Control of Vergence and Accommodation

The supraoculomotor region of the midbrain is directly involved in the control of the movement of the two eyes in opposite directions, i.e., vergence. This region is located just dorsal and lateral to the oculomotor nuclei. These neurons provide a direct, ipsilateral projection to the medial rectus subdivision of the oculomotor complex. In addition, there may also be a projection to the Edinger-Westphal nuclei on the same side, and this could account for the accommodative responses found in Edinger-Westphal neurons. The supraoculomotor region receives direct projections from the interposed nuclei of the cerebellum as well as from portions of the frontal and parietal lobes of the cerebral cortex. The cerebral regions that are known to project to this region include the prearcuate frontal cortex (area 8), area LIP, and areas MT and MST of the temporal-parietal lobes. The neurons in this supraoculomotor region are termed near-response cells because they increase their firing rate for near viewing, but not during vertical or horizontal conjugate eye movements. In addition, most of these neurons were tested in tasks that could distinguish vergence from accommodation. These neurons fired in association with changes in both vergence and accommodation. A small subset of the supraoculomotor neurons was sensitive solely to accommodation or vergence.

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