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This thesis investigated some of the factors involved in the representation, focus, and movement of covert attention in visual space. Existing research has shown that visual cues produced a facilitation of reaction times (RTs) to visual targets appearing at a cued location for up to 300ms. At longer stimulus-onset-asynchronies (SOAs), this was followed by an increase in RTs relative to the uncued location. This has been termed inhibition of return (lOR). Experiments 1-6 used LED cues which were the same as, or spatially distinct from, a target LED, and were either informative or uninformative about the target location. Results were inconsistent. Where discrimination problems existed, the cue/target probabilities altered the results: from evidence of inhibitory effects, to no significant cue-side effects. Where no discrimination problems existed, facilitatory effects were apparent, and were enhanced by an increased cue/target probability. Experiments 7-10, using exogenous cueing, manipulated attentional focus and the presence of cue-markers. Altering the focus size did not substantiate previous findings of a reciprocal relationship between focus and performance. The removal of cue-markers resulted in increased amounts of inhibition not supporting current single- or dual-component views of lOR. Previous work has shown that informative symbolic visual cues produced costs and benefits of RTs to visual targets. Experiments 11-16, using static endogenous cueing with targets framed in central and peripheral locations, attempted to demonstrate object-based attentional representation. All six experiments showed significant effects due to SOA and cue validity, however, initial results showed no evidence of stimulus-grouping. Only when target position markers were removed, when peripheral targets were used, and when inside/outside location judgements were required instead of target detection, did results indicate some possibility of grouped attentional representation. Finally, several of the experiments also investigated the nature of attentional movement. Results did not support straightforward analogue explanations.

Over a decade ago Milner and Goodale suggested that perception and action are subserved by two distinct cortical visual streams. The ventral stream projecting from striate cortex to inferotemporal cortex is involved in the perceptual identification of objects. The dorsal stream projecting from striate cortex to posterior parietal cortex is involved in visually guided actions. A series of experiments have been carried out and are presented within this thesis to investigate how various aspects of visuomotor behaviour fit into such a model. A range of techniques were employed, including: (1) behavioural studies with patients with optic ataxia (dorsal stream damage) and visual form agnosia (ventral stream damage); (2) transcranial magnetic stimulation (TMS) in healthy subjects; (3) functional magnetic resonance imaging (fMRI) in healthy subjects. The following conclusions were made: (1) obstacle avoidance behaviour is impaired in patients with optic ataxia due to damage to the dorsal stream; (2) obstacle avoidance is intact in patients with visual form agnosia as damage is restricted to the ventral stream; (3) obstacle avoidance is mediated by the dorsal stream when an immediate response is required, whereas under delayed conditions the ventral stream comes into play; (4) visual form agnosic patients can use looming information to catch moving objects and they are capable of responding to online perturbations due to an intact dorsal stream; (5) V5 / MT+ is involved in motion processing for perception and action and does not belong exclusively to the dorsal or ventral stream; (6) the dorsal stream is only sensitive to orientation changes if the stimuli are graspable. While some modifications of the original distinction are necessary, the experiments presented within this thesis suggest that this model has, for the most part, withstood the test of time and provides a useful framework for understanding various aspects of perception and action.

