Who Goes There?

Our understanding of the visual system has benefited markedly from the hypothesis that visual processing occurs in two separate streams—one for what (or who) and one for where (Ungerleider and Mishkin 1982). A dorsal processing stream mediates spatial processing through a circuit including the visual cortex (which contains the primary visual cortex and spatial processing areas, such as area MT) and the parietal cortex. A ventral processing stream mediates object processing through a circuit including the visual cortex (which contains the primary visual cortex and object processing areas, such as area V4) and the inferotemporal cortex. These two processing streams converge in the prefrontal cortex. However, in recent years, it has become clear that these two streams are not strictly parallel. For example, cells in the lateral intraparietal (LIP) area, an area associated traditionally with the dorsal stream, are also modulated by the shape of an object, a computation associated traditionally with the ventral stream (Sereno and Maunsell 1998).