Abstract

Much research supports the existence of top-down effects on visual perception. Within this thesis, two accounts which seek to explain top-down effects on perception are investigated: The Action-Specific Account, which asserts that perception is affected by an individual’s ability to act, and the Attentional Account, which asserts that perception is affected by attention towards an object. These accounts were investigated because they potentially challenge cognitive impenetrability – the assertion that perception is free from cognitive influences. These accounts therefore have ramifications for the concept of modularity and the current understanding of how the mind is organised. However, no research demonstrating top-down effects on perception has successfully avoided the methodological pitfalls which provide alternative explanations for their results. As such, the central aim of this thesis was to develop a method of measuring one example of these top-down effects - changes in object size perception - whilst avoiding said methodological pitfalls. Perception of food size was selected because previous research suggested that changes in perceived food size may affect subsequent eating behaviours. The central finding of this thesis is that attention, independent of action-specific influences, affects perception. Importantly, although top-down attention affects perceptual experience, it is suspected this occurs indirectly through small changes in visual input - and therefore may not challenge cognitive impenetrability. To investigate the extent of top-down effects on perception, this thesis also addressed inconsistencies within the broader food perception literature. It was observed that health outcomes related to over-eating were better predicted by food-specific, than domain-general, self-reports. Interestingly, the impulsivity assessed by these food-specific measures did not predict perceived food size. In summary, this thesis addresses methodological issues within the object perception literature and further research is proposed to assess its limitations.