Effective spatial orientation and postural control requires the integration of proprioceptive, vestibular and visual inputs. Increased visual dependence has been reported in older adults but the underlying mechanisms remain unknown. The aims of this research were to investigate the methodological issues relevant to assessment of visual dependence in healthy younger (18-40 yrs) and older (>60 yrs) subjects and also the effect of sensory manipulation (vibratory and auditory) on visual dependence and postural control. Visual dependence is commonly assessed by the Rod and Disc Test (RDT) and used throughout the thesis. Postural control was assessed by postural sway when standing on a force plate during static (one and two legged stance) and dynamic (stepping) tasks with eyes open (viewing static and rotating images) and closed. It was found in Chapter 2 that presentation shape (rectangular or round) had no effect on visual dependence. However pixellation on a TV screen could provide orientation cues to judge visual vertical i.e. decreased visual dependence as judged by the RDT and thus underestimates the amount of visual dependence. This was similar in both age groups. Visual stimulation can impair balance and there are reports of habituation in response to repetitive stimulation and also that the habituation is less in healthy older people. Little is known about the effects of repeated visual stimulation on visual dependence. There was no effect on visual dependence of repeated visual stimulation by the RDT with two recovery intervals (~7 and 10min) in either age group (Chapter 3). These findings suggest that visual dependence is less sensitive to visual stimulation than balance control. Whole body vibration (WBV) is a sensory stimulus that can improve muscle power and balance after a single session but its effects on sensory systems is less understood. Five minutes of WBV improved visual dependence in the younger group for 20 minutes (p=0.03 – 0.05) but had no effect in the older one (Chapter 4). No such compensation occurred in older people who may have a less flexible and slower sensory reweighting ability. In Chapter 5 balance control was assessed during standing on one and two legs when looking at both a static and rotating image also with eyes closed. There was a non-significant trend for balance improvement in both age groups. The sensory stimulation provided by WBV through proprioceptive and vestibular inputs may reduce reliance on vision. Divided attention may increase visual dependence and disturb balance, particularly dynamic balance. . Auditory distraction, in the form of recordings of real street sounds, did not change the level of visual dependence or balance control during static bipedal stance or dynamic (stepping) in either age group. However, it did increase postural sway during the more challenging task of standing on one leg in younger subjects (p=0.048) and showed a non-significant trend (p=0.07) to do so in the older subjects (Chapter 6). This was the only study to find higher levels of visual dependence in the older age group (p=0.02 – 0.0001). A key finding of majority of studies was the relatively similar level of visual dependence and response to sensory manipulation in the two age groups compared with the published literature, although the subjects studied were often older. They subjects were healthy and very active both physically and mentally and so may not be representative of their age group. This suggests that ageing per se may not necessarily lead to increased visual dependence in healthy people but relates to the development of specific pathologies.

Faces are highly complex visual stimuli that playa disproportionate role in social interactions. Based on visual information from faces alone, humans can judge an individual's race, gender, age, identity, mood and intentions. The first aim of this thesis is to investigate aspects of the mechanisms that underlie the extraordinary sensitivity of human face perception. In the first study, sensitivity to various face features was quantified. Evidence was found of enhanced sensitivity to external (head-shape, hairline), relative to internal (nose, mouth, eyes, eyebrows) features, with the lowest sensitivity for features most affected by face dynamics. Discrimination thresholds were significantly higher when features were embedded within an average face context, relative to isolated presentation. Although inversion significantly reduced sensitivity, the external feature advantage remained. These results have implications on the nature of holistic processing mechanisms for unfamiliar faces. In a different study, face perception was investigated in peripheral vision. Discrimination thresholds for full-faces, internal features and external features were lower in the left visual field, relative to all other visual field locations, consistent with the cortical lateralisation of the face-sensitive fusiform face area. Sensitivity to headshapes, on the other hand, was greatest in the lower visual field. This is consistent with the premise of non-overlapping processing mechanisms for the general shape of objects and specific object categories, such as faces. The aim of a further study was to investigate the neuronal representation of faces by determining the effect of adaptation on face discrimination. Face adaptation significantly reduced unfamiliar face discrimination sensitivity. This effect was, however, highly identity-specific; adaptation did not transfer between different face identities. The magnitude of this adaptation effect increased monotonically with adapting face distinctiveness (how much an individual face differs from an average/mean face), but only up to a limit. These results suggest that individual face identities are encoded by discrete neuronal populations that respond with increasing magnitude as faces become progressively more different from an average face but exclude more extreme face caricatures. Based on these results, the final part of this thesis outlined the development of a new clinical test of face perception. This new test is designed to overcome the limitations of existing face tests such as restricted testing ranges and confounding factors (e.g. memory, familiarity). The range of normal face sensitivity was determined in a large group of young adults. Older adults showed preserved face discrimination sensitivity. A case study of a patient with suspected developmental prosopagnosia demonstrates that the test has the potential to accurately identify impairments of face perception enjoying a substantially higher sensitivity than standard face tests.

