606 RADIATION BIOLOGY 



RESPONSES OF THE OPTIC TRACT, GENICULATE BODY, 



AND CORTEX 



We now leave the retina and turn our attention to events in the optic 

 pathways. Pioneer investigations have been made by Wang and Lu 

 (1936a,b, 1937), Bishop and O'Leary (1938, 1942), Bartley (1941), 

 Marshall and Talbot (1942), Adrian (1946), and Noell (1951), among 

 others. These investigations have in common the purpose to follow, by 

 electrical recording, the course of visual excitation from the eye to the 

 cortical projection areas. It is immediately apparent that in this form of 

 recording there is more variability and less precise localization than is 

 characteristic of single-fiber preparations. Some of the principal findings 

 are as follows: 



1. Optic-tract Potentials. In mammals these typically show several 

 peaks, including two large ones (the "early-on" and "late-on" waves) 

 with latencies of about 15 and 50 msec, respectively, after the beginning of 

 stimulation by light. Off-responses may also be complex, especially after 

 prolonged stimulation by fight. The latency of the largest off-response 

 may lie between 20 and 50 msec. Mammalian optic-tract potentials 

 often appear to have latencies lower than the latency of the action poten- 

 tial, and the action potential often fails to show any off-effect similar to 

 that of the optic tract. The two on-waves differ markedly with respect 

 to the effects of stimulus intensity; the late on- wave has a lower abso- 

 lute threshold, but it is diminished or even abolished at high stimulus 

 intensities. 



2. Geniculate Potentials. Bishop and O'Leary (1942) have made an 

 analysis of the responses that may be picked up in the vicinity of the 

 dorsal nucleus of the lateral geniculate in the cat. These are more 

 spiked, with less prominent separate waves. Furthermore the geniculate 

 responses are more variable from time to time than are the optic-tract 

 potentials, and their wave form is dependent on the exact position of the 

 recording electrodes. 



3. Cortical Potentials. Electrodes on the occipital cortex reveal spon- 

 taneous activity including, most prominently, the a rhythm of 3-12 waves 

 per second in various animals. In response to flashes of light (or to elec- 

 trical stimulation) several response waves are seen, sometimes lasting for 

 a second or more. The form of the response shows great variability, 

 even when elicited under uniform experimental conditions. It is difficult 

 to assign any one component of the cortical response to any definite 

 location in the cortex. 



Some of the experimental factors that influence cortical response poten- 

 tials are the following: 



1. Strychnine initially causes an increase in the first portion of the 

 response, then raises the threshold of stimulation and causes a diminution 



