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Stereopsis
Ophthalmologists and visual scientists are fully aware that the neurophysiological basis of stereopsis depends on the existence of binocular disparity receptors at a cortical level. However, no discrete “stereopsis centre” has been found similar to those for other visual functions such as colour and motion detection. Instead, disparity receptors are found in several regions of the visual cortex such as V1, V2, V3, and in other areas such as MT (middle temporal area) (V5) and MST (medial superior temporal area). This raises the question as to the nature of the role of disparity receptors in each of these areas and also indicates that identification of disparity receptors does not automatically mean that they are involved in depth perception. They could have other functions related to their original location in the visual cortex. Now, a recent report in the journal Nature (1998;394:677–80) has shown that perception of stereoscopic vision and electrical activity in disparity selective neurons in area MT do in fact correlate—that is, that these particular disparity selective neurons carry information concerning depth perception. The studies were carried out in monkeys who were trained to perceive depth under specific conditions during which neuronal electrical activity was recorded from MT neurons stimulated by a moving random dot stimulus covering their receptive fields while fixation was maintained on a small spot. The authors are quick to point out that correlation between MT neurons and stereopsis does not exclude neurons in other centres from similar function.
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