CENTRAL CONTROL OF EVE MOVEMENTS 



II07 



possible to dismiss this observation as contaminated 

 by ideas of verticality from the edges of the projec- 

 tion area, since afterimages of three vertical points 

 observed in the primary position do appear tilted 

 when the eye moves to a tertiary position. 



Sherrington's interpretation however has not been 

 generally accepted. Most ophthalmologists have fol- 

 lowed von Helmholtz who wrote at a time when 

 proprioceptive mechanisms w^ere not understood, and 

 the longstanding confusion over their existence in 

 the eye muscles strengthened the belief that pro- 

 prioceptors were unnecessary. 



It is quite clear that we have little or no direct 

 conscious knowledge of the movements of our eyes. 

 The most striking evidence is the surprise which 

 greeted the first observations of saccadic movement 

 in reading, movements of which the subject is com- 

 pletely unaware. The knowledge that the eye move- 

 inent is jerky is irrelevant to perception in reading. 



It has long been reported that an attempt to move 

 the eye by means of a paralyzed muscle, for example 

 attempting to move the right eye to the right with 

 paralysis of the right lateral rectus muscle, results in 

 apparent displacement of the visual field to the right. 

 The objection that the covered normal eye has made 

 a mov'ement can be met by studying patients with 

 one eye completely immobile and with paralysis of 

 one muscle of the other (8g). Attempts to move the 

 eye in the direction of its paralyzed muscle still give 

 rise to apparent movement of the visual field. The 

 same results follow attempts to move the eyes after 

 paralyzing or weakening the muscles with cocaine 

 (92) or tubocurarine (75; \Valsh, unpubli.shed ob- 

 servations). 



These experiments support the outflow theory 

 rather than the inflow theory. It is not unreasonable 

 to suggest that, when we start a voluntary movement, 

 we expect the visual field to change in accordance 



with the eye movement and prepare to interpret 

 the retinal data accordingly. Sherrington's argu- 

 ments for "inflow' can perhaps be interpreted in 

 terms of monitoring of the outflow to the muscles in 

 the tertiary positions of the eyeball. Others who have 

 ascribed some importance to the eye muscles in 

 judgment of visual space, e.g. Tschermak and his 

 pupils, have used the phrase "myosensory tension' 

 and have avoided taking sides in the controversy 

 (140). 



The proprioceptors in the eye muscles then can- 

 not be shown to play any part in the conscious ap- 

 preciation of eye movement and do not seem to be 

 responsible for any form of stretch reflex. They may 

 modify adversive movements initiated by retinal 

 stimuli, and the muscle spindles in man, one may 

 speculate, may play a part in maintaining fixation 

 or at least in signaling the eye movements in fixation 



One possible role for muscle spindles, i.e. endings 

 the sensitivity of which can be changed by the cen- 

 tral nervous system by altering the gamma efferent 

 discharge to the intrafusal muscle fibers, is to signal 

 small movements when a target is being fixated. It 

 is possible that the discharge to the intrafusal fibers 

 is increased whenever fixation occurs. In this way 

 the sensitivity of the spindles could be increased at 

 any point in the whole range of movements available 

 to the eyeball. Such a mechanism would be more 

 sensitive for the detection of small movements than 

 for signaling the position of the eyeball relative to 

 the head. In the goat the eye muscles can probably 

 signal a movement of 0.5° per sec. (36). This is ap- 

 proximately the minimal movement which can be 

 detected by the human eye without a fixation point 

 (19). The sensitivity of detection of eye movements 

 should be as great as this, since to be sure that a 

 target has moved upwards on a blank field, one must 

 be sure that the eve has not moved down. 



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