template: a preparatory representation of relevant target features. But which features are relevant in complex, real-world category search? Previous research suggests that this template must be flexible to account for variations in naturalistic stimulus properties such as size and occlusion, and that shapes of diagnostic parts of objects are a likely candidate. Here, in three experiments, we systematically evaluated the contribution of diagnostic object parts and whole object shape to the category-level search template. Our hypothesis was that features that better match the active search template will capture attention during search more strongly than partially-matching features. Results showed that while whole objects captured attention reliably and globally across the visual field, diagnostic parts failed to do so in all three experiments. This suggests that whole object shape is a necessary component of the category-level search template.

Cognitive control can involve proactive (preparatory) and reactive (corrective) mechanisms. Using a gaze-contingent eye tracking paradigm combined with fMRI, we investigated the involvement of these different modes of control and their underlying neural networks, when switching between different targets in multiple-target search. Participants simultaneously searched for two possible targets presented among distractors, and selected one of them. In one condition, only one of the targets was available in each display, so that the choice was imposed, and reactive control would be required. In the other condition, both targets were present, giving observers free choice over target selection, and allowing for proactive control. Switch costs emerged only when targets were imposed and not when target selection was free. We found differential levels of activity in the frontoparietal control network depending on whether target switches were free or imposed. Furthermore, we observed core regions of the default mode network to be active during target repetitions, indicating reduced control on these trials. Free and imposed switches jointly activated parietal and posterior frontal cortices, while free switches additionally activated anterior frontal cortices. These findings highlight unique contributions of proactive and reactive control during visual search.

Is a distractor represented distinctly like a target in visual working memory (VWM) with an additional 'tag' that this should be rejected once it has been identified during search? Or is there no distinct representation, perhaps even suppression, of the distractor feature in sensory brain areas during the preparatory period? The current study is the first to provide evidence that visual features of anticipated distractors are not represented more distinctly than irrelevant features (that will not appear in the search display) in early visual cortex (EVC), supporting the hypothesis that a template for rejection is functionally different from a target template.

A 'target template', specifying target features, is thought to benefit visual search performance. Setting up a 'negative template', specifying distractor features, should improve distractor inhibition and also benefit target detection. In the current fMRI study, subjects were required to search for a target among distractors enclosed in coloured circles. Before search, one of three colour cues appeared: a positive cue indicating the target will appear in the same colour, a negative cue indicating only distractors will appear in the same colour, or a neutral cue indicating that the same colour will not appear in the search display. fMRI results revealed down-regulation of neural processing in large parts of visual cortex following negative compared to positive cues. We further found a general attention inhibition mechanism in SPL/precuneus for neutral cues compared to positive and negative cues. These results suggest a cortical distinction between target templates, negative templates, and task-irrelevant distractor inhibition.

Visually perceiving a stimulus activates a pictorial representation of that item in the brain, but how pictorial is the representation of a stimulus in the absence of visual stimulation? Here I address this question with a review of the literatures on visual imagery (VI), visual working memory (VWM), and visual preparatory templates, all of which require activating visual information in the absence of sensory stimulation. These processes have historically been studied separately, but I propose that they can provide complimentary evidence for the pictorial nature of their contents. One major challenge in studying the contents of visual representations is the discrepant findings concerning the extent of overlap (both cortical and behavioral) between externally and internally sourced visual representations. I argue that these discrepancies may in large part be due to individual differences in VI vividness and precision, the specific representative abilities required to perform a task, appropriateness of visual preparatory strategies, visual cortex anatomy, and level of expertise with a particular object category. Individual differences in visual representative abilities greatly impact task performance and may influence the likelihood of experiences such as intrusive VI and hallucinations, but research still predominantly focuses on uniformities in visual experience across individuals. In this paper I review the evidence for the pictorial content of visual representations activated for VI, VWM, and preparatory templates, and highlight the importance of accounting for various individual differences in conducting research on this topic.

Little is known about the cortical regions involved in representing task-related content in preparation for visual task performance. Here we used representational similarity analysis (RSA) to investigate the BOLD response pattern similarity between task relevant and task irrelevant feature dimensions during conjunction viewing and target template maintenance prior to visual search. Subjects were cued to search for a spatial frequency (SF) or orientation of a Gabor grating and we measured BOLD signal during cue and delay periods before the onset of a search display. RSA of delay period activity revealed that widespread regions in frontal, posterior parietal, and occipitotemporal cortices showed general representational differences between task relevant and task irrelevant dimensions (e.g., orientation vs. SF). In contrast, RSA of cue period activity revealed sensory-related representational differences between cue images (regardless of task) at the occipital pole and additionally in the frontal pole. Our data show that task and sensory information are represented differently during viewing and during target template maintenance, and that task relevance modulates the representation of visual information across the cortex.

Access to visual awareness for human faces is strongly influenced by spatial orientation: Under continuous flash suppression (CFS), upright faces break into awareness more quickly than inverted faces. This effect of inversion for faces is larger than for a wide range of other animate and inanimate objects. Here we asked whether this apparently specific sensitivity to upright faces reflects face-specific detection mechanisms or whether it reflects perceptual expertise more generally. We tested car experts who varied in their degree of car and face expertise and measured the time upright and inverted faces, cars, and chairs needed to overcome CFS and break into awareness. Results showed that greater car expertise was correlated with larger car inversion effects under CFS. A similar relation between better discrimination performance and larger CFS inversion effects was found for faces. CFS inversion effects are thus modulated by perceptual expertise for both faces and cars. These results demonstrate that inversion effects in conscious access are not unique to faces but similarly exist for other objects of expertise. More generally, we interpret these findings as suggesting that access to awareness and exemplar-level discrimination rely on partially shared perceptual mechanisms.

