Lateralisation of Categorical Perception of Colour.

The debate over language and thought has traditionally been framed by two opposing approaches: Universalism and Relativism. Universalism holds that language is shaped by universals of human cognition. In contrast, Relativism holds that language shapes thought. Colour categorical perception has been the main area in which the two theories are tested. The current thesis presents a series of experiments examining the nature and origin of colour categorical perception. The focus is on the categorical perception (CP) of colour defined as better discrimination of colours that straddle a category boundary than that of colours within categories. It has recently been argued that colour CP is represented in the. left hemisphere and this reflects explicit language processes. The current thesis tests this account in a series of experiments using a task that loads heavily on perceptual processes. The results show that there is a predisposition for colour CP in pre-lingual infants and that is based in the right hemisphere. In contrast, in adults, colour CP is represented in the left hemisphere. A cross-cultural approach allowed for a further test of the role of language in colour CP comparing speakers of languages that make different categorical colour distinctions. CP effects were found only in marked boundaries and it was lateralised to the left hemisphere. Finally, using a learning paradigm, the findings showed that learning to categorise stimuli alters their perception at an early stage of visual processing and this is restricted to the left hemisphere. Overall, the results presented in this work suggest that colour CP is neither strictly perceptual nor strictly language dependent. Instead, the findings suggest that a subtle interplay between the two positions is required to explain the nature of colour CP. Colour CP is present in the absence of language, but language may modulate the location of categorical perception as colour terms are acquired