CENTRAL MECHANISMS OF VISION 



733 



(4) are shown in figure 13, in which it is indicated 

 that the rhythm could be followed through at least 

 about two cycles. Since maximal stimuli were used, it 

 was inferred that the behavior of the optic nerve as a 

 whole represented the way the single parallel channels 

 in it react. This deduction rested upon the idea that 

 all parallel channels in the nerve were activated 

 simultaneously. This cycle represented the rhythm of 

 single channels while being the rhythmicity of the 

 system as a whole under these conditions. 



The more specific portion of the evoked response to 

 brief stimulation is followed by a long-lasting surface- 

 negative potential (14). It is during this time that a 

 .second brief peripheral impingement evokes either no 

 response or else one of reduced amplitude ('25). The 

 repetition of the cycle implied here may be demon- 

 strated at the frequency of the spontaneous alpha 

 sequence. Use of repetitive stimulation at twice alpha 

 frequency (4) showed that an original inability of the 

 system to respond at intervals half the alpha value 

 slowly changed into submaximal response following 

 each stimulus. The shift was as if the channels avail- 

 able for response became differently distributed in 

 time, so that finally part were ready to respond at the 

 presentation of one stimulus and the other part at the 

 presentation of the succeeding stimulus. .Stimulation 

 at higher multiples of the alpha rate resulted in what 

 appeared to be a further redistribution, such that 

 each stimulus was responded to in some degree but, 

 of course, more weakly than when rates were slower 

 (see fig. 14). 



This phenomenon could be expected to have a 

 parallel in perceptual response. Whereas a certain 

 rate of intermittent stimulation (c.f f) is required to 

 obliterate flicker fully once the visual system is ex- 

 posed to a considerable number of photic pulses, an 

 even slower rate may fail to be responded to as indi- 

 vidual pulses at the very onset of the stimulus train. 

 Wilkinson (68) studied this problem and found that 



the rate at which the first few pulses were seen as indi- 

 \idual flashes or produced flicker was much below 

 the rate at which the pulses could still produce definite 

 flicker after the train had progressed for awhile. The 

 perceptual responses to the first few pulses manifested 

 some of essentially the same irregularity as was mani- 

 fested in the cortical response (4) under the same 

 conditions. 



In some cases, as in the rabbit, the long-lasting 

 surface-negative wave is replaced by a series of 

 briefer waves (25). Something like this was observed 

 in the cat by Bishop & O'Leary (26). Prior to the 

 onset of the depression following a specific cortical 

 response, it was found that a short period of facilita- 

 tion to a second stimulus occurs in the radiation re- 

 sponse (27). For example, as the strength of an initial 

 stimulus is increa.sed, the response of the radiation 

 increases more rapidly than does the tract response. 

 With a second stimulus, it and the first, being 'maxi- 

 mal' for the tract, may elicit a larger radiation re- 

 sponse than a single stimulus. Even below maximal, a 

 second stimulus is typically more effective than the 

 first when falling within the short time limits implicit. 

 This indicates that spatial and temporal summation 

 are operative in the geniculate even with "maximal 

 stimuli.' This, although indubitable, is inconsistent 

 with the idea of a one-to-one fiber channel from retina 

 to cortex in the functional sense. It is consistent with 

 the theory of partially shifted overlap suggested by 

 Lorente de No (53). Similar results were reported by 

 Marshall & Talbot (56). 



Facilitation may occur also at the cortical level. The 

 matter is far more complex, however, for at least two 

 reasons. In the first place, the t\pe of facilitation just 

 described for the geniculate occurs at each cortical 

 synapse. That is, facilitation builds up step by step 

 at each synapse in the sequence, even though the 

 facilitation at each synapse in the cortex may be no 

 greater than the geniculate synapse facilitation. The 



I 2 3 4 5 6 7 e 9 10 II 12 13 14 15 16 17 18 

 REORGANIZATION OF RESPONSE OF CORTEX TO INTERMITTENT STIMULATION 



FIG. 14. The response of the optic cortex of the rabbit to rapidly repeated stimulation of the 

 optic nerve. At first the pulses are delivered more frequently than the corte.\ is able to respond. 

 Later to this same rate, the several channels capable of being activated become distributed in 

 time in such a way that no single channel needs to respond to successive pulses for there to be a 

 cortical response. [From Hartley (4).] 



