Human Brain Regions Involved in Direction Discrimination

Cornette, L., P. Dupont, A. Rosier, S. Sunaert, P. Van Hecke, J. Michiels, L. Mortelmans, and G. A. Orban. Human brain regions involved in direction discrimination. J. Neurophysiol. 79: 2749–2765, 1998. To obtain further evidence for the functional specialization and task-dependent processing in the human visual system, we used positron emission tomography to compare regional cerebral blood flow in two direction discrimination tasks and four control tasks. The stimulus configuration, which was identical in all tasks, included the motion of a random dot pattern, dimming of a fixation point, and a tone burst. The discrimination tasks comprised the identification of motion direction and successive direction discrimination. The control tasks were motion detection, dimming detection, tone detection, and passive viewing. There was little difference in the activation patterns evoked by the three detection tasks except for decreased activity in the parietal cortex during the detection of a tone. Thus attention to a nonvisual stimulus modulated different visual cortical regions nonuniformly. Comparison of successive discrimination with motion detection yielded significant activation in the right fusiform gyrus, right lingual gyrus, right frontal operculum, left inferior frontal gyrus, and right thalamus. The fusiform and opercular activation sites persisted even after subtracting direction identification from successive discrimination, indicating their involvement in temporal comparison. Functional magnetic resonance imaging (fMRI) experiments confirmed the weak nature of the activation of human MT/V5 by successive direction discrimination but also indicated 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 provide further evidence for the functional specialization of the human visual system because the cortical regions involved in direction discrimination partially differ from those involved in orientation discrimination. They also support the principle of task-dependent visual processing and indicate that the right fusiform gyrus participates in temporal comparison, irrespective of the stimulus attribute.

Stimulus Comparison Strategies and Task Demands in Successive Discrimination

This research isolates two distinct strategies used to identify stimuli presented at different rates in successive discrimination tasks. The short-term strategy (used at rapid rates) compares current stimuli to immediately prior stimuli present in sensory or other short-term storage, whereas the long-term strategy (used at slow rates) compares current stimuli to a standard stimulus in long-term memory. In Experiment 1, subjects detected a medium-duration tone amid both long and short distractors. Analysis of false alarms indicated that strategy choice is strongly influenced by event rate, and therefore by the presence or absence of sensory traces of preceding stimuli. Experiment 2 was designed to force subjects to use the short-term strategy (to detect targets, present stimuli had to be compared to their immediate predecessors), and varied the event rate. Subjects were able to maintain a high level of performance throughout the task only at the fastest event rate. This pattern of results suggested that when the task demanded it, subjects could use a particular strategy, but if the event rate (and consequently the availability or otherwise of relevant memorial traces) was not favourable to that strategy, then performance was disadvantaged.