58 research outputs found

    Attending to multiple objects : the dynamics of attentional control in multi-target stimulus arrays

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    In this thesis, the cognitive and neural mechanisms of attentional control are examined, with a specific focus on investigating the temporal dynamics of these mechanisms in scenarios where multiple objects must be attended. Event-related potential (ERP) measures are used to track the continuous time course of visual responses in the brain, and the N2pc component is employed as a marker for the attentional selection of target objects. Two broad lines of research are presented. The first line examines the attentional selection of multiple rapidly presented instances of a single target object defined by varying properties, revealing very rapid and flexible brain responses triggered independently to the appearance of each target. The second line investigates the speed and qualitative nature of strictly serial attention shifts when they are guided by stimulus features or only by location information, revealing the availability of different attentional control mechanisms for these different shifts. In the context of these findings, this thesis attempts to improve the cognitive and neural understanding of how attentional control operates. The attentional template, a working memory representation of currently task-relevant properties, is proposed to flexibly allow for the preparatory enhancement of the activity of neurons that respond to these target-defining properties, allowing for the independent allocation of attention to each instance of a target in real-time. The properties that can be maintained by the attentional template are not restricted to being visual in nature, but can consist of more complex semantic and category-related information. Importantly, the experiments of this thesis demonstrate that attentional control is a highly flexible cognitive mechanism that can be rapidly altered on the basis of current goals, and can rapidly influence the processing of incoming visual informatio

    Attending to multiple objects : the dynamics of attentional control in multi-target stimulus arrays

    Get PDF
    In this thesis, the cognitive and neural mechanisms of attentional control are examined, with a specific focus on investigating the temporal dynamics of these mechanisms in scenarios where multiple objects must be attended. Event-related potential (ERP) measures are used to track the continuous time course of visual responses in the brain, and the N2pc component is employed as a marker for the attentional selection of target objects. Two broad lines of research are presented. The first line examines the attentional selection of multiple rapidly presented instances of a single target object defined by varying properties, revealing very rapid and flexible brain responses triggered independently to the appearance of each target. The second line investigates the speed and qualitative nature of strictly serial attention shifts when they are guided by stimulus features or only by location information, revealing the availability of different attentional control mechanisms for these different shifts. In the context of these findings, this thesis attempts to improve the cognitive and neural understanding of how attentional control operates. The attentional template, a working memory representation of currently task-relevant properties, is proposed to flexibly allow for the preparatory enhancement of the activity of neurons that respond to these target-defining properties, allowing for the independent allocation of attention to each instance of a target in real-time. The properties that can be maintained by the attentional template are not restricted to being visual in nature, but can consist of more complex semantic and category-related information. Importantly, the experiments of this thesis demonstrate that attentional control is a highly flexible cognitive mechanism that can be rapidly altered on the basis of current goals, and can rapidly influence the processing of incoming visual informatio

    The spatially global control of attentional target selection in visual search

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    Glyn Humphreys and his co-workers have made numerous important theoretical and empirical contributions to research on visual search. They have introduced the concept of attentional target templates and investigated the nature of these templates and how they are involved in the control of search performance. In the experiments reported here, we investigated whether feature-specific search template for particular colours can guide target selection independently for different regions of visual space. We employed behavioural and electrophysiological markers of attentional selection in tasks with targets defined by specific colour/location combinations. In Experiment 1, participants searched for pairs of colour targets in a particular spatial configuration (e.g., red in the upper and blue in the lower visual field). In Experiment 2, they searched for single colour-defined targets at specific locations (e.g., red on the left or blue on the right). Target displays were preceded by non-informative cues containing target-colour items at task-set matching or mismatching locations. Contingent attentional capture was observed only for matching cues. However, both matching and mismatching cues elicited identical N2pc components, indicating equivalent attentional capture. This shows that the rapid deployment of attention towards target features is spatially non-selective, and that selection of colour/location combinations occurs at later post-perceptual stages. This was further corroborated in search displays where targets were accompanied by target-colour distractors at nonmatching locations. Here, spatial biases towards the target emerged late and were strongly attenuated relative to displays without such distractors. These results demonstrate that attentional templates for target-defining features operate in a spatially-global fashion. Feature-based guidance of visual search cannot be restricted to particular locations even when this is required by the demands of an attentional selection task

