7IO HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY' I 



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FIG. 2. Postsynaptic responses following single stimuli to the optic nerve from the pretectal area 

 (records i to 3), from the colliculus (i? to ;o) and from both (^ to 7). The presynaptic responses 

 are usually too low to detect. Time scale is in 10 msec, intervals. In each column the first record 

 is below threshold for the major responses to be recorded but nearly maximal for the second tract 

 spike. For record 2, stimulus strength is 1.5 times that for ;, for 3 it is twice. For 5 to 7, strengths 

 are 3, 5, and 6 times that for 4. For g to 10, strengths are 3 and 5 times that for 8. Ratios of thresh- 

 olds depend materially upon durations of square-wave pulse stimuli, the ratios increasing with 

 shorter duration of pulses. First column: Records from electrodes, one at the surface above pretectal 

 area and one below surface. The wave form varies widely with different positions of electrodes, and 

 the spikes recorded here are not usually as prominent. Second column: Critical electrode in anterior 

 border of superior colliculus where it evidently recorded activity both of pretectal character (with 

 latency of 7 msec, shown in l\\e first column) and of colliculus type (latency 11 msec, third column^ 

 Reference electrode in the medial geniculate body. Note growth of second potential in records 6 

 and 7 as a wave starting beyond the crest of the first (starts marked by vertical lines) when stimulus 

 strength is increased. Third column: The second only of the two waves of record 6 is recorded from a 

 critical electrode just below the surface of the colliculus proper against a reference electrode at a 

 distance. Critical electrode negative. Record 8 is at higli amplification, q and 10 at one quarter of 

 this amplification. [From Bishop & Clare (24).] 



the first response. Layers A and Ai arc homologous in 

 function, both being activated by the first tract spike 

 and both relaying to the cortex. Both retinas, how- 

 ever, send fibers to each colliculus, though many more 

 arrive from the contralateral retina than from the 

 other (24, 59). 



Bishop el al. (29) and Bishop & McLeod (30) also 

 have studied the response of the lateral geniculate 

 body, finding it repetitive under the conditions used. 

 They attribute repetitiveness to origins outside the 

 geniculate, possibly to excitation over reverberatory 

 circuits leading back from the cortex. Such paths have 

 not been established anatomically. 



The responses of structures beyond the relay nuclei 

 (the cortex, the lateral thalamic nucleus and the 



tectum) differ from those up to and including the relay 

 nuclei. Instead of being mainly spikes, they are ex- 

 tended responses involving complex pictures of chains 

 of neurons, each link active in turn. 



In the cat, it is at present feasible to distinguish 

 histologically only three fiber-size ranges in the optic 

 tract. The large-size group includes fibers ranging 

 from 8 to 12 /j, and these pass only to the dorsal 

 geniculate nucleus and are included in the first of the 

 four functional groups of fibers already mentioned. 

 The middle histological group with fibers ranging 

 from 4 to 8 |i also makes up some of those in the first 

 functional group as well as some in the second spike 

 group. The small-fiber group constitutes the fibers in 

 the third and fourth functional groups. Although the 



