Regions in the human brain activated by simultaneous orientation discrimination: a study with positron emission tomography

In order to compare regional cerebral activity involved in simultaneous as opposed to successive orientation discrimination, we used positron emission tomography to measure regional cerebral blood flow, in two threefold sets of conditions, in a large number of subjects. The first such triad involved simultaneous orientation discrimination, orientation identification and de tection, with all tasks using the same pair of gratings. The second triad c onsisted of successive orientation discrimination with its corresponding id entification and detection tasks. Comparisons between tasks within each tri ad isolate attention to orientation and, respectively, spatial or temporal comparison. The subtraction of detection from simultaneous discrimination r evealed activation of right fusiform, right lingual, left precentral, left cingulate and left temporal cortex, in addition to right insula, cerebellum and left thalamus. Only the fusiform, insular and precentral activations r emained when the corresponding identification was subtracted from simultane ous discrimination. In contrast, most of the non-visual activation sites re mained when simultaneous discrimination was compared with successive discri mination, which also revealed a left lingual activation. These experiments provide further evidence for task-dependent processing in the human visual system and suggest that the right fusiform cortex is involved in spatial as much as temporal comparisons.