HUMAN BRAIN ACTIVITY RELATED TO SPEED DISCRIMINATION TASKS

The regional cerebral blood-flow (rCBF) pattern of the human brain was measured using positron emission tomography (PET) while subjects view ed, detected, judged the speed of a moving random dot pattern (RDP) or compared speeds of successive RDPs. In all four conditions, retinal i nput was identical. Two additional conditions, continuous presentation of a moving and a stationary RDP, were included to identify human MT/ V5 (hMT/V5). Both speed discrimination tasks involved the right cuneus and right lingual gyrus and to a lesser degree the left lingual gyrus and a more anterior lingual region in the right hemisphere. There was , however, little or no differential activity over hMT/V5 during eithe r speed discrimination. Direct comparison of the two speed discriminat ion tasks revealed higher activity in the right middle fusiform gyrus, a result reminiscent of that obtained in earlier studies using orient ation and direction as the attribute to be discriminated. These result s confirm that processing in the human visual cortex is task dependent and underscore the role of the middle fusiform gyrus in temporal comp arison of simple attributes.