CENTRAL MECHANISMS OF VISION 72 1 



CORTEX UPPER 



CORTEX MIDDLE 



CORTEX LOWER 



OPT. RADIATION 

 THALAMUS 



/W 



MSEC 



14 OPT. NERVE 



FIG. 5. Left: Tentative inferences concerning the origin of cortical responses drawn from experi- 

 mental data. Roman numerali at lejl indicate conventional cortical layers and furnish a scale of depth. 

 Numerals / to J refer to cortical spikes; ,-1+ refers to underlying surface-positive waves; B— refers 

 to late surface-negative wave which appears to arise from lower layers of cortex, .4— represents the 

 early surface-negative wave only occasionally seen well-developed in normal cortex but large under 

 strychnine where it becomes the most prominent potential clement of the record. [From Bishop & 

 Clare (20).! 



FIG. 6. Right: Diagram of responses of the optic pathway of the rabbit. At least four elements of 

 the response, following the activation of the optic nerve, can be distinguished in some records although 

 any two adjacent elements, each presumably complex, may be confluent in a single response. The 

 last of these four may be repeated several times at inter\-als of about 0.2 sec. following a single 

 shock. There is a discharge of the corticofugal fibers during at least the first of these repetitive cortical 

 discharges which appears to facilitate the thalamic neurons to a second discharge from the optic 

 nerve. This is indicated by the Ims' vertical arrows pointing downward. (Abscissae, time; ordinates, 

 voltage.) [From Bishop & O'Leary (25).] 



Chang attributes deflections 2, 3 and 4 in his record 

 to the activities of three different geniculocortical 

 pathways and suggests that they may conduct the 

 respective impulses of trichromatic vision. He uses to 

 support this interpretation the findings of Pieron (60) 

 to the effect that latencies for seeing the three funda- 

 mental colors are different. This is thought to be 

 evidence that the impulses signalling these travel at 

 independent and different velocities. 



We prefer to follow Bishop & Clare (19, 20, 23), 

 Clare & Bishop (37) and Bishop & O'Leary (25, 26). 

 Bishop & Clare (19) made it a point to check the 

 findings of Chang & Kaada in regard to the kind of 

 potentials found in the geniculocortical radiation and 

 found onK a single and rapidly conducting spike. 

 When a later tract spike resulting from use of higher 

 stimulus strengths is elicited, it represents impulses 

 distributed mainly to the pulvinar, pretectal area and 

 colliculus. Hence they conclude that all the successive 

 spikes up to five in number, except the first one, that 

 can be recorded from the cortex represent groups of 

 neurons active within the cortex itself. 



Bishop & Clare (22) stimulated the optic and 

 parietal cortex in cats at various depths below the 

 surface. They found that when the cortex is stimu- 

 lated at the surface, the response obtained from two 

 electrodes, one at the surface and the other at any 

 depth, is a simple negative wave. When stimulation is 

 presented i:)elow the surface, a diphasic wave with its 

 initial phase surface-positive is obtained. When 

 stimulation is presented half way or more down 

 through the cortex, first a single and then two or 

 three short spikes are manifested in the response. These 

 are comparable to those elicited from activation of 

 the radiation pathway. As the radiation terminals are 

 approached, the complete cortical response to pe- 

 ripheral afferent stimulation is simulated. This pro- 

 cedure is thus a way of showing the transition from 

 direct cortical stimulation to the indirect or pe- 

 ripheral. 



The main difference between direct and indirect 

 stimulation, in addition to the possible simultaneous 

 activation by the direct stimulus of elements that 

 respond successively to indirect stimulation, pertains 



