CENTRAL MECHANISMS OF VISION 



/-'5 



of Schouten and Ornstein, and of Fry and colleagues 

 are the ones referred to. 



Clare & Bishop (34) noted that even during the 

 deepest depression, a second stimulus may find that 

 a first one, though maximal, may have left some ele- 

 ments that were not stimulated. This is as if at no time 

 can any stimulus deliverable to the optic nerve fire all 

 channels in the radiation pathway. The opposite ex- 

 treme of this may take place. A second stimulus may 

 cause a large geniculate response at any instant fol- 

 lowing a maximal first one. From this, they infer that 

 many, though not all, elements represented in the first 

 response are likewise involved in the second. When 

 the second stimulus is made weaker, the second re- 

 sponse manifests first a more marked diminution than 

 when the first stimulus is applied alone. Ultimately, 

 the response disappears completely at a stimulus 

 strength otherwise able to elicit a large response if not 

 preceded by an earlier stimulus. If, now, the first 

 stimulus is diminished the response to the second 

 grows, indicating the acti\ation of elements not ac- 

 tivated by the first stimulus. 



ACTIVITY IN REGIONS OTHER THAN OPTIC CORTEX. 



Clare c& Bishop (35) studied the activity of a portion 

 of the lower half of the medial wall of the suprasylvian 

 gyrus in the cat. This is an area responding only sec- 

 ondarily to activity in the striate cortex. They re- 

 corded from this area, first to optic nerve stimulation 

 and second to direct electrical stimulation of points in 

 the striate area itself. 



They found that this region responds quite similarly 

 to the striate area, but with an amplitude about one 

 eighth or less. The response with all its components 

 appears about i msec, later than does the striate 

 response, when the activation is induced by the 

 impulses in the optic nerve. The response to striate 

 stimulation is late by only a very short conduction 

 time. Experimentation showed that this region was 

 fired by the discharge of the second major spike of the 

 optic cortex response to optic nerve stimulation, 

 which is to say, the activity of the pyramids in layer 

 IV and probably in layer II. This activity probably 

 represents an association area interrelating acoustic 

 and optic activities. 



Jasper et al. (48), utilizing repetitive activation pro- 

 ducing local convulsive acti\ity in ihe striate cortex 

 of the monkey, did not disclo.se active pathways acro.ss 

 the cortex from striate to parastriatc and other areas. 

 They did find activity transmitted to the pulvinar. 

 When this region is activated by direct experimental 



m 



'iXTXt 



FIG. 9. Schematic diagram of 

 cell network suggested by analy- 

 sis of spike responses. Solid l,„es: 

 A, afferent radiation; B to D 

 sequence of cell groups. Differ- 

 ences between short a.xon cells 

 and pyramids are ignored in the 

 interest of diagrammatic sim- 

 plicity. Dashed lines: Collateral 

 ■jumpers' impinging on cells fall- 

 ing later in sequence than next 

 adjacent cell. Dotted lines: Affer- 

 ents from sources other than 

 geniculate. Axons leaving the 

 cortex are omitted. Lines ending 

 blindly indicate synaptic con- 

 nections similar to those repre- 

 sented. This fundamental as- 

 sumption is required to render 

 the diagram functionally applica- 

 ble; insofar as impulses at syn- 

 apses are equivalent, a minimal 

 number (more than one) is re- 

 quired to fire a cell at which they 

 arrive simultaneously. From A simultaneous impulses arrive 

 at B and C. B will be activated if .-1' fires with A, and C will 

 not until activated by synchronous firing of B and B', or A and 

 B, etc. Activated by a synchronous volley of impulses from ra- 

 diation fibers, a simple succession of synchronized discharges 

 should ensue. Activated by a barrage of impulses from same 

 source, a much more scattered discharge should result, owing 

 to arrival of impulses at each level over different paths Qsolid 

 and dashed /m«), and with different delays. Arrival of im- 

 pulses from other sources {dotted lines') should further mod- 

 ulate the patterns. [From Bishop & Clare (23.)) 



stimulation, it was found to activate areas of corte.x 

 alongside the optic cortex. 



Marshall et al. (57}, on the other hand, in exploring 

 the cat's cortex found an area of the cortex over- 

 lapping with the acoustic cortex. From it, they were 

 able to obtain response both to the acoustic and to 

 optic stimuli. Bishop & Clare (24) found that the 

 relay fibers responding to the second spike in the 

 optic tract terminate in the thalamus. 



Chang & Kaada (33) interpreted the three spikes 

 of the cortical record following single shock stimula- 

 tion as attributable to separate groups of fibers from 

 the thalamus. They assume that each of the three 

 spikes is followed in turn by a surface positive wave, 

 but that these three longer-lasting effects sum in the 

 record to a single surface positive wave. Bishop & 

 Clare (19) re-examined the matter. To do this, they 

 recorded the impulses in the radiation directly, fol- 

 lowing optic nerve stimulation, using diphasicity as a 

 criterion for propagation in radiation fibers. It was 



