"What" and "where" streams
 

• A combination of anatomical, physiological, and lesion studies suggest that visual cortex is organized into two major functional streams - a so-called "what" pathway proceeding ventrally and a so-called "where" pathway proceeding dorsally.

 
• The "what" pathway is involved in processing information about object shape and form, independent of location, movement, etc. Cells along this pathway are selective to aspects of object shape (area V4, IT complex). Lesions to these areas result in visual agnosia, the inability to properly identify objects.

 
• The "where" pathway is involved in processing information about spatial relationships and motion of objects in the environment. Cells along these pathways are selective to object motion independent of object shape (areas MT, MST), or to location relative to the position of the eyes (area 7a). Lesions to these areas result in the inability to judge spatial relationships, as well as spatial neglect in which the subject ignores one half of visual space.

 

Modular/hierarchical organization
 

• The visual cortex of the macaque monkey contains at least 30 distinct areas within it. These areas can be roughly arranged in a hierarchical fashion, according to how many synaptic stages they are removed from direct visual input.

 
• Topographic organization and response properties change as one moves to higher areas within the visual cortical hierarchy. For example, the lowest level in the hierarchy, area V1, contains a clear topographic map of visual space and cells respond to local image features such as the orientation of luminance edges. But as one proceeds towards higher levels of the hierarchy, the topographic organization becomes fuzzier (i.e., receptive fields become large and unordered according to position), and neurons respond to more global and complex image features such as curves, complex motion, and faces.

 
• In addition to the hierarchical organization, there appears to also be a somewhat modular organization, in which different areas are specialized for different aspects of visual processing. For example, area MT contains many cells tuned for motion but not for shape or color, whereas area V4 contains neurons tuned for complex shape and color but not for motion. Areas within the inferotemporal complex (IT) contain neurons that are selective for complex shapes such as hands and faces. These areas are known to be important for object recognition. Areas within the posterior parietal complex (PP) contain neurons with very large receptive fields that have little or no feature selectivity. Some of these neurons, such as those in area MST, respond to complex optic flow patterns, such as dilation and contraction, or rotation. Others of these neurons, such as those in area 7a, are modulated by eye position and are thought to be important in forming a head-centered or body-centered representation of space. It should be emphasized, though, that what role these different areas of the visual cortex play in perception is only roughly known. Obtaining a full understanding the computations carried out in these areas lies many years ahead.