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To investigate the processing of visual form in human cerebral cortex, we used the PET (positron emission tomography) activation technique to compare the human brain regions that are involved in a visual detection task and two orientation discrimination tasks: the temporal same-different (TSD) task, which includes a short-term memory component, and the identification (ID) task, which is without this component. As a control task we used passive viewing. Stimuli were identical in all four tasks. Subtraction of passive viewing from detection showed that the detection task activates early visual cortical regions (areas 17/18) as well as several motor brain regions, while decreasing activity in several higher order frontal, temporal, and parietal regions. Comparing the ID task to the detection task revealed no further visual cortical activation, while comparison of the TSD task to the detection task revealed an activation of several right visual cortical regions, one of which remained signifcant after the subtraction of ID from TSD (right area 19). These experiments demonstrate the task dependence of visual processing, even for very closely related tasks, and the localization of the temporal comparison component involved in orientation discrimination in human area 19.Previous positron emission tomography (PET) studies of visual processing have made use of stimuli with different attributes to reveal the function of different cortical areas (1-4). The opposite approach, in which the stimulus attribute is held constant while the task-dependent computations to be performed on the visual attribute are manipulated, has yet to be followed, although it has been used in language studies (5) and studies using complex (multidimensional) visual stimuli (4). Yet the task dependency is implied by the seminal hypothesis of Ungerleider and Mishkin (6), proposing that stimulus localization and identity are processed along different visual pathways. Furthermore, task dependency could be a key factor in explaining the existence of the large number of visual cortical areas in the primate brain (7), along with specialization for attributes (3,(8)(9)(10).Several lines of evidence have suggested that comparison of different discrimination tasks in which the subjects use a single attribute, here orientation ofa static visual pattern, can be used to explore the task dependency of visual processing. The discrimination tasks to be contrasted have to differ in computational complexity, as is the case for the temporal same-different (TSD) task and the identification (ID) task. In the former task, subjects have to base their discrimination on the comparison of stimuli presented successively in time,The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. whereas in the latter task, subjects are confronted with a single stimulus that may or may not be the reference. Since the attribute on ...

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