Colors in the Brain – Brains in Colors

Nouchine Hadjikhani

NMR Center, Department of Radiology,
Massachusetts General Hospital

Several neurological and neuroimaging studies have suggested the presence of a color-selective area in human inferior occipital cortex. It has been proposed that this area is homologous to macaque area V4, which also maybe color selective. In humans, we compared the location of this color-selective region to the retinotopically defined areas in the same subjects, using fMRI and cortical flattening techniques. We found that the color selective area coincides with a crudely retinotopic area (V8) which includes a distinct representation of the fovea as well as both upper and lower visual fields. Throughout visual cortex, we found that color stimuli produced relatively more activity towards the foveal representation. This is consistent with color processing by cone photoreceptors which are concentrated in the central retina. These results lead to a second experiment, in which we used equiluminant color-varying gratings in both scotopic and photopic conditions: in the scotopic condition, in which the stimuli appeared achromatic, we found much less activity in all foveal representations, including V8. Finally, we studied the location and the time course of activity produced by a color afterimage illusion, and found that V8 shows activity during this illusion that coincides with the location of the functional responses to a real colored stimulus. In conclusion, we found that there is an area anterior to V4v, which is distinct from it by both its half visual field representation and its own foveal representation. The results of these different experiments, which all show a preferential activation of V8 during color vision compared to luminance or scotopic conditions, demonstrate the role of V8 in the processing of color in the human brain. In a second set of study we looked at the cortical projection of the two basic photoreceptors of the retina, the rods and the cones. Using a single stimulus viewed at different light levels, we were able to segregate rod- and cone-mediated visual input at scotopic and photopic conditions. Scotopic activation was selectively absent in the foveal representation of lower tier areas, and in the cortical area V8, selectively activated by colored stimuli.

The lecture will take place:

in the Lecture Hall, Room 203, 44 Cummington St.
on Wednesday, April 19, 2000
at 1:00 pm