Brain Systems

In order to understand visual perception, it is useful to consider some of the major brain systems (see Gazzaniga, Ivry, & Mangun, 1998, or Tovee, 1996). It is important to note that an oversimplified view is presented here. There are more than 30 visual areas in the cortex, and over half of the area of the cortex responds to visual stimuli. Goldstein (1996, p. 97) provided a useful overview of the visual system:

As we travel farther from the retina, neurons require more specific stimuli to fire. Retinal ganglion cells respond to just about any stimulus, whereas end-stopped cells respond only to bars of a certain length that are moving in a particular direction .. .this specialisation increases even further as we move into other visual areas of the cortex.

The great majority of ganglion cells in the primate retina are M (magnocellular or large-bodied) and P (parvocellular or small-bodied) cells. The axons of these ganglion cells come together to form the optic nerve, which projects to the lateral geniculate nucleus. The lateral geniculate nucleus is organised into six layers, each of which receives input from one eye. Layers 1 and 2 receive inputs from large M ganglion cells, whereas layers 3-6 receive inputs from the smaller P ganglion cells.

Some indication of the functions of the lateral geniculate nucleus was obtained by Schiller, Logothetis, and Charles (1990). They destroyed parts of the magno or parvo layers in monkeys. Magno lesions greatly impaired movement detection, whereas parvo lesions produced loss of the ability to perceive colour, fine textures, and detailed objects.

"Where" pathway

"Where" pathway

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