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In order to act on the world around us, the brain needs to encode the location of visible objects. This begins with representing the location of visual features on the retina. However, a retinal representation alone does not provide information about the direction of visual features with respect to the head or body whenever the eyes, head and body move. Therefore, successful action requires more than a retinal representation of location. One possibility is that the locations of visible objects may be encoded in retino-centric, head-centric and body-centric frames of reference. To investigate this, we used a well-known visual phenomenon: the tilt aftereffect (TAE). We investigated whether visual feature tilt is represented beyond a retino-centric representation, using a gaze contingent adaptation paradigm. The results of seven experiments yielded four key findings: 1) The TAE was contingent on gaze, suggesting that the TAE is not just retino-centric. We found evidence of a head-centric representation of tilt, but no evidence of a body-centric or world-centric representation of tilt. 2) The gaze contingent TAE showed different characteristics compared to the conventional TAE. While the conventional TAE was sensitive to test stimulus duration, the gaze contingent TAE was not. The gaze contingent TAE was also significantly smaller in aftereffect magnitude. Both TAEs shared similar characteristics with respect to stimulus contrast. 3) We found no evidence that either TAE was modulated by attention. 4) Our findings support the conclusion that the conventional TAE is the result of adaptation of two mechanisms: a tilt sensitive mechanism and a gaze direction encoding mechanism. This result fits with neurophysiological findings of neurons jointly sensitive to tilt and gaze direction (Trotter & Celebrini, 1999). Concluding, our results in this thesis provide psychophysical evidence that our impression of the world is based on head-centric visual representations.

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This thesis is an exploration of the oscillatory changes occurring in the visual cortex as measured by a functional imaging technique known as Synthetic Aperture Magnetometry (SAM), and how these compare to the BOLD response, across a number of different experimental paradigms. In chapter one the anatomy and physiology of the visual pathways and cortex are outlined, introducing the reader to structures and terms used throughout the thesis whilst chapter two introduces both the technology and analysis techniques required to record MEG and fMRI and also outlines the theory behind SAM. In chapter three the temporal frequency tuning of both striate and extrastriate cortex is investigated, showing fundamental differences in both tuning characteristics and oscillatory power changes between the two areas. Chapter four introduces the concept of implied-motion and investigates the role of area V5 / MT in the perception of such stimuli and shows, for the first time, the temporal evolution of the response in this area. Similarly a close link is shown between the early evoked potential, produced by the stimulus, and previous BOLD responses. Chapter five investigates the modulation of cortical oscillations to both shifts in attention and varying stimulus contrast. It shows that there are both induced and evoked modulation changes with attention, consistent with areas previously known to show BOLD responses. Chapter six involves a direct comparison of cortical oscillatory changes with those of the BOLD response in relation to the parametric variation of a motion coherence stimulus. It is shown that various cortical areas show a linear BOLD response to motion coherence and, for the first time, that both induced oscillatory and evoked activity also vary linearly in areas coincidental with the BOLD response. The final chapter is a summary of the main conclusions and suggests further work.

Identifying moving objects while we are moving is an important everyday skill. This ability allows us to monitor our surroundings, successfully interact with objects or people, and avoid potential hazards. Self-movement generates optical flow on the retina that complicates the recognition of moving objects from retinal motion alone. Rushton and Warren (2005) proposed a purely visual solution to this problem. They suggest that, in order to assess scene-relative object movement, the brain identifies and parses out (globally subtracts) patterns of visual flow that are consistent with self-movement. Existing research has demonstrated evidence of this flow parsing process in central vision (i.e. Warren & Rushton, 2008). This thesis aims to characterise the role of peripheral visual flow in this process. Research from the wider self-motion literature has often distinguished between central and peripheral vision. Some researchers have claimed that peripheral vision is specialised for self-motion perception, whilst more recent studies have challenged this assertion. This thesis investigates whether peripheral visual motion, traditionally considered to be a strong cue to self-movement, also contributes to flow parsing. The experimental work employed a simulated self-movement paradigm to isolate retinal motion from other non-visual cues. Thus, observers remained stationary whilst computer generated stimuli moved to produce patterns of retinal motion associated with actual self-movement. In the first set of experiments, I demonstrate that peripheral flow simulating forward or backward self-movement gives rise to characteristic flow parsing effects. This finding generalises to rotational observer motion (Chapter 3). Chapter 4 considers whether peripheral flow contributes to parsing for judgements of object size change. Finally, Chapter 5 investigates whether there is a benefit of peripheral information under conditions where central flow is potentially ambiguous. The results indicate that peripheral visual flow contributes to the flow parsing process. The contribution of flow in the near periphery appears to be maximally important.