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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



a given visual end result is occasioned upon such and 

 such events in the bodily mechanism involved. 



It is fortunate that the same stimulus conditions 

 can and have been used to study the behavior of the 

 two categories of events. This is to say that not only 

 are photic stimuli used in both cases, but the very 

 same manipulations are used and seem effective in 

 giving us the data we need. 



The following remarks have to do with modes of 

 studying both vision and the neural mechanisms that 

 underlie them. Vision can be studied only in the intact 

 or near-intact animal. Neurophysiological mecha- 

 nisms can be studied in the reduced animal and in 

 animal preparations. Vision is to be studied only by 

 use of retinal stimulation with photic radiation. 

 Neurophysiological mechanisms (including central 

 mechanisms) may be studied by stimulating either 

 the retina or by eliminating it and stimulating the 

 optic nerve directly by electrical energy, or by directly 

 stimulating regions farther along in the pathway. The 

 electrical method provides for stricter control than 

 the photic and, though 'unnatural' in the temporal 

 pattern of impulses delivered, the central end of the 

 pathway is helpful in analyzing the nature of the 

 mechanisms involved. Eliminating the retina elimi- 

 nates the selective feature^ employed by it in pro- 

 ducing the optic nerve discharge. The retina selects 

 or emphasizes certain channels in the optic nerve 

 according to the distribution of the discharge into the 

 various channels in keeping with its own principles of 

 divergence and convergence in its neural circuits. 

 Virtually no discharge initiated by the retina involves 

 all channels simultaneously. They are activated in 

 temporal succession of some sort or another. 



When the optic nerve is stimulated directly 

 (electrically), all available channels may be activated 

 together in time and thus a very different reception 

 of the afferent input may occur at the stations along 

 the pathway. To discover just how nearly alike the 

 two forms of stimulation, in effect, can be is a matter 

 of empirical test. 



At this point, it may be appropriate to point out 

 one of the more salient features of the retina-initiated 

 optic nerve discharge, namely that it is composed of 

 three temporal orders, a) One is the maintained dis- 

 charge, a series of impulses lasting throughout the life 

 of the photic impingement on the retina. 6) Another 

 is the on-off discharge, occurring at the beginning and 

 also at the termination of the photic impingement, 

 c) The third type is the off discharge, occurring only 

 following the termination of impingement. There are 

 certain visual end results that seem to be explained 



on the basis of these differences. The retina is also 

 responsible for an unexpected end result, the seeing 

 of two flashes when the photic impingement (pulse) 

 is brief, moderate in intensity and singular, owing to 

 the fact that the two sets of sense cells do not have 

 the same latency. 



Modes of study in\olve not only the two forms of 

 setting up the optic nerve message but also manipula- 

 tions in the photic impingements themselves. In 

 general, three forms of timing may be employed : a) 

 single i-solated stimuli; /;) paired stimuli, in which the 

 two members of the pair are variously separated in 

 time; and r) trains of stimuli, often called intermittent 

 stimulation. In intermittent stimulation, time inter- 

 vals between stimuli may be varied, and the ratio 

 between the stimulus (pulse) duration and the length 

 of the cycle of intermittency may also be varied. 



These three forms of manipulation have turned out 

 to be much more than empty differences in form of 

 stimulation as will be seen later. The use of method 

 a provides for a response from a resting system, at 

 least as far as intended activation is concerned. 

 Method b proxides for the determination of the effect 

 of the first stimulus on the second, or otherwise stated 

 it provides for discovering how long it takes for the 

 reacting system to complete its response and reassume 

 status quo. Method c provides for still another aspect 

 of the reacting system to become manifest. Since the 

 optic pathway consists of a number of parallel chan- 

 nels, each with finite limits in the rate at which it can 

 be reactivated, it is possible that, when a whole train 

 of stimuli is dcli\ered at a rate beyond which single 

 channels can repeatedly respond, a redistribution of 

 the relationships between repeated pulses and the 

 responses to them occurs as stimulation progresses. 



Phenomena of Vision 



The phenomena of vision are the items to be ulti- 

 mately accounted for, hence it is necessary that we 

 have in mind what they are. Vision consists in the 

 appreciation of the surrounds via the use of the eye, 

 the nervous system, and in turn the effector muscles. 

 The feature of the environment to which response is 

 made is, of course, photic radiation. The dimensions 

 involved are spatial, intensive and temporal. Hence 

 it could easily be supposed that these w-oujd be the 

 experimental variables to be used. 



In vision a field is responded to in terms of intensive 

 components that, when they evoke experience, are 

 perceived as lightness and darkness of various degrees. 

 These qualities need not be stable but may be per- 



