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This thesis is a systematic investigation of preparatory reorienting of task-relevant spatial attention. Task switching experiments typically report a performance overhead when the current task is different to that performed on the previous trial relative to when the task repeats. This ‘switch cost’ tends to reduce as participants are given more time to prepare (consistent with an active reconfiguration process) but a ‘residual’ switch cost usually remains even at very long preparation intervals (often interpreted as evidence of carryover of response selection parameters from the previous trial which are immune to preparation). Although attentional selection of perceptual attributes is often considered to be part of task-set and is included in some models of task-set control, little research has investigated the dynamics of this component in detail. Over a series of seven experiments in which tasks were consistently mapped to screen locations, eye-tracking was used to systematically investigate task-relevant spatial selection of perceptual attributes during the preparation interval and early after stimulus onset. Experiment 1 revealed a switch-induced delay in appropriate attention orientation and a measure of ‘attentional inertia’ which could not be explained by task-independent re-orienting to locations or low-level oculomotor phenomena but were markers of task-relevant spatial selection. Experiment 2 provided a sensitive measure of both of these attentional handicaps and demonstrated that they both contribute to the switch cost (including its residual component). Although attentional inertia reduced with preparation, both handicaps were present at the longest preparation intervals. The constancy of the delay in attending to the relevant attribute reflects the effort to re-allocate attention, rather than peculiarities of spatial orienting when the cue and stimulus are presented near-simultaneously on trials with short cue-stimulus intervals. The presence of attentional inertia in blocks with long preparation intervals suggested some component of inertia immune to preparation (though see Experiments 5 and 6 below). Experiments 3 and 4 investigated the extent to which attentional selection can be decoupled from other task-set components. Cues which explicitly provided location information reduced (or eliminated) the attentional effects found in Experiment 2 indicating that attentional selection can be decoupled from other task-set components. However, Experiment 3 found that the ‘natural’ state is for attentional selection to be coupled at least to a degree (and accessed via) task-set. Experiment 5 combined eye-tracking with ERPs to investigate the relative order of attentional selection and reconfiguration of other task-set components. A well-documented ERP marker of task-set preparation always followed onset of the first fixation on the currently relevant stimulus element indicating that (at least some) task-set components are reconfigured in a serial order with spatial selection preceding other components (e.g., loading of S-R rules or other parameters into working memory). Experiments 6 and 7 investigated the nature of attentional inertia. In Experiment 6 participants were given ultimate control over the duration of the preparation interval which eliminated attentional inertia (at least as indexed by preferential fixation of the previously relevant element on switch trials). In Experiment 7 the stimulus comprised three items which were from perceptually distinct classes (digits, letters, objects) to investigate whether the presence of task-specific features would elicit extra attentional inertia and whether early spatial selection was effective enough to block the processing of task-irrelevant features once the stimulus was presented. Although there was some evidence that the previously relevant stimulus element ‘captured’ attention, this tendency was modest in the fixations and absent in performance measures (response congruence effects).

The question of whether words can be identified without spatial attention has been a topic of considerable interest over the last five and a half decades, but the literature has yielded mixed conclusions. Some studies show substantial effects of distractor words which are argued to appear outside of spatial attention, whereas a small number of other studies show no evidence of such effects. I argue that at least some of the discrepant results can be understood in terms of failures to optimally focus attention at the cued location. The present experiments manipulated the proportion of valid trials to encourage distributed (Experiments 1 and 3) or focused (Experiments 2 and 4) spatial attention. Participants read aloud a target word, and the impact of a simultaneously presented distractor word was assessed. Semantic and repetition distractor effects were present when conditions promoted distributed spatial attention, but distractor effects were absent when conditions promoted focused spatial attention. These data are consistent with the proposal that (1) the allocation of spatial attention across displays is strongly context-dependent and (2) spatial attention is a necessary preliminary to lexical/semantic processing.

The present thesis investigated how words are processed within the context of visual search. Both explicit and implicit measures were used to assess whether spatial attention is a prerequisite for words to undergo processing. In the explicit search task, subjects searched a display and indicated whether a word was present or absent among nonword distractors. In the implicit task, priming was employed to index word processing. Subjects viewed the same search displays that were used in the explicit task, however, the displays were presented briefly and were followed by a single target letter string to which subjects performed a lexical decision. In Experiments 3 through 6, in which the target was always presented at fixation, no priming was evident. In Experiments 7 and 8 when the location of the target moved from trial to trial, priming was observed. It is argued that attentional resources are narrowly allocated to a location in visual space when target location is certain but diffusely allocated when target location is uncertain. Furthermore, processing only occurs for words that fall within the suffusion of this strategically pliable attentional beam. The results are also interpreted within the domains of perceptual cuing and attentional capture.

