734 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



second reason is that the size of the cortical response 

 varies independently from the amplitude of the re- 

 sponse of the geniculate, owing to the phase of the 

 spontaneous alpha rhythm that is involved at the 

 time. The greater responses are elicitable during the 

 final surface-positive phase of the cycle (i8). 



The depression phase following activation of a 

 pathway may be a more general thing than merely a 

 phenomenon of the optic pathway. Such a phenom- 

 enon was dealt with by Pitts (6i) in respiratory func- 

 tion. Phasic fluctuations of response to a second click 

 stimulus have been observed in the auditory tract by 

 Ro.senzweig (63). 



As an extension of the description of the phasic 

 nature of cortical activity both spontaneous and 

 evoked, and thus as a furtherance of the understanding 

 of the mechanisms that possibly underlie brightness 

 enhancement, we shall describe the changes in spon- 

 taneous activity of the cortex following a single optic 

 nerve stimulus. 



The spontaneous picture in the rabbit and cat dif- 

 fers (21). Whereas in the rabbit extended periods of 

 alpha wave activity are t\pical, in the cat similarh 

 dealt with one may find short trains of alpha waves, 

 but more often this is replaced by a continuous rapid 

 sequence of low-amplitude waves varying in fre- 

 quency from 20 to 80 per sec. This sort of sequence 

 tends to appear during the intervals between the 

 "spindles' of alpha wave activity. 



If stimulation is presented during this fast-wave 

 activity, the cortex undergoes a characteristic altera- 

 tion. The waves just mentioned disappear and slowly 

 come back over a period of from 100 to 200 msec. At 

 the end of this period, the amplitude of these waves 

 mav be far above normal in some cases. Sometimes 

 the waves may coalesce into longer waves as if two 

 or more had summed. These may be of a higher am- 

 plitude and are frequently diphasic. The whole se- 

 quence, in amplitude variation and in temporal 

 features, often presents the over-all envelope of the 

 typical alpha wave. 



In the rabbit also, the t\pical alpha wave may be 

 replaced at times by three or more peaks with the 

 same over-all duration as the alpha wave (26). The 

 differences in the two animals presumably consists in 

 a smoother summation in the one than the other, 

 rather than in the presence or absence of the alpha 

 cycle. The depression cycle in both animals seems to 

 be a recovery from the peripheral input and a return 

 to spontaneous activity. 



When submaximal rather than maximal stimuli are 

 u.sed, the response to a second stimulus of a pair is 



le.ss depressed. The spontaneous activity is also less 

 depressed. The amplitude of the specific response 

 during the depression period is a function of depres- 

 sion in both the geniculate and cortex, but the re- 

 covery in the cortex seems to be dependent upon 

 events in the cortex alone. 



The basic picture of how the overall systems must 

 react to intermittent inputs was summarized by 

 Bartley (6) in what he called the "alternation of re- 

 sponse' theory. The essentials of the theory are as 

 follows, a) There is a fixed number of parallel channels 

 in the optic pathway from eye to brain. 6) These 

 channels can be activated simultaneously or they can 

 be activated according to the various temporal distri- 

 butions, f) Certain maximally intense, but abrupt and 

 brief, stimuli may activate all available channels while 

 submaximal stimuli do not. d') Any given single chan- 

 nel from eye to cortex cannot be reactivated until it 

 has recovered. This requires about 0.2 sec. for the 

 rabbit and o. i sec. for man. i) The period represented 

 in the cycle is of the same length as the animal's alpha 

 rhythm. In fact, it is the alpha rhythm, as was indi- 

 cated in the work of Bartley (15). /) Brief stimuli de- 

 livered at the alpha rate would be expected to produce 

 maximal brightness effects, g) Not only must the 

 stimuli be intense (maximal or in the upper range of 

 intensity) but they must be relatively brief, else they 

 would involve not only the initial activation of chan- 

 nels but also the reactivation of the same channels or 

 the activation of still others, tending to spread the 

 over-all activity out in time and reduce the number of 

 channels participating in the responses at any single 

 instant. /;) Since the available channels, as has already 

 been said, may be activated not only in unison but 

 also in various temporal distributions, various patterns 

 of the latter would result in corresponding levels of 

 sensory brightness. This is to say that with continuous 

 stimulation the activity of the available channels be- 

 comes uniformly distributed throughout the cycle. 

 There would be as many channels going into action 

 at all instants as are going into rest. This would pro- 

 vide for continuous uniform visual (sensory) response 

 to continuous illumination. In fact, sensory continuity 

 and steadiness may result before the full uniformity of 

 channel activity is achieved. 



Bartley (11, 12, 13, 16, 17) and colleagues ha\e 

 performed a number of sensory experiments, and in 

 all cases the expectations of the alternation of response 

 theory have been met. 



It may be said, then, that in brightness enhance- 

 ment and in the group of findings in regard to the 

 way in which the optic pathway is able to react to 



