Corollary Discharge

Vision

Well-understood corollary systems reside in the visual and eye movement networks of vertebrates, particularly in primates. A common visuomotor behavior of the primate that requires corollary discharge is the saccade, a fast eye movement that occurs about 2 to 3 times per second as an animal scans a scene. Saccades are beneficial because they permit rapid relocation of the fovea, but are costly because each saccade results in two unfortunate consequences for the visual system: an intrasaccadic smearing of the image and a trans-saccadic displacement of the image across the retina. With each saccade, the world should appear as both blurry and jumpy because this is what the retina is actually reporting. Instead, the world appears focused and stable despite the retinal report because the visual system receives advance warning of each saccade in the form of a corollary discharge.

Corollary discharges of saccadic eye movements emerge from oculomotor structures that span virtually all levels of the nervous system and impinge on visual areas throughout the brain, early and late, subcortical and cortical. At their point of termination, the corollary discharges interact with recipient sensory areas to minimize the effects of the eye movement-induced sensory inputs. For the case of retinal blur, the corollary discharges participate in a mechanism known as saccadic suppression. Neurons participating in saccadic suppression are transiently inhibited by corollary discharges at the time of the saccade, thus reducing the amount of visual information they convey and ultimately the amount of blur that is perceived. As [Page 326]for the displacement problem, one putative compensatory mechanism involves neurons that shift their receptive fields before each eye movement. Instructed by corollary discharge, these neurons sample a new part of the visual field before the eyes begin to move. By sampling the same portion of space both before and after the eye movement, shifting receptive fields effectively seem to test whether the external world moves during the sac-cade. If the presaccadic and postsaccadic samples match, the world is judged as stable. The corollary discharge that shifts receptive fields, at least those of neurons in the frontal cortex, is known to arise from the midbrain superior colliculus.

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