The question of whether words can be identified without spatial attention has been a topic of considerable interest over the last five and a half decades, but the literature has yielded mixed conclusions. Some studies show substantial effects of distractor words which are argued to appear outside of spatial attention, whereas a small number of other studies show no evidence of such effects. I argue that at least some of the discrepant results can be understood in terms of failures to optimally focus attention at the cued location. The present experiments manipulated the proportion of valid trials to encourage distributed (Experiments 1 and 3) or focused (Experiments 2 and 4) spatial attention. Participants read aloud a target word, and the impact of a simultaneously presented distractor word was assessed. Semantic and repetition distractor effects were present when conditions promoted distributed spatial attention, but distractor effects were absent when conditions promoted focused spatial attention. These data are consistent with the proposal that (1) the allocation of spatial attention across displays is strongly context-dependent and (2) spatial attention is a necessary preliminary to lexical/semantic processing.

The present thesis investigated how words are processed within the context of visual search. Both explicit and implicit measures were used to assess whether spatial attention is a prerequisite for words to undergo processing. In the explicit search task, subjects searched a display and indicated whether a word was present or absent among nonword distractors. In the implicit task, priming was employed to index word processing. Subjects viewed the same search displays that were used in the explicit task, however, the displays were presented briefly and were followed by a single target letter string to which subjects performed a lexical decision. In Experiments 3 through 6, in which the target was always presented at fixation, no priming was evident. In Experiments 7 and 8 when the location of the target moved from trial to trial, priming was observed. It is argued that attentional resources are narrowly allocated to a location in visual space when target location is certain but diffusely allocated when target location is uncertain. Furthermore, processing only occurs for words that fall within the suffusion of this strategically pliable attentional beam. The results are also interpreted within the domains of perceptual cuing and attentional capture.

Synaesthesia is a fascinating condition whereby individuals report extraordinary experiences when presented with ordinary stimuli. In this thesis, we examine an individual (L) who experiences time units (i.e., months and hours) and numbers as occupying specific spatial locations (e.g., January is 30º to the left). This type of spatial-form synaesthesia has been recently investigated by Smilek et al. (2007), demonstrating that synaesthetic time-space associations are highly consistent, occur regardless of intention, and can direct spatial attention. We extended this work in Chapter 2 by showing that for L, her time-space vantage point changed depending on whether the time units were seen or heard. For example, when L saw the word JANUARY, she reported experiencing January on her left side, however when she heard the word "January" she experienced the month on her right side. In this thesis, we validated L’s subjective reports using a spatial cueing task. The names of months were centrally presented followed by targets on the left or right. L was faster at detecting targets in validly cued locations relative to invalidly cued locations both for visually presented cues (January orients attention to the left) and for aurally presented cues (January orients attention to the right). We replicated these vantage-point dependent cueing effects also using hours of day. In Chapter 3, we further explored whether synaesthetic number forms could bias spatial attention using a spatial cueing and SNARC-type task. Two synaesthetes (L and B) both described experiencing the numbers 1 through 10 running vertically from bottom to top. Both experiments confirmed their synaesthetic number forms, such that when making odd-even judgments for the numbers 1, 2, 8, and 9, they showed SNARC-compatibility effects for up-down movements (aligned with their number form), but not left-right (misaligned) movements. Likewise in the spatial cueing task, both synaesthetes showed significantly faster response times to detect targets on the bottom of the display if preceded by a low number (1,2) and the top of the display if preceded by a high number (8,9), whereas they showed no cueing effects when targets appeared on the left or right (misaligned with their number forms). Both synaesthetes were, however, reliably faster to detect left targets following the presentation of numbers 10, and 11, and right targets following numbers 19 and 20 (running from left to right). Hence, we demonstrated that cueing and SNARC tasks could be used to empirically verify synaesthetic number forms. Moreover, we showed that numbers could direct spatial attention to idiosyncratic locations similar to time-units, replicating and extending our findings from Chapter 2. Lastly, Chapter 4 was aimed to explore the automaticity and involuntary nature of L’s number-forms. We continued to use the spatial cueing task and sought to eliminate any influence of strategy on L’s performance by: (1) shortening the interval between the cue and target onset to only 150 ms and (2) having the targets only fall in synaesthetically cued locations on 14.2% of trials. As a result, these manipulations should eliminate any cuing effects if L’s performance was attributable to intentionally using the cue to predict target location. However, our findings still showed an attentional bias consistent with L’s synaesthesia. We attributed L’s resilient cueing effects to the automaticity of her number-forms, thus demonstrating one of the hallmark attributes of synaesthesia. Overall, this series of studies convincingly demonstrated the reality of time-space and number-form synaesthesia and Chapter 5 concludes by discussing how this work has significantly contributed to the synaesthesia literature and to the study of perception overall.

