ELECTRICAL PHENOMENA IN VISION 



593 



discriminating mechanisms of the eye are responsible for any particular 

 component of the action potential, though this possibility must not be 

 overlooked in future research. The form of the 6- wave does not appear 

 to change with variations in wave length of stimulation. 



Area Effects and the Electroretinogram. Attempts have been made 

 (Cooper et at., 1933; Adrian, 1946) to stimulate foveal regions of the eye 

 without involving peripheral regions. These attempts have met with 

 little success, however, in arousing a measurable action potential. Fry 

 and Bartley (1935) were able to show why this is true. Working with 

 the rabbit, they found that only an extremely bright flash of light would 



WBi Retinitis 





0.1 S0C. 



Fig. 13-7. Comparative responses of a normal eye and an eye affected by retinitis 

 pigmentosa. 



ehcit the action potential when directed upon a small patch of the retina. 

 When two such patches were stimulated alternately, there was no action 

 potential. Hence it was concluded that under these conditions the action 

 potential is not produced by the region directly stimulated but by the 

 relatively large nonfocal area to which light is reflected and scattered 

 within the eyeball. 



This result, amply confirmed in experiments on the human eye (Boyn- 

 ton and Riggs, 1951), makes it diflftcult if not impossible to elicit an action 

 potential from photopic receptors without a large involvement of scotopic 

 ones as well. Scotopic responses may be suppressed to some extent by 

 the use of brief exposures, moderate degrees of light adaptation, and 

 stimuli made up of the longer wave lengths of light. 



Clinical Use of the Electroretinogram. When the contact-lens electrode 

 was first introduced, the suggestion was made (Riggs, 1941) that clinical 

 cases might be more easily investigated by this means. Karpe (1945, 

 1948a, b) has reported the results of testing a number of patients with 

 various retinal disorders, and a number of such observations have been 



