Where and what streams

The parallel processing beyond V5 and V4 answers from the segregation of information into the two streams. The dorsal stream, largely from MT, goes to the MST (i.e., medial superior temporal) and PP (i.e., posterior parietal) cortex. The PP cells display selectivity for size and orientation of objects, their firing based on where an animal is looking and they fire as an animal grasps an object. The lesions to MST and PP in primates outcomes in optic ataxia, in which the visuospatial tasks are profoundly affected, however object recognition is not affected.

By contrast, the ventral stream from V4 to the inferotemporal cortex (IT) is vital for object recognition. The IT cells are sensitive to form and color but are relatively unfussy about retinal position object size, or orientation. Many respond selectively to particular objects like hands or faces. Unusually for visual cortex, the IT region has no retinotopic map. The lesions of the IT cortex cause visual agnosia in which the animals fail at tasks requiring object recognition. The Visuospatial tasks are not affected. Since of their various functions the dorsal and ventral streams are frequently referred to as where and what streams correspondingly. The clinical data suggests that a similar dichotomy also exists in humans.

The bilateral loss of V1 causes total loss of visual perception. Though, there are primates and humans with this damage who are capable to avoid obstacles whilst moving via space is much better than by chance. This phenomenon is known as blindsight. The humans with it report that they are totally unaware of the visual world and do not understand how they are capable to navigate via space. The blindsight might be mediated by a pathway which goes directly from the magnocellular LGN to the thick stripe of V2. This gives input to the where system. The implication of this situation is that V1 is needed for conscious visual perception.