Synaesthesia is a fascinating condition whereby individuals report extraordinary experiences when presented with ordinary stimuli. In this thesis, we examine an individual (L) who experiences time units (i.e., months and hours) and numbers as occupying specific spatial locations (e.g., January is 30º to the left). This type of spatial-form synaesthesia has been recently investigated by Smilek et al. (2007), demonstrating that synaesthetic time-space associations are highly consistent, occur regardless of intention, and can direct spatial attention. We extended this work in Chapter 2 by showing that for L, her time-space vantage point changed depending on whether the time units were seen or heard. For example, when L saw the word JANUARY, she reported experiencing January on her left side, however when she heard the word "January" she experienced the month on her right side. In this thesis, we validated L’s subjective reports using a spatial cueing task. The names of months were centrally presented followed by targets on the left or right. L was faster at detecting targets in validly cued locations relative to invalidly cued locations both for visually presented cues (January orients attention to the left) and for aurally presented cues (January orients attention to the right). We replicated these vantage-point dependent cueing effects also using hours of day. In Chapter 3, we further explored whether synaesthetic number forms could bias spatial attention using a spatial cueing and SNARC-type task. Two synaesthetes (L and B) both described experiencing the numbers 1 through 10 running vertically from bottom to top. Both experiments confirmed their synaesthetic number forms, such that when making odd-even judgments for the numbers 1, 2, 8, and 9, they showed SNARC-compatibility effects for up-down movements (aligned with their number form), but not left-right (misaligned) movements. Likewise in the spatial cueing task, both synaesthetes showed significantly faster response times to detect targets on the bottom of the display if preceded by a low number (1,2) and the top of the display if preceded by a high number (8,9), whereas they showed no cueing effects when targets appeared on the left or right (misaligned with their number forms). Both synaesthetes were, however, reliably faster to detect left targets following the presentation of numbers 10, and 11, and right targets following numbers 19 and 20 (running from left to right). Hence, we demonstrated that cueing and SNARC tasks could be used to empirically verify synaesthetic number forms. Moreover, we showed that numbers could direct spatial attention to idiosyncratic locations similar to time-units, replicating and extending our findings from Chapter 2. Lastly, Chapter 4 was aimed to explore the automaticity and involuntary nature of L’s number-forms. We continued to use the spatial cueing task and sought to eliminate any influence of strategy on L’s performance by: (1) shortening the interval between the cue and target onset to only 150 ms and (2) having the targets only fall in synaesthetically cued locations on 14.2% of trials. As a result, these manipulations should eliminate any cuing effects if L’s performance was attributable to intentionally using the cue to predict target location. However, our findings still showed an attentional bias consistent with L’s synaesthesia. We attributed L’s resilient cueing effects to the automaticity of her number-forms, thus demonstrating one of the hallmark attributes of synaesthesia. Overall, this series of studies convincingly demonstrated the reality of time-space and number-form synaesthesia and Chapter 5 concludes by discussing how this work has significantly contributed to the synaesthesia literature and to the study of perception overall.

