Color signals in the primary visual cortex of marmosets

This study concerns the input from short-wavelength sensitive (S) cone photoreceptors to the primary visual cortex (striate cortex, Brodmann area 17, area V1) in marmosets. Signals from S-cones are thought to reach V1 by way of the koniocellular layers of the dorsal lateral geniculate nucleus. However, it is not known whether the S-cone afferent signals cause selective activation of cytochrome oxidase-rich cortical "blob" domains. To address this question, intrinsic optical signals and extracellular responses of V1 neurons were recorded. Stimuli consisted of drifting achromatic gratings and gratings that stimulated selectively either the S-cones or the medium-long wavelength sensitive (ML) cones. All stimuli produced contrast-dependent activation throughout the imaged regions of V1. The S- and ML-cone-selective stimuli produced activation levels of respectively 30% and 80% of that to achromatic gratings. No spatial variation in the strength of S-cone activation was apparent, and the ratio of S to ML activation was constant across all imaged regions. Consistently, in all of the single neurons recorded from V1, the functional input from S-cones was weaker than the input from ML-cones. We conclude that in the primary visual cortex of marmosets, S-cone signals are uniformly distributed.