1 104 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



II I 



I2i 



(31 



Time 



Time 



FIG. 1 1 . The improv ement in following movements made by 

 the human eye in following a target moving sinusoidally. (/) 

 Acutal motion of the target. (2) First solution of tracking prob- 

 lem. {3) Second (velocity) solution of tracking problem. {4} 

 Third (acceleration and velocity) solution. [From Stroud (130).] 



movements the subject gets very close to simple 

 harmonic movement of the eyes (130). This rapid 

 improvement with practice was also observed by 

 Dodge in 1907 (58) who watched subjects following 

 the movements of a pendulum. He pointed out that 

 if the pendulum is obscured for part of its course, 

 the eye lags and makes large saccades to catch up as 

 the pendulum reappears. Similar behavior is ob- 

 served in all tasks where the eyes must follow a mov- 

 ing target. At first the eyes lag by at least the 

 reaction time of about 0.2 sec, or nearer 0.5 sec. if 

 the stimulus falls at first on the peripheral field. If 

 the target movement is regularly repeated, its move- 

 ment is rapidly learned and the lag between eye 

 movement and target movement disappears. At this 



stage eye movement becomes a special case of the 

 problem of the human being as an operator in a 

 servo system, and the general principles described 

 by Craik (44) apply. The subject makes periodic 

 attempts to reduce the error between target and 

 fixation point, and any procedure tried out and 

 found to minimize this error, be it positional change, 

 steady moxement or velocity change, is continued 

 until further errors accumulate. 



Integration of Eye Movements 



It is remarkable that there is no coherent view on 

 the relation between reflex mechanisms originating 

 in the neck, labyrinth and eyes, which act on the 

 eye muscles. As far as the neck and labyrinth are 

 concerned, algebraic summation of their effects has 

 been suggested by Magnus. The relation of cortical 

 mechanisms to .subcortical postural mechanisms is 

 much more oljscure. It is generally held that the 

 labyrinthine mechanisms are of little importance in 

 the monkey and in man, and that cortical mecha- 

 nisiTis are so much more important that they can 

 completely compensate for the absence of those of 

 labyrinthine origin. In body posture control, Bieber 

 & Fulton (12) have suggested that vestibular mecha- 

 nisms are inh bited by the cortex normally and that 

 only after removal of the motor and premotor cortex 

 do vestibular attitudes appear in the monkey. There 

 seems to be much more to be said for an alternative 

 view that eye mo\'ements are controlled at a series 

 of levels in the central nerxous system in the Jack- 

 sonian sense, but that at each level some degree of 

 stabilization is attained, a degree of stabilization 

 which is essential if the commands of the next higher 

 level are to he effectively carried out. 



After complete destruction of the \cstibular ap- 

 paratus, there mav lie a considerable degree of corti- 

 cal compensation, especially in young subjects, but 

 the reaction to sudden passive movement is seriously 

 and permanently defective. Some patients, in whom 

 the vestibular fibers of the eighth nerve were bi- 

 laterally destroyed by Dandy, complained that 

 their \ision was "jumbled' while they were pushed 

 over rough ground in a wheel chair, although at 

 rest vision was perfectly normal. While being pushed 

 along a hospital corridor they were unal^le to recog- 

 nise the faces of friends (68). In the history of a pa- 

 tient who apparently developed bilateral and isolated 

 degeneration of the vestibular nuclei, one of the 

 earlier symptoms was difficulty in drixing a car 



