ELECTRICAL PHENOMENA IN VISION 591 



THE HUMAN ELECTRORETINOGRAM 



A convenient method of recording the human retinal or action potential 

 is by the use of an electrode embedded in a plastic contact lens (Riggs, 

 1941'; Karpe, 1945; Antrum, 1950). This type of electrode may be used 

 to provide a stable and comfortable attachment to the human eye for a 

 period of several hours (see Fig. 13-1). 



Ejfects of Dark Adaptation. Wald (1945) has described the remark- 

 able series of events by which the retinal receptors are able to restore 

 the photosensitive substance that is broken down by light. The course 

 of this restorative process may be followed by measuring the height of 

 the human action potential in response to a test flash delivered to the 

 eye at successive intervals. Karpe and Tansley (1948) measured the 

 increase in this height as a function of time in the dark and proceeded to 

 compare their results with the decrease in psychophysical threshold which 

 occurred under the same conditions. They concluded that the two pro- 

 cedures yielded nearly parallel results. They recognized, however, that 

 this parallelism was somewhat arbitrary, since it involved a comparison 

 between a set of response magnitudes (electroretinogram) with a set of 

 stimulus magnitudes (psychophysical thresholds) over a somewhat 

 restricted range of times in the dark. 



Johnson (1949) made a thorough study of the effects of dark adap- 

 tation on the human electroretinogram. He used red, yellow, blue, and 

 white test stimuli over a wide range of intensities and determined the 

 intensity necessary to produce a given response magnitude at each time 

 in the dark. He found that the fo-wave of the electroretinogram in man, 

 as in the rat (Charpentier, 1936) and the frog (Riggs, 1937), changes 

 importantly in wave form as well as in magnitude as dark adaptation 

 proceeds. This fact makes it inadvisable to use &-wave height alone as 

 an index of state of adaptation (Johnson and Riggs, 1951). Furthermore 

 the 6-wave steadily increases during the later stages of dark adaptation, 

 by which time the psychophysical threshold has nearly ceased to decline. 



Psychophysical data (Hecht, 1934) on dark adaptation show a clear 

 difference between an early stage, for which the cone receptors are 

 believed responsible, and a later stage representing the rod receptors. 

 When red light is used for the test flash in these psychophysical experi- 

 ments, only the cone portion of the dark adaptation is revealed, since the 

 rods are never much more sensitive than the cones for red stimulation. 

 In the case of the 6-wave of the electroretinogram, however, the same 

 rate of dark adaptation is shown by red test flashes as by other colors of 

 test flash, though it is necessary to use very high intensities of red to 

 achieve the necessary response (Johnson, 1949). The interpretation of 

 'these facts is simple, namely, that the 6-wave of the human electroretino- 

 gram is almost exclusively a product of rod-receptor activity. The 