A computational cognitive neuroscience approach was used to examine processes of visual attention in the human and monkey brain. The aim of the work was to produce a biologically plausible neurodynamical model of both spatial and object-based attention that accounted for observations in monkey visual areas V4, inferior temporal cortex (IT) and the lateral intraparietal area (LIP), and was able to produce search scan path behaviour similar to that observed in humans and monkeys. Of particular interest currently in the visual attention literature is the biased competition hypothesis (Desimone & Duncan. 1995). The model presented here is the first active vision implementation of biased competition, where attcntional shifts are overt. Therefore, retinal inputs change during the scan path and this approach raised issues, such as memory for searched locations across saccades, not addressed bv previous models with static retinas. This is the first model to examine the different time courses associated with spatial and object-based effects at the cellular level. Single cell recordings in areas V4 (Luck et al., 1997; Chelazzi et al., 2001) and IT (Chelazzi ct al., 1993, 1998) were replicated such that attentional effects occurred at the appropriate time after onset of the stimulus. Object-based effects at the cellular level of the model led to systems level behaviour that replicated that observed during active visual search for orientation and colour feature conjunction targets in psychophysical investigations. This provides a valuable insight into the link between cellular and system level behaviour in natural systems. At the systems level, the simulated search process showed selectivity in its scan path that was similar to that observed in humans (Scialfa & Joffe, 1998; Williams & Reingold, 2001) and monkeys (Motter & Belky. 1998b), being guided to target coloured locations in preference to locations containing the target orientation or blank areas. A connection between the ventral and dorsal visual processing streams (Ungerleider & Mishkin. 1982) is suggested to contribute to this selectivity and priority in the featural guidance of search. Such selectivity and avoidance of blank areas has potential application in computer vision applications. Simulation of lesions within the model and comparison with patient data provided further verification of the model. Simulation of visual neglect due to parietal cortical lesion suggests that the model has the capability to provide insights into the neural correlates of the conscious perception of stimuli The biased competition approach described here provides an extendable framework within which further "bottom-up" stimulus and "top-down" mnemonic and cognitive biases can be added, in order to further examine exogenous versus endogenous factors in the capture of attention.

This study employed adaptation paradigms to explore visual motion processing during smooth pursuit eye movement. Two classes of model, classical models and reference signal models, employ estimates of retinal motion and pursuit to estimate head-centred world motion. The pursuit estimate in classical models is purely extra-retinal. In reference signal models the pursuit estimate is additionally modulated by retinal feedback. Chapters 2 and 3 investigated the motion aftereffect (MAE) following adaptation to simultaneous retinal motion and smooth pursuit. In chapter 2 adaptations to either horizontal retinal motion or vertical smooth pursuit respectively resulted in retinal or extra-retinal MAE. Simultaneous orthogonal adaptation to both motions resulted in a unidirectional MAE that bisected the individual MAE directions. Adaptation to a head-centred motion signal (perceived direction) was not supported by the recorded directions for adapting motion and resulting MAE. An explanation employing separate lower level adaptations was favoured. Chapters 3 and 4 examined motion perception following collinear motion adaptation. Additionally, the retinotopic nature of retinal motion adaptation was exploited to limit the effects of retinal sensor adaptation during the test phase. The two classes of model then make differing predictions: Reference signal models predict a pursuit estimate that is modulated by retinal motion, whilst classical models do not. In chapter 3 varying the background motion during adaptation did alter the physical eye movement. However, the properties of the resulting MAE were not modulated by retinal feedback and a classical model was supported. Chapter 4 used a moving test to quantify the perceived stability of a background during smooth pursuit using a two alternative forced choice paradigm and staircase procedure. Either a phantom velocity aftereffect or a modified reference signal model was suggested as modulating the Filehne illusion in Experiment 6. Two control experiments failed to find evidence for phantom adaptation. Experiment 9 demonstrated a potential retinotopic location bias for background motion when applying a reference signal model, background motion above the test area did not alter perceived stability judgments.

This research explored two aspects of visually guided walking (1) what is the role of optic flow in the recalibration of misperceived direction while walking, and (2) how does a change in perceived direction map onto a change in walking direction. Data from five studies investigating adaptation to displaced direction (by prism glasses) suggested the following. First, optic flow is important in the recalibration of perceived direction. Further, processing optic flow is attentionally demanding, such that when cognitive load is increased, recalibration decreases. The results also demonstrated that the timecourse of recalibration changed as a function of the presence, or absence, of optic flow. With regards to the relationship between egocentric direction and walking direction, we demonstrated that a change in visual straight ahead could be mapped onto a change in target-heading error. We found that this relationship held when we unpacked the data according to the direction of displacement to which observers were exposed. The important relationship between visually perceived direction and walking direction was also highlighted in a patient study, using patients whose perception of direction was endogenously shifted after a right hemisphere stroke. Taken together, the results of this thesis help to highlight the role of optic flow in the recalibration of perceived direction, and the role of perceived direction in the visual guidance of walking. It is argued that optic flow promotes rapid recalibration of visual direction, and that change in perceived visual straight ahead can be mapped onto a changed in walking direction.

