THE CEREBRAL-ACTIVITY RELATED TO THE VISUAL-PERCEPTION OF FORWARD MOTION IN-DEPTH

We have used the technique of PET to chart the areas of human cerebral cortex specifically responsible to an optical pow stimulus simulating forward motion in depth over a pat horizontal surface. Tile optical f low display contained about 2000 dots accelerating in radial direction s away from the focus of expansion, which subjects fixated at the cent re of rite display monitor Dots remained of constant size, but their d ensity decreased from the horizon, lying across the middle of the scre en, to the foreground at the lower screen margin; the top half of the display was void. For the control stimulus the dot motions were random ized removing any sensation of motion ill depth and diminishing the im pression of a pat terrain. Comparison of the regional cerebral blood p ow (rCBF) elicited bt the optical pow and control stimuli was thus int ended to reveal any area selectively responsive to the radial velocity field that is characteristic of optical pow in its simplest natural f orm. Six subjects were scanned, and analysed as a group. Four subjects were analysed as individuals, their PET data being co-registered with MRIs of the cerebrum to localize rCBF changes to individual gyri and sulci. There were three main areas of activation associated with optic al flow: the dorsal cuneus (area V3) and the latero-posterior precuneu s (or superior parietal lobe) in the right hemisphere, and the occipit o-temporal ventral surface, in the region of the fusiform gyrus, in bo th hemispheres. There was no significant activation of V1/V2, nor of V 5. These results show that higher stages of motion take place in both the 'dorsal' and 'ventral' visual pathways, as these are commonly conc eived and that both may be fed by area V3. The information potentially derivable from optical flow concerns the direction of heading, and th e layout of the visual environment, a form of three-dimensional struct ure from-motion. The perceptual division of labour between the various activated areas cannot be directly, inferred though it is a reasonabl e supposition that the parietal activation reflects the utility of opt ic pow for guiding self-motion.