422 Comparative Animal Physiology 



frog's eye, and Fig. 115). In some photoreceptors the cessation of illumina- 

 tion is followed by a further increase in negativity, called the d-wave. In the 

 electrical action potentials of most vertebrate photoreceptors, the b-wave is 

 preceded by a brief low magnitude potential in the positive direction, the 

 a-wave. Although this terminology has been applied to the retinal action 

 potentials of all photoreceptors, a 'strict homology of the various waves has 

 not been demonstrated. 



For the purpose of measuring the action potential of the retina, electrodes 

 may be connected to the front and back of the eye. By means of such con- 

 nections, either on an isolated eye or on one in situ, the electrical changes 

 are recorded by appropriate instruments. A record of the retinal action poten- 

 tial obtained in this manner is called an electroretinogram (ERG). The wave 

 form of the electroretinogram has been studied as a function of intensity of 

 stimulating light, of the degree of dark- or light-adaptation, of the duration of 

 the flash of light, of the frequency of a flickering light, of temperature, and of 

 the effect of various drugs.^^- ^^^ As a result of such analyses it is postulated 

 that the ERG represents the algebraic sum of potentials from three sources, 

 which are referred to as the three components, P (meaning process) I, P II, 

 and P III. The exact site of origin of these electrical components has not been 

 determined, but at least one of them exists in the sense cell.^'^ This has been 

 well demonstrated for the sense cells of Limulus,^'^ the compound eye of the 

 grasshopper, Trimerotropis,^^^ and for Dytiscus,^ and it undoubtedly occurs in 

 all photoreceptors. 



The simplest electroretinograms obtained are from the eye of Limulus 

 (Fig. 127), an eye lacking lateral connections between sensory neurones. 

 The response to illumination is a simple negative wave, followed by a lower 

 sustained potential as long as stimulation persists. The a-, b-, c-, and d-waves 

 are conspicuously lacking, but the wave form is quite similar to that of P II 

 in the component analysis of the vertebrate retinal action potential. Measure- 

 ments of retinal electrical activity have usually been made in terms of the 

 magnitude of the b-wave. Measurements of c-wave magnitudes^^^- ^'^'^ seem 

 to parallel the b-wave measurements, at least in some respects. The former 

 are more difficult to obtain. 



Electrical Measurement of Visucil Functions. The relation between in- 

 cident light and electrical response indicates the possible dependence of the 

 retinal electrical event upon the known properties of the photochemical 

 system. 



ELECTRICAL MEASUREMENT OF SPECTRAL SENSITIVITY. The magnitude of 



the electrical action potentials of photoreceptors depends (within hmits) 

 rather direcdy on the intensity and on the duration of the light stimulus. 

 This dependence of magnitude on intensity suggests the possibility of meas 

 uring the spectral sensitivity of this response, using relatively monochromatic 

 light corrected for equal ener[;v. Techniques for such measurement have 

 been described.**^' ^"'^ The ideal procedure is to select a series of bands 

 throughout the visible spectrum (with a monochromator or with color filters) 

 and adjust the intensity so that it is equal for all the wave length bands. (The 

 bands should be as narrow as possible). The magnitude of the photoreceptor 

 electrical response (b-wave) elicited bv constant duration test flashes of the 

 various spectral bands will vary. The resultant plot of magnitude against 



