Color signals in human motion-selective cortex.

The neural basis for the effects of color and contrast on perceived speed was examined using functional magnetic resonance imaging (fMRI). Responses to S cone (blue-yellow) and L + M cone (luminance) patterns were measured in area V1 and in the motion area MT+. The MT+ responses were quantitatively similar to perceptual speed judgments of color patterns but not to color detection measures. We also measured cortical motion responses in individuals lacking L and M cone function (S cone monochromats). The S cone monochromats have clear motion-responsive regions in the conventional MT+ position, and their contrast-response functions there have twice the responsivity of S cone contrast-response functions in normal controls. But, their responsivity is far lower than the normals' responsivity to luminance contrast. Thus, the powerful magnocellular input to MT+ is either weak or silent during photopic vision in S cone monochromats.