We have used positron emission tomography to investigate the cortical areas
of the normal human brain involved in processing inward (Expansion) and ou
tward (Contraction) radial motion simulated with an optic flow stimulus. Th
e optical flow display was made out of dots moving radially away from or to
ward the center of the display monitor. In the Control condition, the dots'
motion was randomized in order to remove any sensation of radial motion. I
n the Expansion condition, several loci of activation were observed: visual
areas V2-V3 and the superior parietal lobule (BA 7), predominantly in the
right hemisphere. In the Contraction condition activation sites were found
in the same visual areas (V2 and V3) in the right hemisphere but the increa
se in rCBF in these regions was much lower than in the Expansion condition.
BA 7 was activated in both hemispheres. When the motion component of the s
timulus was isolated by subtracting the static condition from the incoheren
t motion condition, we obtained activations of areas V2, V3, and MT (putati
ve V5). These results indicate that the detection of radial motion derived
from an optic flow stimulus is mediated by structures forming the dorsal pa
rt of the visual cortical system and confirm that area MT is not specifical
ly involved in flow analysis. (C) 2001 Academic Press.