The selectivity of responses to red-green colour and achromatic contrast in the human visual cortex: An fMRI adaptation study

Abstract

© 2015 Federation of European Neuroscience Societies and John Wiley and Sons Ltd. There is controversy as to how responses to colour in the human brain are organized within the visual pathways. A key issue is whether there are modular pathways that respond selectively to colour or whether there are common neural substrates for both colour and achromatic (Ach) contrast. We used functional magnetic resonance imaging (fMRI) adaptation to investigate the responses of early and extrastriate visual areas to colour and Ach contrast. High-contrast red-green (RG) and Ach sinewave rings (0.5 cycles/degree, 2 Hz) were used as both adapting stimuli and test stimuli in a block design. We found robust adaptation to RG or Ach contrast in all visual areas. Cross-adaptation between RG and Ach contrast occurred in all areas indicating the presence of integrated, colour and Ach responses. Notably, we revealed contrasting trends for the two test stimuli. For the RG test, unselective processing (robust adaptation to both RG and Ach contrast) was most evident in the early visual areas (V1 and V2), but selective responses, revealed as greater adaptation between the same stimuli than cross-adaptation between different stimuli, emerged in the ventral cortex, in V4 and VO in particular. For the Ach test, unselective responses were again most evident in early visual areas but Ach selectivity emerged in the dorsal cortex (V3a and hMT+). Our findings support a strong presence of integrated mechanisms for colour and Ach contrast across the visual hierarchy, with a progression towards selective processing in extrastriate visual areas. We measured responses to red-green color and achromatic contrast in human visual cortex with fMRI adaptation. Cross adaptation between the two contrast types occurred across the visual hierarchy indicating integrated color and achromatic responses. Selective adaptation for color (i.e. greater adaptation to color than achromatic contrast) emerged in ventral cortex, particularly in areas V4 and VO, whereas selective adaptation to achromatic contrast (i.e. greater adaptation to achromatic than color contrast) was evident in dorsal cortex (V3a, hMT+). Our findings suggest a progression from integrated color and achromatic responses in early visual cortex to more selective processing in extrastriate visual areas.