Synaesthesia is a cognitive trait in which stimuli of one sensory modality are automatically and consistently experienced in conjunction with perceptions in a separate modality or processing stream. Investigations of synaesthesia may help determine the neural processing required in the generation of a conscious experience. In order to gain the most complete understanding of synaesthesia, we have applied an integrated neurophenomenological approach.udIn Chapter 2 we present an extended case study of spatial-form synaesthesia (SFS) phenomenology. This investigation goes significantly beyond the rudimentary accounts of provided elsewhere, and provides novel observations on inducer-concurrent relationships,udsuggesting that guided introspection techniques can provide neurobehaviourally relevant information.udIn Chapters 3-5 we investigate neural activity in grapheme-colour synaesthesia (GCS). In Chapter 3 we demonstrate that activation in colour selective areas during synaestheticudcolour processing is dependent on individual differences in phenomenology, thereby reconciling previous attempts to replicate this key finding in the GCS literature. In Chapterud4 we find no evidence for trait level differences in context specific functional connectivity in GCS, however, we demonstrate that localisation of the synaesthetic concurrents modulate connectivity between colour and low-level visual areas. In 5 we replicate findings ofudtrait level differences in resting state fronto-parietal networks, suggesting that the RFPN may be a significant network in aspects of the synaesthetic experience common to all participants. We demonstrate that localisation of concurrents also modulates resting state visual networks, whilst automaticity of concurrents modulates parietal networks. Both Chapters 4 and 5 support a model of synaesthesia in which localisation of concurrents isudmodulated by bottom-up connectivity, between colour and early visual areas.udThis thesis demonstrates that individual differences in synaesthetic phenomenology significantly impact neural activity. We propose that future investigations place emphasis on the phenomenological experience of the participant in the interpretation of neural effects.
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