Monitoring the trustworthiness of social interaction partners is a cornerstone of social cognition. However, the mechanics of learning about trust during online interactions as a result of a person’s behaviour can be difficult to explore. The current experiments use a gaze cueing paradigm where faces provide either valid (always shift their gaze towards the location of a subsequent target), or invalid cues (always shift their gaze to a different location). Following gaze cueing, participants rate valid faces as more trustworthy than invalid faces. We show that this incidental trust learning is sensitive to the emotional expression of the face, is specific to assessments of trust, occurs outside of conscious awareness, and is driven primarily by a decrease in trust for invalid faces (Chapter 2), perhaps reflecting a cheater detection module. Memory for incidentally learned trust is surprisingly durable, is affected by the familiarity of the cueing faces (Chapter 3), and does not affect memory for the faces’ physical features, nor does the trustworthiness of the face generalise to other stimuli (Chapter 4). Furthermore, learning is modulated by top-down knowledge of social group membership − when group identity is made experimentally salient, participants default to a group-level representation as a heuristic for social judgements (Chapter 5), while using naturally occurring group memberships (i.e. race) results in better learning for in-group members than out-group (Chapter 6). Finally, while there is evidence that trust learning is driven by learning about eye-gaze behaviour, this cannot be explained purely by disruptions to visuomotor fluency (Chapter 7), which suggests that this phenomenon is part of an active social monitoring framework that relies on physical changes or behaviours in a face to affect subsequent social judgements.

Research is divided as to what degree visually unattended objects are processed (Lachter et al., 2008; Carrasco, 2011). The hybrid model of object recognition (Hummel, 2001) predicts that familiar objects are automatically recognised without attention. However under perceptual load theory (Lavie, 1995), when objects are rendered unattended due to exhausted attentional resources, they are not processed. The present work examined the visual processing of images of everyday objects in a short-lag repetition-priming paradigm. In Experiments 1-3 attention was cued to the location of one of two objects in the first (prime) display, with the unattended sometimes repeated in the second (probe) display. ERP repetition effects were observed which were insensitive to changes in scale (Experiment 1) but sensitive to slight scrambling of the image (Experiment 2). Increasing perceptual load did not modulate these view-specific repetition effects (Experiment 3), consistent with the predictions of automatic holistic processing. In Experiments 4-7 a letter search task was used to render the flanking object image unattended under high load. In Experiment 5 distractor processing was observed in ERP even under high load. In Experiments 4, 6 and 7 a pattern of view sensitive/insensitive and load sensitive/insensitive repetition effects on RT (Experiment 4) and ERP amplitude (Experiments 6, 7) were observed that were difficult to interpret under either the hybrid model or perceptual load theory, but may reflect fast view-based and slow view-independent processing of objects. Overall, the properties of the view-sensitive repetition effects were generally consistent with those associated with the automatic/pre-attentive processing of the holistic route of the hybrid model. However, differences between the processing of objects rendered unattended via a spatial cue or perceptual load indicate that the bottom-up driven hybrid model and perceptual load theory may benefit from the consideration of the interaction of top-down biasing of processing (Tsotsos et al., 2008).

Spatial orientation is the ability to maintain knowledge of our position with respect to other cues within an environment. This is an essential skill, forming the foundation of other abilities, including spatial navigation. Previous research has identified that virtual environments impede participants’ ability to orient accurately. Research exploring the role of environment type, specifically nested environments, has further identified a situation which hinders orientation ability. This thesis seeks to link these research bodies, exploring orientation ability within virtual nested environments. Across a series of experiments, it was found that participants struggled to accurately orient within these environments, especially when a link to the external environment was unavailable. The addition of orienting cues within the environment, however, reduced this difficulty. Participants provided with additional cues recorded significantly lower orientation error. This effect is apparent following either active exploration or a passive video tour. Subsequent studies illustrated that other factors such as anxiety, as manipulated via the use of stereotype threat, also influenced orientation accuracy within a nested environment. Geovisualisations were explored to examine whether orientation difficulties are observed in symbolised, rather than realistic, virtual environments. Participants reported orientation difficulties and demonstrated an inability to accurately track their position within symbolised space. Results suggest that geovisualisation users, similar to users of virtual nested environments, require increased support to efficiently orient. Results support that orientation within digital nested environments is difficult due to the lack of consistent visual cues within the multiple aspects of the environment.