CENTRAL MECHANISMS OF VISION 719 



FIG. 3. Responses from the lateral nucleus of the thalamus to the second postsynaptic volley 

 from dorsal nucleus of the geniculate with single optic nerve stimuli. The first record in each column 

 was from a stimulus just below threshold for the second postsynaptic spike as recorded from the 

 dorsal nucleus. Strength of stimulus for record 2 was 5 times that for / ; for 4, 3.5 times that for 3; 

 for 6, 2.5 times that for j. Record 7 is a duplicate of (J at 1.5 and higher amplification. In the second 

 column the cortical record appears on the second oscillograph beam. In the third column the response 

 consists of a sequence of brief spikes. Time scale for thalamic records in i o msec, intervals. Latency 

 of response cannot be accurately determined but is not over 6 msec. Form of response varies widely 

 with location of electrodes and with depth below the surface of the thalamus, and varies considerably 

 at one locus following identical stimuli. All records presented were from a critical electrode in the 

 dorsal or dorsolateral region of the lateral nucleus approximately at the level of the anterior tip 

 of the dorsal nucleus of the geniculate. The reference electrode v/as deeper in the thalamus or in 

 white matter lateral to it. [From Bishop & Clare (24).] 



locations of the fiber groups in the cross section of the 

 tract are known (24), it is not necessary to delineate 

 them here. 



CORTICAL RESPONSE. The cortical response of the cat 

 to a peripheral input as simple as it is possible to 

 deliver is exceedingly complex. The simplest pattern 

 may be shown by the recorded events in the optic 

 cortex following single stimuli to the stump of the 

 optic nerve. The afiferent radiation fillers conduct 

 impulses mainly to the fourth layer of the cortex. 

 Activity, of course, immediately spreads to the other 

 cortical layers. This is pictured in the record as a 

 sequence of three definite spikes interpretable as indi- 

 cating that three groups of cell bodies are discharging 

 in sequence (20). More intimate examination (23) of 

 the early part of the response shows that a second spike 

 sequence also occurs. It, of course, is less prominent 

 than the one just mentioned. In the record, the second 

 series (the small spikes) alternates with the first. The 

 authors have reason to infer that the sinall spikes 

 represent the short-axon cells of the cortex. These 

 cells do not possess long apical dendrites as do the 

 pyramid cells. Their axons are short and mingle with 

 the adjacent pyramid cells. It is supposed that they 



conduct activity from one group of pyramids to 

 another. 



The model of activity that Clare & Bishop (37) 

 suggest is as follows and is pictured in figure 4. Af- 

 ferent radiation fibers first activate the short axon cells 

 of the fourth layer of the cortex. These, in turn, in- 

 nervate a group of pyramid cells at about the same 

 cortical level. These cells discharge into their axons. 

 The main branches of these leave this level of the 

 cortex via the subcortical white matter, activating 

 other parts of the central nervous system. The pyra- 

 mid-cell axons possess recurrent branches that arbor- 

 ize within the cortex activating a second group of 

 short axon cells. Tiicse, then, activate a second group 

 of pyramidal cells. This alternation occurs until the 

 sequence first mentioned has been completed. Since 

 the synaptic periods between each two successive 

 spikes is less than i msec, the transmission is thought 

 to be from axon to cell body. 



When activation of the pyramidal cells is intense 

 enough, the dendrites of these cells are definitely in- 

 volved. When so, they conduct their effects toward 

 their terminals. Clare & Bishop (37) state that this 

 produces the slow wave sequence typical of the re- 

 sponse of the visual cortex. If stimulation is slight. 



