Herrick, Lecture Notes on Attention. 7 



Experiment. — Fix the single eye upon a faintly illuminated screen with per- 

 forations near the margin of the field and a shutter behind in such a way that 

 a ray of strong light may be revealed at will. Note that the axis of the eye in- 

 voluntarily turns toward the illuminated spot Vary the experiment by using 

 small colored images painted with transparent color and projected upon a trans- 

 parent part of the screen, etc. Now determine to resist this tendency and keep 

 the eyes immovably fixed on the original spot. Observe whether there is a 

 feeling of tension in the eye, a strain on antagonistic muscles of the neck or in 

 the scalp or even a sense of tension in the brain. Interpret the results. Ob- 

 serve what happens when the subject is encouraged to expect the new impres- 

 sion on the right and then unexpectedly it appears on the left. A wider angle 

 than the actual retinal one may profitably be used in these experiments while in 

 other cases a diaphragm will be needed. In some it is an advantage to have 

 the head fixed while it might be instructive in the deviation experiments in 

 some cases to have the weight of the head counterpoised by braces under chin 

 and occiput suspended to a swivel in the ceiling 



What is the result if the eye is centred on a bright, a varying, or a barely 

 legible object ? Is there a conflict of interest ? 



We have then in the eye a device for differentiating a part 

 of the field of vision by reason of which that portion is more 

 vividly reproduced in the brain and we have an anatomical adjust- 

 ment of the muscles of the eyeball to enable this sensitive 

 point to be turned upon any object which, by reason of the in- 

 herent intensity, usurps the position. More than this, there is 

 a very complicated system of nervous correlation reflexes which 

 must have developed pari passu with the anatomical structure 

 by which the visual coordinations are affected anatomically.' 



It is of fundamental importance to decide whether the 

 turning of the eye upon the vivid point in the margin of the 

 field is due to volition or is spontaneous and a pure reflex. It 

 seems certainly to be the latter. It may be noticed that in the 

 fixing of both eyes on any field there is a complicated coordina- 

 tion which has to be learned but passes out of voluntary con- 

 trol, so far as the details of the process are concerned, very 

 early. When any particular object is to be " fixed " in vision 

 the two maculae luteal must be properly adjusted as "identical" 

 spots of the retina and when this is done all other parts of the 

 two images will fall on "identical" spots. Now when a bright 

 or vivid spot appears near the margin of the field we might 



^ The instructor will review the visual-motor coordinating systems. 



