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University of Toronto Scarborough
Matthias Niemeier

Neuronal mechanisms of visual stability Robert H Wurtz INTRODUCTIONoccurrence of rapid or saccadic eye movements recognized widely by Alfred Yarbusrecorded shifts of eye while subject looked at a visual scenefound hundreds of saccades that rapidly move the fine grained receptors of fovea from pt to pt in visual scenea series of schematic saccades visual motor sequence has 2 phases 1 fixation at a particular location when virtually all vision occurs and 2 rapidly moving saccades when virtually no vision occursvisual system consequently receives series of high resolution snapshots centered on diff points in the scenesnapshots transmitted up optic nerve with successive fixations all centered on fovea no info in this transmission abt where in scene snapshot is locatedinterspersed amongst the snapshots are blurred images of the wide field motion produced by rapidly moving eye during saccade white patchessince 1600stopic is how our sense of visual stability survives such disruptions of visual inputonly since techniques became available to record neuronal activity in brains of awake animals able to move their eyes has it been possible to look at brain mechanisms underlying this stable visual perception2 major questions o 1 displacement of retinal image resulting from the saccade o 2 suppression of the blurmotion during the saccade Signals that contribute to visual stabilityonly 3 signals and the interactions bw them are used to make the distinction bw real world movementeye movemento first signal visual reafference from retinaeye movement generates visual motiondisplacement signals that can indicate eye movement o second signal an extraretinal onewhen eye muscles contract proprioceptors are activatedthese indicate the eye has movedthis proprioception is an inflow signal from the periphery into brain just as is visual input o third signal another extraretinal onea corollary or copy of the command to move the eyes referred to as an outflow signal bc it copies what flows out to activate eye muscles21 Visual Reafferencewhole visual field moves with eye movement this optic flow can be taken as an indication of self motion rather than motion in environment this visual inputreferred to as reafference in recognition of its self generated nature in contrast to the exafference which results from motion in real world in front of a stationary eyew saccadesfull field motion usually blurred bc of their rapidity but a visual cue remains bc whole field is displaced for slower eye movements such as w movement of subject thru environment such optic flow is criticaloptic arrayregarded as central factor for stability but for saccades the contribution is probably minimal as indicated by evidence presented below related to corollary discharge22 Proprioceptioneye position info from eye muscle proprioceptors could flow into brain after eye movesinteract with visual input to produce visual stabilityproprioception does not make a substantial contribution to perceptual stability critical test to artificially change position of eye by electrically stimulating brain while monkey was preparing a saccade to a visual target after stimulation the saccade had to be made to the same visual target but from a new positionIf proprioception provides position information every time stimulation changed the initial eye position proprioceptive feedback should have indicated that new position subsequent visually guided saccade shouldve compensated for the change in position
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