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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



movcnient of tlie bod\', it is possiljle for the fixation 

 reflex to operate to kec[) the target on the fovea. 



When the stabihzation by the labyrinth breaks 

 down, as in side-to-side movements of the head 

 faster than q per sec, then the visual field will appear 

 to move and visual acuity goes down. The same 

 arguments concerning stabilization at each level 

 may ije applied to the cortical control of eye move- 

 ment. The occipital cortex, occipitocollicular tract 

 and superior coUiculus form a system by which an 

 image of a point in the periphery is brought to the 

 fovea and held there. If the image wanders off the 

 fovea, appropriate correcti\e eye movements return 

 it to the fovea again. Very rarely, this mechanism 

 can be interrupted without cortical blindness by 

 lesions of the occipitocollicular tract in the pul- 

 vinar (83). The patient then can move his eyes to 

 command but cannot retain 'visual grasp' of objects. 



On the other hand the fixation mechanism can be 

 inhibited by turning the attention to other objects 

 or as a result of voluntary movement of the eyes. 

 This requires the activity of the parietal region and 

 probably of the frontal eye field. In lesions of these 

 areas the patient is able to fixate on an object but 

 has great difficulty in 'letting go' of it and may have 

 to blink to change his fixation point. The highest 

 center must be able to enhance or inhibit the ac- 

 tivity of subcortical reflexes, as Graham Brown 

 most clearly pointed out; in order that both head 

 and eyes may be moved to the side, the upper frontal 

 area for eye movement must be able to inhibit the 

 subcortical orientation reflex which would otherwise 

 keep the visual areas fixed in space while the head 

 turns. The relation of the cerebellum to these levels 

 of organization of eye movement is still a subject 

 for speculation. The existence of cerebellar nystag- 

 mus is denied by some but affirmed by Holmes (84) 

 who holds that it is exaggerated during attempted 

 fixation whereas vestibular nystagmus is more marked 

 when fixation is made impossible, de Kleijn & 

 Magnus (52) showed that righting reflexes persisted 

 after destruction of the cerebellum. Unfortunately, 

 this was distorted by textbook writers into the state- 

 ment that the cerebellum has nothing to do with 

 postural reflexes. Rademaker (112) pointed out that 

 movements controlled by midbrain mechanisms 

 were poorly executed after lesions of the cerebellum 

 and that its 'regulating' function applied as much to 

 movements mediated by the midbrain as by the 

 cortex. 



Evidence is accinnulating that the precise per- 

 formance of vestibulo-ocular reflexes requires cere- 



bellar assistance, and there is little doubt that 

 voluntary movement of the eyes also requires cere- 

 bellar cooperation. 



Proprioceptors (uid Sensation 



•Since \()n Helmholtz (139) it has been realized 

 that we have some knowledge of the position of the 

 eye relative to the head. This can he shown dra- 

 matically by^ wearing a 12° prism in front of one eye 

 with the other eye closed. If the hand is brought 

 quickly from behind the back to grasp at an object 

 in the visual field, the subject's hand misses the 

 target by nearly 25 cm. If, however, the hand is 

 brought up slowly, then the movements are corrected 

 and the target is attained. If, after wearing the prism 

 for some minutes, the subject removes it and repeats 

 the attempt to grasp an object, he now- misses by an 

 equal distance on the other side. It is not difficult 

 to show that this knowledge of the direction of the 

 visual axis is unaffected even if the subject can not 

 see his own nose or cheek. The judgments of 'subjec- 

 tive horizonlar and 'subjective mid-line' have long 

 been thought to depend on the activity of the eye 

 muscles, and there has been much controversy on the 

 reliability of these judgments. Bourdon (ig) found 

 subjects could be consistent to 1.5° on the hori- 

 zontal and the mid-line, but there is considerable 

 variation between subjects. Ludvigh (100) states 

 that errors up to 6° may be made but Walsh (un- 

 published observations) found this is an outside 

 figure as the standard deviation is 3 to 4°. 



Clearly then we have some knowledge of the direc- 

 tion of the visual axis. What is not clear is whether 

 this is due to knowledge of the number of motor 

 impulses reaching the eye muscles — the 'outflow 

 theorv' — or some knowledge of the afferent impulses 

 arising in the proprioceptors of the eye muscles — the 

 'inflow theory.' 



Sherrington (124) observed that three points ar- 

 ranged vertically do not appear to tilt when ob- 

 served in tertiary positions of the eyes. The images 

 of these points must be on retinal points forming an 

 angle with those originally stimulated, and he con- 

 cluded that the new interpretation implied that some 

 information about the rotation of the eyeball was 

 available to the brain and was derived from pro- 

 prioceptors. It is notewortlu that Sherrington's 

 situation is one in which the observer is called on to 

 make a perfectly familiar judgment in which the 

 position of the eyeball is an essential feature. It is not 