    Distractor intrusions are the result of delayed attentional engagement: a new temporal variability account of attentional selectivity in dynamic visual tasks

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    When observers have to identify targets among distractors in a rapid serial visual presentation (RSVP) stream, distractor intrusion errors are frequent, demonstrating the difficulty of allocating attention to the right object at the right moment in time. However, the mechanisms responsible for such intrusion errors remain disputed. We propose a new attentional engagement account of selective visual processing in RSVP tasks. Engagement is triggered by the pre-attentive detection of target-defining features. Critically, the success versus failure of target identification is determined by the speed of such engagement processes on individual trials. To test this account, we measured electrophysiological markers of attentional engagement (N2pc components) in three experiments where observers had to report the identity of a target digit in one of two lateral RSVP streams. On most trials, the target was immediately followed by a digit distractor, resulting in many post-target distractor intrusions. Critically, N2pcs components measured on distractor intrusion trials were significantly delayed relative to trials with correct target reports. This was the case regardless of whether the target was defined by a shape cue or by its colour, and even when the location of shape-defined targets was known in advance. These findings show that distractor intrusions are the result of delayed attentional engagement. They demonstrate that temporal variability in attentional selectivity across trials can strongly affect visual awareness and perceptual reports. Our temporal variability account of attentional engagement offers a new framework for assessing the temporal dynamics of attention in visual object recognition

    EPS mid-career award 2014: the control of attention in visual search - cognitive and neural mechanisms

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    In visual search, observers try to find known target objects among distractors in visual scenes where the location of the targets is uncertain. This review article discusses the attentional processes that are active during search and their neural basis. Four successive phases of visual search are described. During the initial preparatory phase, a representation of the current search goal is activated. Once visual input has arrived, information about the presence of target-matching features is accumulated in parallel across the visual field (guidance). This information is then used to allocate spatial attention to particular objects (selection), before representations of selected objects are activated in visual working memory (recognition). These four phases of attentional control in visual search are characterized both at the cognitive level and at the neural implementation level. It will become clear that search is a continuous process that unfolds in real time. Selective attention in visual search is described as the gradual emergence of spatially specific and temporally sustained biases for representations of task-relevant visual objects in cortical maps

    Humans can efficiently look for but not select multiple visual objects

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    The human brain recurrently prioritizes task-relevant over task-irrelevant visual information. A central question is whether multiple objects can be prioritized simultaneously. To answer this, we let observers search for two colored targets among distractors. Crucially, we independently varied the number of target colors that observers anticipated, and the number of target colors actually used to distinguish the targets in the display. This enabled us to dissociate the preparation of selection mechanisms from the actual engagement of such mechanisms. Multivariate classification of electroencephalographic activity allowed us to track selection of each target separately across time. The results revealed only small neural and behavioral costs associated with preparing for selecting two objects, but substantial costs when engaging in selection. Further analyses suggest this cost is the consequence of neural competition resulting in limited parallel processing, rather than a serial bottleneck. The findings bridge diverging theoretical perspectives on capacity limitations of feature-based attention

    Space-based and feature-based attentional selection in perception and working memory

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    In order to manage the high amount of sensory input we experience, attention processes enable the selective prioritization of goal-relevant information over irrelevant distractions. Two fundamental ways in which this is accomplished is by focusing attention at particular locations in the environment (spatial attention) or by focusing on specific forms of information (feature-based attention). Despite many decades of research examining these mechanisms, however, they have been seldom directly compared particularly in relation to their underlying neural mechanisms. In this thesis, the neural correlates of spatial and feature-based attentional selection for perception and working memory maintenance processes are contrasted. Event-related potential (ERP) components from electroencephalography (EEG) recordings are used as markers of such processes. The N2pc component is used to measure lateralised attentional selection to targets defined by one or a combination of spatial locations and features in perceptual tasks, whilst the CDA component is used to measure the active maintenance of target objects/locations in working memory tasks. In total, this thesis contains three lines of investigation. The first line compares these ERP components for attentional selection to targets defined by spatial locations and features and reveals that in many contexts, spatial attention is processed similarly to featural attention with a few notable exceptions (Chapter 2). The second line of enquiry examines how spatial configural information affects feature-based attentional selection when it is a critical component for successful goal-directed search, revealing that such information can guide attentional selection for some feature dimensions (Chapter 3). Finally, the third line of enquiry compares how spatial and feature-based attention influences visual perceptual and post-perceptual working memory processes (Chapters 4 and 5). This investigation lead to the observations that spatial attentional templates are quicker to guide attention when there is no SOA between the cue and target display onset, and that the two types of attention have similar working memory capacity limitations These findings culminate to provide one of the first direct comparisons of the neural correlates of attention to spatially or featurally-defined information, thereby expanding the current understanding of how spatial/feature-based attention operates. By measuring real-time event-related responses during these task contexts, the present thesis highlights the independent nature of spatial and feature-based attention and their qualitative similarities, but also how they interact upon one another under some circumstances. The findings aid the literature by shedding light on the argument perceptual and post-perceptual processes involved in spatial attention are qualitatively different from featural attention processes

