Cortical function and structure of human colour perception

Chromatic and achromatic signals in the brain have historically been studied independently. This thesis examines the combinations of chromatic and achromatic signals using both functional Magnetic Resonance Imaging (fMRI), structural MRI, and behavioural measurements to understand how these essential facets of human vision interact. Chapter one utilises random luminance masking to examine functional cortical responses to colour stimuli, and whether they can be used to predict behavioural threshold measurements. First, I examine the potential independence and interdependence of chromatic and achromatic signals in primary visual cortex. I found evidence of interdependence in a manner consistent with a normalising effect of achromatic contrast on chromatic signals, which can accurately predict behavioural thresholds. Then I used the same methods to examine the differing responses to L-M and S cone signals at two spatial frequencies, between which the behavioural sensitivity difference is greater for S cone than L-M stimuli. I found responses in early visual cortex which aligned well with behavioural sensitivity. In both projects described in chapter one, I have shown that chromatic responses do not reduce dramatically up the visual hierarchy, while achromatic responses do. In chapter two, I present research collected from a cohort of rare participants with congenital achromatopsia, who have no functional cone-mediated vision from birth, leading to a total loss of chromatic and central vision. This cohort, known as rod achromats, represents a unique opportunity to investigate the effects of chromatic deprivation on the brain. Using structural MRI, I found widespread reduction in cortical surface area in the achromat group compared to healthy controls, suggesting that the loss of chromatic vision or the loss of high acuity foveal vision significantly affects cortical morphology. I conclude with some areas for future research which would further expand the understanding of the links between chromatic perception and the structure and function of the human brain.