The dorsal stream is a dominant visuomotor pathway that conappropriate motor output. Since the late 1960s two major nects the striate and extrastriate cortices to posterior parietal areas. visual pathways have been implicated in the spatial control In turn, the posterior parietal areas send projections to the frontal of movement. As originally defined these two systems were primary motor and premotor areas. This cortical pathway is hyhypothesized to consist of the retinal projection to the supepothesized to be involved in the transformation of a visual input rior colliculus for localization of stimuli in space and the into the appropriate motor output. In this study we used functional geniculostriate system for the identification of stimuli (1). magnetic resonance imaging ( f MRI) of the entire brain to deter-Although this initial hypothesis was an oversimplification, mine the patterns of activation that occurred while subjects perthe distinction between object identification and spatial loformed a visually guided motor task. In nine human subjects, calization-between ''what'' and ''where''-has persisted. f MRI data were acquired on a 4-T whole-body MR system Ungerleider and Mishkin (2) mapped this functional dichotequipped with a head gradient coil and a birdcage RF coil using a T * 2 -weighted EPI sequence. Functional activation was deter-omy onto two diverging projections or ''streams'' from the mined for three different tasks: (1) a visuomotor task consisting striate cortex. Based on experimental work in primates, these of moving a cursor on a screen with a joystick in relation to various authors proposed that appreciation of an object's qualities targets, (2) a hand movement task consisting of moving the joyand of its spatial location depends on the processing of difstick without visual input, and (3) a eye movement task consisting ferent kinds of visual information. The former involves striof moving the eyes alone without visual input. Blood oxygenation ate and extrastriate projections into the inferior temporal level-dependent (BOLD) contrast-based activation maps of each cortex, the ''ventral stream,'' and the latter striate and extrasubject were generated using period cross-correlation statistics. striate projections into the posterior parietal cortex, the ''dor-Subsequently, each subject's brain was normalized to Talairach sal stream.'' Many recent positron emission tomography coordinates, and the individual maps were compared on a pixel (PET) studies of the visual system are based on this twoby pixel basis. Significantly activated pixels common to at least cortical-stream theory, attempting to differentiate between four out of six subjects were retained to construct the final funcobject identification and object localization (3-5). tional image. The pattern of activation during visually guided movements was consistent with the flow of information from stri-
In Ungerleider and Mishkin's model, the distinction beate and extrastriate visual areas, to the posterior parietal complex, tween the two pathways is based primarily on perceptual and then to frontal motor areas. The extensive activation of this aspects of different stimulus attributes. Recently, the distincnetwork and the reproducibility among subjects is consistent with tion between the ventral and dorsal streams has been apa role for the dorsal stream in transforming visual information proached rather differently. Milner and Goodale (6, 7) suginto motor behavior. Also extensively activated were the medial gest that two different visual systems have evolved in the and lateral cerebellar structures, implicating the cortico-pontoprimate: (1) a visuomotor system associated with the dorsal cerebellar pathway in visually guided movements. Thalamic actistream that transforms visual information into action and vation, particularly of the pulvinar, suggests that this nucleus is
(2) a perceptual system associated with the ventral system an important subcortical target of the dorsal stream. ᭧ 1998 Academic that is involved in off-line object recognition and is indirectly Press linked to action via cognitive processes. Within this concep-