In everyday settings, spatial attention helps listeners isolate and understand individual sound sources. However, the neural mechanisms of auditory spatial attention (ASpA) are only partially understood. This thesis uses within-subject analysis of functional magnetic resonance imaging (fMRI) data to address fundamental questions regarding cortical mechanisms supporting ASpA by applying novel multi-voxel pattern analysis (MVPA) and resting-state functional connectivity (rsFC) approaches. A series of fMRI studies of ASpA were conducted in which subjects performed a one-back task in which they attended to one of two spatially separated streams. Attention modulated blood oxygenation level-dependent (BOLD) activity in multiple areas in the prefrontal, temporal, and parietal cortex, including non-visuotopic intraparietal sulcus (IPS), but not the visuotopic maps in IPS. No spatial bias was detected in any cortical area using standard univariate analysis; however, MVPA revealed that activation patterns in a number of areas, including the auditory cortex, predicted the attended direction. Furthermore, we explored how cognitive task demands and the sensory modality of the inputs influenced activity with a visual one-back task and a visual multiple object tracking (MOT) task. Activity from the visual and auditory one-back tasks overlapped along the fundus of IPS and lateral prefrontal cortex (lPFC). However, there was minimal overlap of activity in the lPFC between the visual MOT task and the two one-back tasks. Finally, we endeavored to identify visual and auditory networks using rsFC. We identified a dorsal visual attention network reliably within individual subjects using visuotopic seeds. Using auditory seeds, we found a prefrontal area nested between segments of the dorsal visual attention network. These findings mark fundamental progress towards elucidating the cortical network controlling ASpA. Our results suggest that similar lPFC structures support both ASpA and its visual counterpart during a spatial one-back task, but that ASpA does not drive visuotopic IPS in the parietal cortex. Furthermore, rsFC reveals that visual and auditory seed regions are functionally connected with non-overlapping lPFC regions, possibly reflecting spatial and temporal cognitive processing biases, respectively. While we find no evidence for a spatiotopic map, the auditory cortex is sensitive to direction of attention in its patterns of activation.

This thesis encompasses five chapters. Chapter 1 details the background literature for how the inhibition of return effect manifests in 3D environments. Additional summary is provided about how positioning cues and targets within the boundary of the same objects affects the spread of IOR when compared to when attention is cued in empty space. Finally, the literature review also provides a background for whether the IOR effect is affected by a viewer-centered or world-centered frame of reference. Chapters 2-4 are empirical chapters. Chapter 2 observes a depth-specific IOR effect in a 3D composed of pictorial depth cues. Chapter 3 suggests that this depth-specificity can only occur when cues and targets are positioned in different objects rather than when positioned within a single object. Chapter 4 investigates how the distances between viewer and cue, viewer and target, and cue and target affect the magnitude of IOR, suggesting that the world-centered reference frame influences IOR. Chapter 5 serves as a general discussion and conclusion chapter, discussing the findings and implications of each empirical chapter. / The distribution of human attention in space can be modulated by spatial and temporal factors. This dissertation studied inhibition of return (IOR), a robust behavioural effect obtained through a spatial cueing paradigm where observers exhibit slower detection times to a target appearing over 300 ms after a cue in a previously cued location. Most research has studied the IOR effect in two-dimensional space; thus, it remains unclear whether, in three-dimensional space (3D) space, slower reaction times occur due to a target appearing in the same world location (defined in 3D coordinates) or in the same retinal location as the cue (i.e., anywhere along an observer’s line of sight to the cue). My thesis examines IOR in a computer-simulated 3D environment, with the location of the cue and target residing in the same versus different depth/distance position either within the same or different object and either relative to the observer or to the world environment. Following a general literature review (Chapter 1), the first empirical chapter (Chapter 2) demonstrates that IOR is depth-specific when the direction of depth switch between cue to target occurs from far-to-near space, suggesting a behavioural advantage for near space in the human attention system. Chapter 3 shows that this depth-specificity and depth-asymmetry of IOR is maintained only when cues and targets are not part of the same object; object membership can therefore override the depth-specific property of IOR in 3D scenes. Chapter 4 introduces motion of the viewpoint, showing that IOR is depth-specific when the cue and target appear in different depth locations in the world environment even when located at the same relative distance from the observer’s viewpoint. Thus, IOR could be the result of an inhibitory tag placed at a location relative to the environment rather than at a location relative to the viewpoint. / Dissertation / Doctor of Science (PhD) / Human attention can be distributed over space and affected by external events. Prior research using 2D environments has shown that some time after the first stimulus (a cue), the reaction time to a subsequent stimulus (a target) appearing in the same location is typically slower compared to when this target appears elsewhere. Thus, attention likely moves away from a previously observed to more novel location of interest. I examined, in a 3D environment, whether this “location” of reduced attention resides in the same 3D location or retinal location as that of the cue. I also assessed the impact on reaction time for when the cue and target belong to the same or different object and when their locations differ in reference to the observer or world environment. My research suggests that humans maintain a higher level of attention for nearer space when the cue previously appears at a farther location.