My research identifies facial adiposity, a measure of weight in the face, as a novel facial cue to attractiveness and health. Previously identified facial cues, such as symmetry, averageness, sexual dimorphism and skin condition, are not consistently related to indices of actual health. I chapter 2 I demonstrate that facial adiposity is reliably associated with judgements of facial attractiveness and health in Caucasians and also with frequency and duration of respiratory infections, antibiotics use and blood pressure, indicating that facial adiposity is a valid cue to health. Additionally, in chapter 3 I identify three quantifiable facial shape cues that are reliably related to Body Mass Index (BMI) and are used by observers to judge weight in Caucasian and African faces. In chapter 4 I show that Western Caucasian women, but not men, prefer a significantly lower facial adiposity when judging attractiveness than when judging health in other women's faces. This difference may reflect the influence of the media, since it was only significant in women's judgements and previous work showed that women internalize media messages about body ideals more than men do. In contrast, African participants in chapter 6 did not show any difference between the optimal facial adiposity for health and attractiveness, which is consistent with the prediction that people living in an environment with a high disease burden will base their concept of attractiveness more closely on cues to health. Importantly, these different patterns of results for Western Caucasian and African participants are unlikely to be due to cultural differences in media ideals of beauty, since the new African body ideal portrayed by the South African media is closely aligned with the Western ideal (chapter 5). Thus, my research suggests that perceptions of facial adiposity may well be influenced by an interaction between environmental factors and media ideals.

Selective attention is a crucial function that encompasses all perceptual modalities and which enables us to focus on the behaviorally relevant information and ignore the rest. The main goal of the thesis is to test well-established hypotheses about the mechanisms of visual selective attention in the auditory domain using behavioral and neuroimaging methods. Two fMRI studies (Experiments 1 and 2) test the hypothesis of feature-specific attentional enhancement. This hypothesis states that when attending to an object or a feature, there should be an enhancement of the response in the sensory region that is sensitive to that object or feature. Experiment 1 investigated feature-specific attentional modulation mainly within the tonotopic fields around primary auditory cortex. Experiment 2 investigated feature-specific attentional modulation mainly around non-primary auditory cortex, when attending to frequency modulation or motion of the same auditory object. Experiment 1 showed evidence for feature-specific enhancement, while Experiment 2 did not. The role of competition among concurrent auditory objects as a necessary factor in driving feature-specific enhancement is discussed. A second hypothesis from vision research is that spatial perception and attention is much more precise in the centre than in the periphery. Experiment 3 used a masking release paradigm to investigate whether the acuity of auditory spatial attention was similarly increased in the midline. Although location discrimination of sounds segregated by inter-aural time differences was more precise at the midline than at the periphery, spatial attention was not. Therefore for this task at least there was no effect of eccentricity on auditory spatial attention. The results of these three studies are discussed in view of selective attention as a flexible process that operates in different ways according to the specifics of the task.

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Selective spatial attention and multisensory integration have been traditionally considered as separate domains in psychology and cognitive neuroscience. However, theoretical and methodological advancements in the last two decades have paved the way for studying different types of interactions between spatial attention and multisensory integration. In the present thesis, two types of such interactions are investigated. In the first part of the thesis, the role of audiovisual synchrony as a source of bottom-up bias in visual selection was investigated. In six out of seven experiments, a variant of the spatial cueing paradigm was used to compare attentional capture by visual and audiovisual distractors. In another experiment, single-frame search arrays were presented to investigate whether multisensory integration can bias spatial selection via salience-based mechanisms. Behavioural and electrophysiological results demonstrated that the ability of visual objects to capture attention was enhanced when they were accompanied by noninformative auditory signals. They also showed evidence for the bottom-up nature of these audiovisual enhancements of attentional capture by revealing that these enhancements occurred irrespective of the task-relevance of visual objects. In the second part of this thesis, four experiments are reported that investigated the spatial selection of audiovisual relative to visual objects and the guidance of their selection by bimodal object templates. Behavioural and ERP results demonstrated that the ability of task-irrelevant target-matching visual objects to capture attention was reduced during search for audiovisual as compared to purely visual targets, suggesting that bimodal search is guided by integrated audiovisual templates. However, the observation that unimodal targetmatching visual events retained some ability to capture attention indicates that bimodal search is controlled to some extent by modality-specific representations of task-relevant information. In summary, the present thesis has contributed to our knowledge of how attention is controlled in real-life environments by demonstrating that spatial selective attention can be biased towards bimodal objects via salience-driven as well as goal-based mechanisms.

Previous research of the effects of perceptual load on visual processing demonstrates reduced perceptual sensitivity and reduced neural activity for task-irrelevant stimuli under high compared to low perceptual load. However, the precise underlying mechanisms for reduced processing remain unclear. The present thesis approaches this question by assessing the interactions between low level visual stimuli and their visual context under different levels of perceptual load. The results from Chapter 2 demonstrate that perceptual load modulates visual perception of orientation not only by reducing overall signal gain but also by broadening the orientation tuning profile. These findings suggest that perceptual load not only reduces the signal strength but also reduces the extent to which the signal is discriminated from noise; thus altering the strength of contextual interaction. Chapter 3 further demonstrates the role of perceptual load in contextual interactions by establishing the effects of load on the tilt-illusion, which is thought to rely purely on interactions among orientations. The results show that high perceptual load increases the impact of context (leading to greater tilt-illusion) for subthreshold context presentation, which precludes top-down suppression of the task-irrelevant context. Chapters 4 to 5 further establish reduced efficiency of separating signal from noise with higher perceptual load, found when signal and noise are presented successively (as in backward masking). The time course of perceptual load effects shows a distinct pattern of both, more effective and longer lasting masking under high compared to low load. This pattern evolves rapidly for pattern masking which indicates low-level integration and demonstrates an early locus for the effects of load. It also persists at later periods for metacontrast and object substitution masking where the stimulus and mask do not spatially overlap, indicating load effects at later processing stages. Together, the results provide mechanistic explanations for reduced perception under high perceptual load.