CENTRAL MECHANISMS OF VISION 



735 



timing of input, we ha\c one of the more fully docu- 

 mented sets of the relationship ijciween sensory be- 

 ha\ ior (perception) and neurophysiology of the 

 central nervous system. 



In the foregoing, it has been shown that stimulus 

 conditions for obtaining various brightnesses and 

 those for obtaining various amplitudes of cortical 

 response are the same, but much still remains to be 

 worked out. For example, the comparison seems to 

 pertain to the quantitative features of continuous 

 over-all brightness and the amplitude of a momentary 

 feature of cortical activity, namely the specific brief 

 response. As yet, there is no characteristic of recorded 

 electrical response of the cortex to indicate the level of 

 cortical activity in response to a continued peripheral 

 stimulus. It is as if we are confined to dealing exclu- 

 sively with momentary and brief effects in the central 

 nervous system. They can be dealt with because they 

 represent an observable change from previous or from 

 subsequent activity as a reference. Continued stimula- 

 tion does not result in any characteristics of prolonged 

 cortical activity that lend themselves to useful quanti- 

 fication. One approach to this is, of course, the com- 

 parison of the appearance of the ongoing cortical 

 activity during stimulation with activity in the absence 

 of an intended experimental input. This, as was 

 implied, does not give anything to quantify in a 

 direct way. The chief difference between cortical 

 records in the two sets of conditions seems to be the 

 disappearance of certain forms of wave-like activity 

 in the 'active' record. Various studies on 'blocking' 

 the alpha rhythm are relevant here (64). They were 

 also relevant in the earlier section on brightness. 



Jasper & C^ruikshank (49) studied the electro- 

 encephalograms of human subjects exposed to a cross- 

 target in a room in which this was the only photic 

 stimulation. They ascertained the change in the cor- 

 tical activity picture to the sudden exposure to the 

 target and the subsequent sequence of changes that 

 followed. They found the following: a) an occasional 

 and \aried short detectal)lc cortical effect arising in a 

 few milliseconds easily confused midst the features of 

 the alpha rhythm; A) 'blocking' of the alpha rhythm 

 after a latency of 160 to 520 msec. ; c) gradual irregular 

 reco\ery of the alpha rhythm if the stimulus continued 

 for more than 3 to 5 sec. ; dj the emergence of a second 

 dubious positive effect that, since it followed the 

 termination of exposure to the target, could be called 

 an 'off' effect; i) sometimes a second 'blocking,' this 

 time of the recovered rhythmic activity, following 

 cessation of the stimulus;/) a continued depression or 

 'blocking' effect during; the existence of reported 



afterimages; g) a partial recovery toward the usual 

 amplitude of alpha waves between successive after- 

 images; and //) a final total recovery of the normal 

 alpha activity following the final afterimage. This 

 recovery typically would begin as a train of small 

 waves of higher frequency than those of the alpha 

 rhythm. The amplitude of the alpha waves might even 

 increase for a while before the full prior status quo 

 would be reached. Here we have a set of results 

 seeming to bear upon several matters: the nature of 

 the cortical activity during continued stimulation, 

 and the fact that one can detect cortical response 

 during afterimages as being different than when they 

 are absent. Others have been interested in the latency 

 of the blocking effect, but we shall forego listing the 

 authors or the exact latencies found. 



To further the understanding of what constitutes 

 the cortical response to continued stimulation, certain 

 reference conditions for inactivity will have to be 

 discovered. From these comparisons can be made. 

 One of the possible leads in this direction may be the 

 study of dendritic behavior. We have progressed from 

 the exclusive concern with and ability to record 

 spike-like, momentary, conducted all-or-none activity. 

 The activity of dendrites seems to fall into the category 

 of sustained potentials (36, 37). While sustained states 

 seem to be 'inactive' ones, since we cannot detect 

 them as ongoing processes, this static aspect may be 

 only the over-all aspect of the whole complex of 

 activities that is in operation, thus for us an 'illusion.' 



When we realize that what is happening in any 

 mass of central nervous tissue is a combination of 

 \arious orders of process, having many origins and 

 sustaining conditions, we may find added use, in our 

 concept, for sustained states. They may be the sub- 

 strata for the interplay of more highly particularized 

 items of activity that occur differently and play dif- 

 ferent roles during one level of sustained potential 

 than during another. It thus might become possible 

 to conceive of the level of sustained dendritic potential 

 in crucial areas as being the correlate of the experi- 

 ential or motor outcome in the ultimate response 

 called perceptual behavior. 



In essence, the idea of a sustained state, varying in 

 significance or potency according to its level, is 

 nothing new. We have long had it in the central 

 excitatory state and in the central inhibitory state of 

 Sherrington. But, to understand the sustained state as 

 being inherent in a neuron rather than in some sort of 

 a chemical matrix outside it is very different. In 

 dendritic activity, we may now have a basis for sus- 

 tained potentials as an activity of neurons themselves. 



