A comparison of the BOLD fMRI response to achromatic, L/M opponent and S-cone opponent cardinal stimuli in human visual cortex: II. chromatic vs achromatic stimuli

Abstract

Aims: To compare the responsiveness of cortical areas to stimulation of the two cone opponent systems and the achromatic system. The appropriate contrast metric for the comparison of color and luminance sensitivity is unknown and so we made the comparison using both a perceptual match between stimuli (in multiples of detection threshold), a direct contrast match (in cone contrast).Methods: MR images were acquired on a 4T Bruker MedSpec. Stimuli were sinewave counter-phasing rings (2Hz, 0.5cpd) and cardinal for the selective activation of the L/M cone-opponent (RG), S cone-opponent (BY) and achromatic (Ach) systems. In different scans, stimuli were matched either at 25x detection threshold (determined psychophysically), or in cone contrast (6%). Both stereotaxic and a region of interest analysis on early visual areas (V1-V4v, and hMT+) were performed, with the latter identified in separate scanning sessions.Results: In extra-striate visual cortex two different response profiles were observed, differential for chromatic and achromatic stimuli. These profiles were independent of the metric by which the stimuli were equated. Two loci in dorsal and lateral occipital cortex, corresponding to areas V3A and hMT+, had a markedly stronger response to achromatic than to chromatic stimuli. This is in contrast to a region in the ventral occipital cortex (beyond V4v) that exhibited stronger fMRI signals to chromatic compared to achromatic stimuli.Conclusions: In areas V3A and hMT+, a selective response to achromatic stimuli was observed, suggesting a predominantly magnocellular input in these motion-sensitive areas. On the other hand, a region in ventral occipital cortex was activated more by chromatic than achromatic stimuli, which may reflect a predominant parvocellular input. These two types of response profiles are independent of the metric used to equate the stimuli. Data on V1-V4v do not follow these patterns and are discussed in a related abstract (Mullen et al.).