HUMAN BRAIN-REGIONS INVOLVED IN DIRECTION DISCRIMINATION

To obtain further evidence for the functional specialization and task- dependent processing in the human visual system, we used positron emis sion tomography to compare regional cerebral blood flow in two directi on discrimination tasks and four control tasks. The stimulus configura tion, which was identical in all tasks, included the motion of a rando m dot pattern, dimming of a fixation point, and a tone burst. The disc rimination tasks comprised the identification of motion direction and successive direction discrimination. The control tasks were motion det ection, dimming detection, tone detection, and passive viewing. There was little difference in the activation patterns evoked by the three d etection tasks except for decreased activity in the parietal cortex du ring the detection of a tone. Thus attention to a nonvisual stimulus m odulated different visual cortical regions nonuniformly. Comparison of successive discrimination with motion detection yielded significant a ctivation in the right fusiform gyrus, right lingual gyrus, right fron tal operculum, left inferior frontal gyrus, and right thalamus. The fu siform and opercular activation sites persisted even after subtracting direction identification from successive discrimination, indicating t heir involvement in temporal comparison. Functional magnetic resonance imaging (fMRI) experiments confirmed the weak nature of the activatio n of human MT/V5 by successive direction discrimination but also indic ated the involvement of an inferior satellite of human MT/V5. The fMRI experiments moreover confirmed the involvement of human V3A, lingual, and parietal regions in successive discrimination. Our results provid e further evidence for the functional specialization of the human visu al system because the cortical regions involved in direction discrimin ation partially differ from those involved in orientation discriminati on. They also support the principle of task-dependent visual processin g and indicate that the right fusiform gyrus participates in temporal comparison, irrespective of the stimulus attribute.