Nouchine Hadjikhani
NMR Center, Department of Radiology,
Massachusetts General Hospital
will speak on
Colors in the Brain - Brains in Colors
Abstract:
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
Hosted by the
Brain and Vision Research Laboratory