EXCITATORY AND INHIBITORY PROCESSES 201 



to whole light at "on", exhibiting depolarization and spiking. However 

 several A cells respond both at "on" and at "off", less frequently at "off" 

 (Arvanitaki and Chalazonitis, 1958a, I960), as was known for visual photo- 

 receptors (Hartline, 1938; Granit, 1950; Kuffler, 1953; etc.). 



The "off" type response to illumination corresponds properly to an 

 inhibition process at "on". The mixed "on" "off" type response manifests 

 the interference of both the excitation and the inhibition processes. Such a 

 response, even to localized illuminations in cells of a given identity, suggests 

 the interference of two opposite actions exerted by the radiations of the 

 whole visible, one or the other dominating, according to the individual con- 

 ditions met in the photoactivated cell. 



Monochromatic activations. As a matter of fact, cells of the same type may 

 differ by the relative concentrations of the two main intracellular pigments, 

 the heme-protein and the carotene-protein. Accordingly, in a given cell the 

 activation by whole light has been compared to that by monochromatic beams 

 at 579 m/Lt specifically absorbed by heme-protein molecules and by mono- 

 chromatic beams at 490 m/it absorbed by carotene-protein molecules. 



Responses to A 579 m/t activation reproduce with a much greater efficiency 

 the general data of the excitatory processes elicited by the whole light acti- 

 vations. In an initially resting soma, with a just suitable intensity, a spike 

 generates after a latency of a fraction of a second. At suprathreshold inten- 

 sities, the latency decreases and subsequent spikes are generated at increasing 

 frequencies. If the intensity is kept constant, the frequency increases as a 

 function of the duration of the illumination. 



Plotting the spikes' maximum frequency reached during illumination 

 against the logarithm of the light intensity, a reasonably good linear relation 

 has been found. Thus, we may write: F = k log / + c. 



With flashes of short durations t, such that r be inferior to the latency time, 

 the number of spikes composing the response increases as a function of the 

 product of intensity by duration: /' r, which is the energy of the light 

 stimulus. To elicit a given response of A^ spikes, / v. t is constant. To elicit 

 this same response, the energy of a monochromatic A 579 m/t activating beam 

 is 10-100 times less than that necessary by whole light. 



If an A soma was autoactive in the dark, its spiking frequency increases 

 on illumination. The maximum frequency reached is a function of the light 

 intensity (Fig. 8). A short illumination of minute incident energy, 10^ cal 

 g mm"^2^ is sufficient to bring about a marked transition. 



The maximum increase of the frequency reached during illumination has 

 been plotted against the logarithm of the light intensity and data have been 

 compared for whole light and for a monochromatic A 579 mix beam in the 

 presence of oxygen, and in the presence of air (Arvanitaki and Chalazonitis. 

 1960). The relations are linear in all cases, but they are significantly distinct 

 in their slope. 



