Photoreception 



433 



potentials elicited by the test flash increases rapidly at first and subsequently 

 more slowly. 



The relation of this recovery to time may be graphically illustrated in two 

 ways: (1) by determining the intensity of the test flash necessary to elicit 

 a single spike potential during the period of dark adaptation, and (2) by 

 measuring the number of impulses elicited bv a constant test flash as a func- 

 tion of time. A series of recovery curves plotted according to the second 

 method are illustrated in Figure 139. These curves were obtained after 

 adaptation to lights of different intensities. It is apparent that recovery after 



Z(?^_, /ntensit/ 

 Fig. 137. The relationship between frequency of spike potentials recorded from single 

 optic nerve fibers (Lzmw/ws) and the logarithm (base 10) of the illuminating intensity. 

 A, Maximal frequency of discharge, shortly after onset of illumination; B, frequency of 

 discharge 3.5 seconds after onset of illumination. From Hartline and Graham.'^ 



adaptation to illumination of a low intensity is much more rapid than re- 

 covery after adaptation to illumination of a higher intensity. A similar series 

 of recoverv curves were obtained after adaptation for varying periods but 

 at constant intensity. These results indicate that the rate of dark adaptation 

 varies with the intensity and duration of light adaptation. In this regard 

 these curves duplicate the complexities of the curves of vertebrate eye rod 

 recoverv (Fig. 115, upper curves). ^^^ The variations in rate of dark adapta- 

 tion are explained in terms of the rhodopsin cycle (page 411), and an 

 analogous explanation may possibly be applied to the Limidus eye. 



Relations between Retinal and Optic Nerve Activity. The foregoing dis- 

 cussion of retinal and optic nerve activity in a variety of photoreceptors in- 

 dicates that both reflect a dependence on the photochemical nature of photo 

 reception. This follows on consideration of spectral sensitivity measurements 



