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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



from this technique that they found only one spike 

 conducting in the radiation from geniculate to cortex 

 and that this was the relay from the first spike in the 

 optic tract prior to the geniculate. 



There are possibly three subcortical pathways the 

 activities of which directly or indirectly affect the 

 optic cortex in a demonstrable way. There is, of 

 course, the direct relay path, the optic pathway itself; 

 the collateral path of the brain stem to association 

 nuclei in the thalamus (65); and possibly a collateral 

 circuit from thalamic relay nuclei to others in the 

 thalamus to the cortex (38, 47). 



Bishop & Clare (24) describe a bundle of fibers 

 lea\ing the pulvinar and terminating in the temporal 

 lobe of the cortex. When the optic nerve is stimu- 

 lated, a weak and extended discharge is recorded in 

 this bundle. The activation of the bundle was pre- 

 sumably from ijrain-stem or thalamic nuclei re- 

 sponding to optic tract activation. The exact origin of 

 the discharge is not yet known, but its temporal 

 features tally with the pretectal or lateral nucleus. 



CORTICAL LOC^Liz.^TiON. Hartley (4), using photic 

 stimulation of the retina in which two discrete retinal 

 areas were tested, showed that one part of the cortex 

 responded maximally to the activity in one retinal 

 area A, and another part of the optic cortex responded 

 ma.ximally to another retinal area B. Stimulation of 

 A would not usually activate the cortical area re- 

 sponding to B, and \ice versa. When any sign of 

 response under these conditions was detected, it was 

 merely an irregular train of indefinite waves. 



When A and B were stimulated simultaneously, 

 the corresponding cortical areas did not manifest any 

 summation. The responses were simply of the usual 

 size. On the other hand, when the two stimuli were 

 separated by an interval of 150 to 175 msec, the 

 respon.se to the second was enhanced; and if the inter- 

 val was lengthened, a value could be found at which 

 inhibition or depression of the second response was 

 manifested. 



At a retinal point C, intermediate between A and 

 B, responses to both stimuli were recordable, both 

 responses being discernible when enough temporal 

 separation was allowed for the two response waves to 

 be seen in two parts of the record. When simultaneous, 

 the record representing the response to the two retinal 

 areas was a single wave larger than either of the two 

 responses individually recorded. 



In one experiment, for instance, cortical point C 

 later came to respond only to the stimulation of the 



second retinal area. At first, the two stimuli produced 

 an enhanced result when simultaneous. When the 

 two were separated by about 15 msec, the entire 

 response almost disappeared. This continued to be 

 the case as the separation was made considerably 

 greater. It did not matter which of the two stimuli 

 was delivered first. When finally the first retinal area 

 failed to produce a response of its own, it still could 

 augment B, when applied simultaneously, and reduce 

 the size of B, when definitely out of phase with it. Still 

 other examples of the interaction of cortical areas 

 were obtained. 



Since this material relates not only to cortical locali- 

 zation but also to the visual experience of movement, 

 it will lie discussed in the section on that subject. 



PROPERTIES OF DENDRITES. Stimulation of the cortex 

 at various depths has led Clare & Bishop (36, 37) to 

 make certain inferences about the behavior of cortical 

 dendrites. These were given in the section devoted to 

 the cortical response. The following is a statement of 

 the picture they paint of the behavior of the various 

 parts of the neuron. In referring to intercortical paths 

 that terminate only on apical dendrites of pyramidal 

 cells, they found cases in which only the terminal por- 

 tions near the cortical surface are activated. When 

 the dendrites are activated indirectly by way of these 

 paths, or directly by artificial stimuli, dendritic con- 

 duction is not of the all-or-none type. The conduction 

 occurs more readily away from the cell body than in 

 the reverse direction. Its rate is less than i m per sec. 



After initial indirect activation, a stimulus finds the 

 dendrites activatable at all times later than the abso- 

 lutely refractory period of the axons involved. Follow- 

 ing a 20 msec, facilitation period, depression sets in 

 and the sign is positive. The authors inferred that, in 

 general, the depression following neuron activation is 

 attributable mainly to its dendrites. Apical dendrites 

 manifest no refractoriness, and so later activation sums 

 with the first. Continuous negativity may be perpetu- 

 ated by repetitive stimulation. All that would be re- 

 quired to produce waves of potential of any temporal 

 proportions would be modulation of stimulation of the 

 dendrites. The authors suggest this principle in the 

 production of the waves of the cortical record. 



The influence of a neuron on its surrounds is 

 brought about only through the impulses it causes to 

 be discharged by the cell body into its axon. These 

 effects are variously distributed via the ramification 

 of axon branches. Activation of the neuron may occur 

 via two avenues. The one is by way of cell-body 

 synapses; the other by way of dendrite synapses. These 



