CENTRAL CONTROL OF EVE MOVEMENTS 



1097 



0-6 sec 



FIG. 6. Records of the excursion of the swing in which tlie subject is seated [iarge smooth sinusoi- 

 dal trace) and of the rolling movements of the eye (smaller broken trace). Disregarding the irregular 

 jerking movements, the eye moves roughly one quarter of the amplitude of the swing and lags by 

 about 15°. In the calibration record, taken with the mica plate attached to the swing, the two 

 traces almost superimpose, showing that the sensitivity of the two records is equal and that dis- 

 tortion is inappreciable at this amplitude. The calibration scale of degrees is approximate. [From 

 Davies & Merton (49).] 



rotation stops, the eye drifts back in i to 2 sec. or 

 moves quickly back if a blink occurs. 



It is convenient to deal here with the responses 

 to rotation of the head about a sagittal axis in ani- 

 mals with forwardly directed eyes. In the decerebrate 

 cat this produces a brisk contraction alternately in 

 the superior and inferior oblique muscles. In man a 

 compensatory movement was described Ijy Mulder 

 (108) and Breuer (21) who used the movement of 

 afterimages against a ruler held by the teeth as a 

 measure of the extent of compensation. These ola- 

 servers remark that the compensatory movements 

 are considerable during head movement up to 30°; 

 but that when head movement stops, the eyes drift 

 back in the next few seconds to a displacement of 

 /^ to 1^2 of the head moveinent. Unfortunately sub- 

 sequent authors have only described the residual 

 compensation (60, 66). The subject has been rein- 

 vestigated recently by Merton (49, 107). When a 

 subject sits in a chair which swings about a ball race 

 on a level with the subject's eye, the horizontal 

 meridian of the eye remains nearly horizontal during 

 rotation of up to 30° (see fig. 6). After the movement 

 is over, the eye catches up all but about i-fo of the 

 movement, with a time constant of about i sec. 

 Visual acuity is not appreciably diininished either 

 during the movement or during the following 2 to 

 3 sec. when the eye 'catches up' with the head. 



If the sul)ject lies on his back with the optical 

 axes vertical, there is compensation during rapid 

 rotation of the whole body in the horizontal plane; 

 but after the movement is over, the eyes 'catch up' 

 and there is no residual rotation. Presumably, there- 

 fore, the residual rotation present in the upright 

 position is due to otolith activity. 



If fixation is prevented, these movements are still 

 present, but the compensation is maintained for a 

 shorter time. The eye 'catches up' in a series of jerks. 

 These reactions are absent in patients with bilateral 

 destruction of the labyrinth (Merton, personal com- 

 munication). Movements of the head about the 

 sagittal axis in animals with laterally directed eyes 

 produces contraction of the levator palpebrae su- 

 perioris and the superior rectus on the lower side of 

 the head and contraction of the inferior rectus on 

 the upper side. 



In man, the monkey and the cat which have 

 forwardly directed eyes, nodding movements aljout 

 a bitemporal axis produce contraction of both recti 

 superiores and the levatores. In animals these reac- 

 tions are brisk and their maintenance is presumably 

 due to stimulation of the otoliths. In man, evidence 

 of this mechanism appears in those patients who have 

 paralysis of voluntary upward movement. If they 

 fixate on an object in the horizontal plane, they may 

 be able to maintain fixation in spite of forward 

 flexion of the head on the neck. This requires con- 

 traction of the elevators of the eyeball. This may 

 occur even when fixation by itself is ineffective, as 

 when the patient fixates an object in the horizontal 

 plane but is unable to follow as it is slowly moved 

 upwards (82). 



NECK REFLEXES 



By the classic inethods of Magnus it is possible to 

 show that bending the neck may produce com- 

 pensatory eye movements. These are clearly seen in 

 rabbits (51). Recently McCouch et al. (104) showed 



