Pathways/streams

The analysis of visual stimuli that begins in V1 and V2 continues through two major cortical systems for processing visual information:

What

The basic function of the ventral stream seems to be to let us consciously perceive, recognize, and identify objects by processing their “intrinsic” visual properties, such as shape and color.

The ventral stream is involved in forming conscious representations of the identity of objects.

The “What” stream gets its input primarily from the small ganglion cells in the fovea

Where

The basic function of the dorsal visual pathway seems to be to let us exercise visual-motor control over objects by processing their “extrinsic” properties—the ones that are critical for handling them, such as their size or their position and orientation in space.

The main function of the dorsal visual pathway is to guide in real time the actions that we direct at objects in the visual world. Most of the processing done by this pathway is believed to be unconscious. The dorsal pathway could thus be described as an “action pathway”, because by integrating the spatial relationships between our bodies and our environment, it lets us interact with this environment effectively.

In the cortical areas that contribute to the ventral system, increasingly complex, specialized representations of the outside world are elaborated.

The Action/Where stream gets its input primarily from the large ganglion cells in the peripheral retina.

Mapping the visual information

There is a correspondence or “mapping” between the arrangement of the elements of the visual field as they strike the retina and their arrangement on the surface of the visual cortex. This mapping onto the visual cortex is called retinotopy, because it is the retina that serves as the reference for the cortical maps of the various visual areas.

In retinotopic maps, the zone of greatest discrimination in the retina—the fovea, a small area at its center—is represented by a disproportionately large area on the cortex. The center of the visual field, covered by the fovea, occupies as large an area of the primary visual cortex,as the rest of the retina.

As visual information arrives at the visual cortex (V1), it is initially mapped as it was in the retina.  As processing continues, the organization gradually shifts to a functional one. As the signals proceed to higher centers, the retinotopic organization is replaced with an organization that reflects perceptual concepts and ultimately intended actions.  As this happens, the number of concepts is reduced, but their content increases.  Consider looking at a picture of the house you grew up in; what started out as a 100 Mpixel retinal image, is reduced to 1 MB in the optic nerve and ends up as a single concept “my old house” that is connected to an enormous web of information and memories, almost all of which is not part of the picture you looked at.

See Our mental model of the world.