ELECTRICAL PHENOMENA IN VISION 



585 



The action potential recorded from these electrodes by oscillographic 

 means, the electroretinogram, is typically a rather complex one whose 

 characteristics depend on the species of animal, the condition of the eye, 

 the electrode placement, the technique used in recording, and the nature 

 of the stimulating light. Such a record from a human eye is shown in 

 Fig. 13-3. There is evidence that the various positive and negative 

 waves of the electroretinogram originate within different retinal struc- 

 tures, but detailed information on 

 this point is lacking. It is clearly 

 not true that the principal com- 

 ponent of the electroretinogram is 

 a mere photoelectric phenomenon, 

 nor is it simply a summation of 

 action potentials within optic nerve 

 fibers. 



EVIDENCE FROM PRIMITIVE 

 EYES 



Fig. 13-3. Records of the human elec- 

 troretinogram. Responses to high, me- 

 dium, and low intensities of light are 

 shown in .4, B, and C, respectively. 

 Time is shown by the faint vertical lines 

 (0.01- and 0. 1-sec units). The moment 

 of stimulation is indicated by the gap in 

 the horizontal line at the bottom (flash 

 duration 0.04 sec). In A the first por- 

 tion of the response is the negative 

 a-wave of very short latency; the princi- 

 pal positive deflection is the 6-wave; and 

 the small, second positive deflection is the 

 c-wave. {Unpublished records of R. M. 

 Boynton.) • 



It is not easy in vertebrate eyes 

 to locate the original site of the 

 action potential. In more primi- 

 tive eyes, however, it appears prob- 

 able that a large monophasic re- 

 sponse originates within the sensory 

 cells of the retina. Hartline (1928) 

 found such a response in the coni- 

 pound eye of Limulus, the horse- 

 shoe crab, whose ommatidia were 

 supposed to contain only first-order 

 (sensory) neurones. In Dytiscus, the water beetle, Adrian (1937) ob- 

 served a similar response, though it was complicated by the. activity of 

 the gangha. Bernhard (1942) was able to separate the retina from the 

 other structures in this preparation. The isolated retinal response, pre- 

 sumably from primary sense cells, was always smooth and monophasic. 

 Therman (1940) obtained similar results with Loligo, a squid, and 

 observed in addition that there were two separate components in the 

 action potential from this eye. The two components, of opposite sign, 

 are distinct in their responsiveness to chemical stimulation, light adap- 

 tation, and other effects. The presence of two distinct retinal systems 

 in another mollusk, Pecten, has been demonstrated by Hartline (1938b; 

 see later). It may be that in such an eye each retinal system has its 

 own action potential as well as its own characteristic manner of respond- 

 ing to the onset or extinction of the stimulating light. 