There is much debate about how detection, categorization, and within-category identification relate to one another during object recognition. Whether these tasks rely on partially shared perceptual mechanisms may be determined by testing whether training on one of these tasks facilitates performance on another. In the present study we asked whether expertise in discriminating objects improves the detection of these objects in naturalistic scenes. Self-proclaimed car experts (N = 34) performed a car discrimination task to establish their level of expertise, followed by a visual search task where they were asked to detect cars and people in hundreds of photographs of natural scenes. Results revealed that expertise in discriminating cars was strongly correlated with car detection accuracy. This effect was specific to objects of expertise, as there was no influence of car expertise on person detection. These results indicate a close link between object discrimination and object detection performance, which we interpret as reflecting partially shared perceptual mechanisms and neural representations underlying these tasks: the increased sensitivity of the visual system for objects of expertise as a result of extensive discrimination training may benefit both the discrimination and the detection of these objects. Alternative interpretations are also discussed.

Theories of visual search postulate that the selection of targets amongst distractors involves matching visual input to a top-down attentional template. Previous work has provided evidence that feature-based attentional templates affect visual processing globally across the visual field. In the present study, we asked whether more naturalistic, category-level attentional templates also modulate visual processing in a spatially global and obligatory way. Subjects were cued to detect people or cars in a diverse set of photographs of real-world scenes. On a subset of trials, silhouettes of people and cars appeared in search-irrelevant locations that subjects were instructed to ignore, and subjects were required to respond to the location of a subsequent dot probe. In three experiments, results showed a consistency effect on dot-probe trials: dot probes were detected faster when they appeared in the location of the cued category compared with the non-cued category, indicating attentional capture by template-matching stimuli. Experiments 1 and 2 showed that this capture was involuntary: consistency effects persisted under conditions in which attending to silhouettes of the cued category was detrimental to performance. Experiment 3 tested whether these effects could be attributed to non-attentional effects related to the processing of the category cues. Results showed a consistency effect when subjects searched for category exemplars but not when they searched for objects semantically related to the cued category. Together, these results indicate that attentional templates for familiar object categories affect visual processing across the visual field, leading to involuntary attentional capture by template-matching stimuli.

Theories of visual selective attention propose that top-down preparatory attention signals mediate the selection of task-relevant information in cluttered scenes. Neuroimaging and electrophysiology studies have provided correlative evidence for this hypothesis, finding increased activity in target-selective neural populations in visual cortex in the period between a search cue and target onset. In this study, we used online TMS to test whether preparatory neural activity in visual cortex is causally involved in naturalistic object detection. In two experiments, participants detected the presence of object categories (cars, people) in a diverse set of photographs of real-world scenes. TMS was applied over a region in posterior temporal cortex identified by fMRI as carrying category-specific preparatory activity patterns. Results showed that TMS applied over posterior temporal cortex before scene onset (-200 and -100 msec) impaired the detection of object categories in subsequently presented scenes, relative to vertex and early visual cortex stimulation. This effect was specific to category level detection and was related to the type of attentional template participants adopted, with the strongest effects observed in participants adopting category level templates. These results provide evidence for a causal role of preparatory attention in mediating the detection of objects in cluttered daily-life environments.

The work presented here is at the meeting point of two branches of visual search research, one of which focuses on the proposition that visual search is guided by preparatory internal representations of targets (i.e., search templates: e.g., Bravo & Farid, 2009; 2012; Castelhano & Heaven, 2010; Duncan & Humphreys, 1989; Malcolm & Henderson 2009; 2010; Schmidt & Zelinsky, 2009; Vickery, King, & Jiang, 2005; Wolfe, 2007; Wolfe, Cave, & Franzel, 1989; Yang & Zelinsky, 2009), and the other of which focuses on investigating target detection in naturalistic search environments (e.g., Delorme, Richard, & Fabre-Thorpe, 2010; Delorme, Rousselet, Macé, & Fabre-Thorpe, 2004; Li, VanRullen, Koch, & Perona, 2002; Peelen, Fei-Fei, & Kastner, 2009; Peelen & Kastner, 2011; Thorpe, Fize, & Marlot, 1996; VanRullen & Thorpe, 2001). The search template for objects presented in naturalistic scenes is relatively unknown in terms of its content and characteristics, neural underpinnings, and individual differences in its representation. This thesis explores these topics in depth using behavioral and neurostimulation methods in four experimental chapters.

Visual search involves the matching of visual input to a 'search template', an internal representation of task-relevant information. The present study investigated the contents of the search template during visual search for object categories in natural scenes, for which low-level features do not reliably distinguish targets from nontargets. Subjects were cued to detect people or cars in diverse photographs of real-world scenes. On a subset of trials, the cue was followed by task-irrelevant stimuli instead of scenes, directly followed by a dot that subjects were instructed to detect. We hypothesized that stimuli that matched the active search template would capture attention, resulting in faster detection of the dot when presented at the location of a template-matching stimulus. Results revealed that silhouettes of cars and people captured attention irrespective of their orientation (0, 90, or 180). Interestingly, strong capture was observed for silhouettes of category-diagnostic object parts, such as the wheel of a car. Finally, attentional capture was also observed for silhouettes presented at locations that were irrelevant to the search task. Together, these results indicate that search for familiar object categories in real-world scenes is mediated by spatially global search templates that consist of view-invariant shape representations of category-diagnostic object parts.