CENTRAL CONTROL OF EYE MOVEMENTS 



103 



the receptor mechanism. If the image is stabilized, 

 which can be accomplished by viewing hght which 

 has been reflected from a contact lens, so that it 

 moves with eye movement, then the visual detail 

 seen rapidly decreases, and contrast in the visual 

 field disappears after some seconds, abruptly reap- 

 pears and again proceeds to fade (55). It was thought 

 that elimination of 95 per cent of the movement of 

 the target relative to the retina would provide ade- 

 c[uate fixation. In fact stabilization has to be much 

 better than this, and an increase in stabilization from 

 99.94 per cent to 99.96 per cent results in a large 

 change in visibility (54)- 



Very fast movement of the visual field diminislies 

 contrast, since for any unit visibility depends on the 

 product of light intensity and time of exposure. For 

 total exposures less than about o.i sec, movement of 

 the image reduces visibility (116); however, for ex- 

 posures lasting over 0.2 sec, movement of the image 

 greatly increases visibility. Vibration of small ampli- 

 tude at 16 cps greatly reduces \isif)ilitv (109). 



Saccadic and Pursuit. Movements 



During reading or scanning of the visual field, the 

 eyes are moved in brief jerks or 'saccades' (30, 138) 

 (see fig. 10). The velocity of movement is constant 

 for a particular amplitude of movement for any one 

 subject and cannot be altered voluntarily. It in- 

 creases somewhat with increasing amplitude of move- 

 ment. The range for normal subjects is about 200 to 

 400° per sec (22, 26, 137). When the eyes are moved 

 from one target to another, there is usually a slow- 

 drift or possibly a flick after the rapid movement is 

 over. The drift and flick may be in the same direc- 

 tion of movement or in the opposite (56). Apparently 

 the eyes carry out a 'preset' movement of approxi- 

 mately the correct amplitude and bring the fixation 

 target to the fovea by the final drift or flick. There 

 is no evidence of check movements of antagonists to 

 stop the movement. In reading continuous lines of 

 print, saccadic movements last about 20 to 50 msec. 

 Each is separated by a fixation pause of about 0.3 

 sec. The frequency of inovements is variable and in 

 reading regression movements frequently occur. 

 During a normal line of 4^^ in. a skilled reader will 

 make about six fixation pauses depending on the 

 difliculty and on the interest in the material read 



(138). 



In addition to saccadic movements, it is also pos- 

 sible to make smooth pursuit movements. If an ob- 



cone. siz-C- 



FIG. 9. Mosements of a retinal image indicated by alter- 

 nating continuous and inlertupted lines. The diameter of the 

 circular retinal area is about half that of the "central territory.' 

 The tremor mo\ement which has been omitted has a median 

 excursus of about 1.3 min. of arc or 6 y.. [From Ditchburn (54).] 



FIG. I o. .Saccadic movements recorded by the corneal reflec- 

 tion method. Records made by a number of different subjects. 

 [From Carmichael & Dearborn (30).] 



server is set to follow a spot which is performing 

 simple harmonic motion, his eye movements change 

 progressively, as shown in figure 1 1 . He at first at- 

 tempts to follow by a series of vertical or horizontal 

 movements at irregular intervals — "positional cor- 

 rection.' After a few repetitions, he follows by move- 

 ment of the eye at uniform \elocity and approxi- 

 matelv correct direction with corrective movements — 

 "velocity control' Finally Ijy altering the velocity of 



