Abstract
Categorical perception (CP) of colour is demonstrated by faster or more accurate discrimination of colours that cross a category boundary, compared to equivalently spaced colours from the same colour category. Despite a plethora of behavioural research exploring the origin and nature of colour CP, the processes involved in the effect are still unresolved. This thesis investigates the time course and underlying mechanisms of colour CP by using the Event-Related Potential (ERP) technique. This is a relatively novel approach that involves the measurement of stimulus evoked brain potentials at the scalp. ERPs were recorded during four studies that used variations of a visual oddball task, where a standard colour stimulus was shown frequently, interspersed by less frequent presentations of 'deviant' colours from the same or different category to the standard. Analysis of adult ERP waveforms in Study 1 established that category effects occur-from as early as 90 ms post-stimulus onset. This finding provides the first clear evidence of an early perceptual basis for colour CP. Additionally, differences in later ERP components also implicate post- perceptual, cognitive mechanisms such as memory and language. Study 2 explored early perceptual stages of processing in more detail, revealing that early category effects are rapid, automatic and pre-attentive. The time course and neural markers of infant colour CP were investigated in Study 3. This study provides the first electrophysiological evidence of pre-linguistic color category effects, demonstrating that processes of attention, recognition memory and novelty detection are involved in infant colour CP. Finally, Study 4 explored the relative contributions of perceptual and post-perceptual processes to acquired adult colour CP following category training. Learning of two new categories led to enhanced discrimination of colour's around the learned category boundary, with this effect only revealing itself in later, cognitive ERP components. This suggests that category training does not affect early perceptual responses, but rather that mechanisms such as memory and language govern acquired colour CP. This thesis demonstrates that categorical information is detected rapidly by the human brain. Additionally, the finding of category effects in infant ERP components offers new evidence of how the infant brain categorises in the absence of language. Finally, the novel finding that learnt category effects reveal themselves in later ERP components emphasises the importance of cognitive strategies in forming categories.