    The preparatory activation of guidance templates for visual search and of target templates in non-search tasks

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    Representations of task-relevant object attributes (attentional templates) control the adaptive selectivity of visual processing. Previous studies have demonstrated that templates involved in the guidance of attention during visual search are activated in a preparatory fashion prior to the arrival of visual search displays. The current study investigated whether such proactive mechanisms are also triggered in non-search tasks, where attentional templates do not mediate the guidance of attention towards targets amongst distractors but are still necessary for subsequent target recognition processes. Participants either searched for colour-defined targets among multiple distractors or performed two other non-search tasks where imperative stimuli appeared without competing distractors (a colour-based Go / NoGo task, and a shape discrimination task where target colour was constant and could thus be ignored). Preparatory activation of colour-selective templates was tracked by measuring N2pc components (markers of attention allocation) to task-irrelevant colour singleton probes flashed every 200 ms during the interval between target displays. As expected, N2pcs were triggered by target-coloured probes in the search task, indicating that a corresponding guidance template was triggered proactively. Critically, clear probe N2pcs were also observed in the Go / NoGo task, and even in the shape discrimination task in an attenuated fashion. These findings demonstrate that the preparatory activation of feature-selective attentional task settings is not uniquely associated with the guidance of visual search but is also present in other types of visual selection tasks where guidance is not required

    The Automaticity of Semantic Processing Revisited: Auditory Distraction by a Categorical Deviation

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    Automatic information processing has been and still is a debated topic. Traditionally, automatic processes are deemed to take place autonomously and independently of top-down cognitive control. For decades, the literature on reading has brought to the fore empirical phenomena such as Stroop and semantic priming effects that provide support for the assumption that semantic information can be accessed automatically. More recently, there has been growing evidence that semantic processing is in fact susceptible to higher level cognitive influences, suggesting that this form of processing is instead conditionally automatic. The purpose of the present study was to revisit this debate using a novel approach: the automatic access to the meaning of irrelevant auditory stimuli was tested through the assessment of their distractive power. More specifically, we aimed to examine whether a categorical change in the content of to-be-ignored auditory sequences composed of speech items that are personally non-significant to participants (e.g., a digit among letters) can disrupt an unrelated visual focal task. In seven experiments, we assessed this categorical deviation effect and its functional properties. We established that distraction by categorical deviation is non-contingent on the activated task set and appears resistant to topdown control manipulations. By suggesting not only that the semantic content of the irrelevant sound can be extracted preattentively, but also that such semantic activation is ineluctable during auditory distraction, these findings shed new light on the automatic nature of semantic processing

    Perceptual competition between targets and distractors determines working memory access and produces intrusion errors in RSVP tasks

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    When a target and a distractor that share the same response dimension appear in rapid succession, participants often erroneously report the distractor instead of the target. Using behavioral and electrophysiological measures, we examined whether these intrusion errors occur because the target is often not encoded in working memory (WM) or are generated at later post-encoding stages. In four experiments, participants either provided two guesses about the target’s identity, or had to select the target among items that did not include the potential intruder. Results showed that the target did not gain access to WM on a substantial number of trials where the distractor was encoded. This was also confirmed with an electrophysiological marker of WM storage (CDA component). These findings are inconsistent with post-encoding accounts of distractor intrusions, which postulate that competitive interactions within WM impair awareness of the target, the precision of target representations, or result in the target being dropped from WM. They show instead that target-distractor competition already operates at earlier perceptual stages, and reduces the likelihood that the target gains access to WM. We provide a theoretical framework to explain these findings and how they challenge contemporary models of temporal attention
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