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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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 IMPLICIT TIME 



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FIG. lo. The relation between implicit time of the rabbit's 

 cortex and the visual angle of the target. Note break in curves 

 at or near 20 degrees. This suggests that the increase in target 

 size beyond this point does not involve further spatial summa- 

 tion at the retina, and that further reduction in implicit time 

 is a continuation of the effect from increasing the intensity of 

 incidental stray illumination of the retina. [From Hartley (3).] 



monkey is not essentially different from that of the 

 normal monke\- or of man. 



The decorticated monkey responds differentially to 

 two equally intense and equally large targets if their 

 distances from the eye are quite different. This is to 

 say, the animal is able to use differences in total flu.x 

 as a factor. This reaction, like the others in which 

 familiarity with the stimulus is ab.sent, has to be 

 learned rather than immediately apprehended. 



Responses to targets differing in shape, preopera- 

 tively established, are lost through the remosal of the 

 occipital lobes. Neither can new differential responses 

 be learned if the new targets are compact and do not 

 differ greatly in any dimensional respect. A decorti- 

 cate animal, taught to respond to a target having a 

 greater amount of flux, will continue to do so regard- 

 less of whether the flux is continuous or intermittent. 

 Thus a source with an on-off rate of 4 per sec. is 

 equivalent to a continuous one just so long as the 

 total flux per second is the same. 



Color vision is permanently lost in the decorticate 

 animal. Kliiver would say, from his many studies, 

 that brightness vision is destroyed in the animal 



without occipital cortices. What such an animal does 

 is to respond on the basis of total flux. 



Area of Target 



When a photic source (target) subtending some 

 known \isual angle at the eye is used as a stimulus, the 

 resulting response of an animal may be that which we 

 could call the perception of brightness, or it may be 

 simply the response to the stimulus as total flux. 

 Brightness is the response to flux per unit area of 

 target, and may be expressed as an experience or as 

 a differential motor response on the i^asis of flu.x per 

 tmit area. It is not difficult to determine which of the 

 two possible responses is being made when the human 

 is a subject, and not too difficult when certain sub- 

 human species are being tested. It is a little more 

 uncertain when inferred in some cases such as from 

 the cortical response to photic stimulation of the 

 retina. 



The experiments of Hartley (3) on manipulating 

 the target area to measure the implicit time of the 

 cortical response will be presented here. Implicit time 

 is the time elapsing between the beginning of stimulus 

 and the peak of the initial large wave of response. In 

 this investigation, target area was varied from i to go 

 degrees of visual angle. Throughout this range incre- 

 ments in visual angle reduced implicit time of the 

 cortical response. One of the significant findings was 

 that the relation between target area and implicit 

 time was not a simple function throughout the range 

 used as was the function of target intensity. To explain 

 this, it was pointed out that the image of the target 

 on the retina was not the only site of retinal stimula- 

 tion. The retina as a whole received stray radiation 

 as well as focused radiation. Thus the cur\e showing 

 the relation between implicit time and stimulus area 

 is a composite of the increasing spatial summation 

 within the image and the increasing intensits' of stray 

 photic flux on the retina as a whole. 



If the target area operates as just indicated, one 

 ought not to expect the resulting curves relating area 

 to implicit time and intensity to implicit time to 

 coincide. .Since spatial interaction and stimulus in- 

 tensity are both varied when target area is manipu- 

 lated and stimulus intensity alone when photic in- 

 tensity is manipulated, the curve for the former would 

 lie to the left of the latter. Thus, if interaction (spatial 

 summation) would operate over part of the range of 

 target manipulation and not over the whole range, 

 then a break in the response curve would be expected. 

 This is what resulted, as appears in figure 10. In all 



