EXCITATORY AND INHIBITORY PROCESSES 



205 



Fig. 6. Subthreshold and threshold "on" responses to local monochromatic 



activations. 



Highly photosensitive medium sized nerve cell ( 1 50 u.), densely pigmented, initially 

 inactive in the dark, in the presence of oxygen. It was submitted to three mono- 

 chromatic (579 mji.) brief irradiations (as indicated by the white horizontal lines 

 under the recordings), of increasing intensities, successively from top to bottom: 

 005, 0-2 and 2 x 10 " cal g mm-2 sec"i. 



At the lower intensity, only a subthreshold photodepolarization (generator 



potential) is initiated. At higher intensities, the rate of the generator potential and 



the frequency of the spiking increase. Note the slow repolarization wave following 



the cessation of the discharge at "off". 



The shape of the subliminal generator depolarization, carefully inspected 

 at high amplification, appeared in many instances to have quite a smooth 

 contour, without any perceivable superimposed potential change. However, 

 it frequently occurred that some bumps of regular shape of about 0-1 mV 



