CENTRAL MECHANISMS OF VISION 



723 



of propagated impulses becomes evident. Marshall 

 ^55) has shown a remarkable degree of temporal 

 summation of the optic pathway to be recordable at 

 the geniculate. Bishop & Clare (19, 20) have done 

 likewise for the effects manifested at the cortex. The 

 latter authors have pointed out that, with graded 

 stimulation, a large effect reaching the geniculate 

 neurons is required to produce even a threshold re- 

 sponse in them. Furthermore, supraliminal activity 

 of the afferent radiation is needed to produce a thres- 

 hold response at the corte.x. Increasing the strength of 

 stimulation upward from this level prolongs the corti- 

 cal response with its spikes and waves. 



They also point out that weak stimulation is, in 

 effect, the stimulation that fails to produce much 

 spatial summation. In normal activity of the optic 

 pathway, this lack of summation ought to be charac- 

 teristic and, accordingly, lead to incomplete or brief 

 cortical responses. But the fact that normal stimula- 

 tion is characteristically prolonged rather than ab- 

 breviated to a very small fraction of a second, would 

 provide for temporal summation that ought to com- 

 pensate for the lack of the spatial variety. Relevant to 

 this, Chang (32) has demonstrated the extreme effec- 

 tiveness of 'potentiation' by photic stimulation of the 

 retina of responses to individual brief stimuli at the 

 geniculate. This steady photic stimulation seemed to 

 maintain a raised level of excitation and this made 

 incidental impingements more effective. We know 

 that exceedingly weak excitation of the retina trans- 

 mits something to the cortex. Not only does this slight 

 effect get through to the cortex but certain effects 

 from adjacent cortical areas are afso produced in 

 order that the activity in the visual cortex be given 

 a context that would provide meaning for the terminal 

 input. Bishop & Clare (23) describe well how the 

 experimental conditions of the laboratory emphasize 

 the effects of spatial interaction at the expense of what 

 may occur via temporal interaction. 



From their work and knowledge of cytoarchitec- 

 ture, Bishop & Clare (23) depict the kind of interac- 

 tion between cortical elements that would plausibh' 

 occur. The description is as follows. The elements in 

 the cortical network can be suppo.sed to constitute a 

 system of both parallel and series connections. Each 

 afferent channel (fiber) at any given synaptic level 

 would be in connection with a number of postsynaptic 

 elements, the arrangement involving definite over- 

 lapping. This would be the parallel set of connections. 

 In addition to this, elements at each synaptic level 

 send axons to the next higher level, and this provides 



series connections. The authors suggest two further 

 features of the system. Afferents from sources collateral 

 to the visual projection system surely connect at some 

 or all levels. What they call 'jumpers' may be in- 

 volved. These are collaterals affecting more than a 

 single synaptic level. An example may be the recurrent 

 axons of the pyramidal cells. Some authors have 

 reported afferent radiation fibers terminating not only 

 at the usual layer IV but also at the two successive 

 layers above it. Bishop & Clare inject an additional 

 assumption, namely, that the mass of impulses tra- 

 versing the one-step circuits are more effecti\e than 

 those involving a jump of two or three synapses. 

 Perhaps the latter pathways should be considered to 

 need assistance even to fire the synapses. The activity 

 just suggested is pictured in figure 8, schematized in 

 figure 9. The evidence for the scheme consists in a 

 double series of spikes, ones of low amplitude alter- 

 nating with ones of high amplitude when certain sub- 

 maximal records are obtained from potentiometer 

 leads and from electrodes subtending only small frac- 

 tions of the total cortical depth. The timing of the 

 spikes is about i .4 msec, between those of the same 

 series, and 0.7 msec, between any two of the alternate 

 series. Bishop & Clare's suggestion is that this double 

 series is made up of pyramidal cells alternating in dis- 

 charge with short axon cells (Golgi II). Since the 

 latter cells are oriented in random fashion, their 

 potentials would tend to be registered by the leads as 

 lower in amplitude. The pyramids extend in a single 

 direction. This description diflfers from that of Thomas 

 & Jenkner (66) in which records were interpreted as 

 evidencing repetitive firing of the same cells. 



ACTIVATION OF CORTEX BY STIMULATION OF RADIATION. 



Instead of initiating activity in the optic pathway by 

 stimulating the optic nerve, it is possible to eliminate 

 the geniculate and stimulate the radiation and note 

 the cortical effects. By stimulating at this site. Bishop 

 & Clare (23) found that the same cortical response 

 was obtainable as when the cortex is stimulated 

 through the geniculate. Certain features of cortical 

 response must be independent of the geniculate cycle. 

 To single stimuli constant in intensity, the initial 

 cortical response spike attributable to radiation axons 

 was constant in amplitude. Throughout a period of a 

 few milliseconds, the specific response to a second 

 stimulus manifested only slight diminution of ampli- 

 tude in its positive pha.se. The early negative wave of 

 the second response was depressed nearly to the 

 vanishing point, and this effect covered the whole de- 



