' IV' 



II WDHdllK OF I'HYSIUl ( ii.Y 



M UROPHYSIOLOGY III 



included), a lineal relationship was found. Since the 

 frequency at which flicker Fusion occurs is propor- 

 iKin. il in the logarithm <>f the brightness of a flicker- 

 in- light, Enroth (and also Granit) make the in- 

 ference i hat brightness is directly related to nerve 

 impulse flow. 



Experiments arc yel to be done in which processes 

 in sensory neural pathways and centers are con- 

 trolled and manipulated directly while behavioral dis- 

 criminations are observed. First steps in this direction 

 have been taken by the utilization of implanted 

 electrodes to record neural activity from or to stimu- 

 late neural centers in unanesihetized animals. 



Another method, one almosl as old as the history 

 of scientific investigation of sensation, has gisen U s 

 valuable data about the pans of sensors systems which 

 are necessar\ for different kinds of sensory dis- 

 crimination and has, moreover, enabled us to make 

 some inferences about the aeurophysiological proc- 

 esses which underlie sensors discrimination. The 

 method is that of testing discriminatory capacity in 

 animals after selective ablation of neural centers or 

 (he transection of neural pathways. The validity of 

 the results obtained b\ use of this method depends 

 on a number of factors, particularly: a) the definition 

 of appropriate neural 'units' to be rendered non- 

 functional by ablation, //) the exactness of surgical 

 procedures, , i the adequacy of pre- and postopera- 

 tive methods of measuring discriminatory capacity 

 and <l) the accuracy with which the damage done 

 surgically is evaluated post-mortem. [For a more 

 complete discussion of the ablation method, see 



\V|| & Diamond ( I ",()).] 



The capacity to discriminate changes in intensity 

 ol siiinuli applied to sense organs has been measured 

 carefully by behavioral techniques in a variety of 

 experimental animals before and after ablation of 

 the sensors projection areas ol all of the sensors sss- 



tems, The visual and auditor) systems base received 

 the most careful attention and consequently the re- 

 sults for these systems are most complete. Tactual 



discrimination has also been measured in a number 



ol studies. Due to difficulty in controlling stimulation 



lack ol adequate data on central pathways and 

 Centers, the remaining senses base been examined 

 onls In a less preliminary experiments. 



I he ( apaens lo discriminate small changes in the 



intensity of light is affected ver) little or not at all 

 lis bilateral ablation oi th< cortical projection areas 

 ol the visual system, Discriminations ol differences in 



intensity Ol visual stimuli base been measured in 



the rat (n8), cat (194), dog (137, 223 and 



monkey ( 1 1 j, 114) after complete bilateral ablation 

 of the visual cortex. 1 " Kliiver has shown for the 

 monkey that postoperative discriminations are made 

 on the basis of differences in total luminous flux 

 rather than on differences in brightness (113, 114). 

 While appropriate control tests were not made in 

 the experiments done on other species, a parsimonious 

 conclusion is that they also discriminate on the basis 

 of differences in total luminous flux. 



Discrimination of changes in sound intensity has 

 been studied for lesser animal species, but for the 

 cat, at least, evidence is fairly conclusive that capacits 

 to discriminate small differences in intensity of sound 

 remains after complete bilateral ablation of auditory 

 cortex (144, 171, 181). Evidence from studies in 

 which other kinds of discrimination sverc measured 

 suggests that similar results might be predicted for 

 the dog and monkey (74, 104, 131). 



Only rather crude measures of capacity to make 

 discriminations of differences in tactual and kines- 

 thetic stimulation have been obtained in experimental 

 investigations (9, 10, 15, 45, 1G0, 188, 232, 233). The 

 evidence that is available supports the conclusion 

 that, for these senses also, capacity to discriminate 

 small changes in intensity remains after bilateral 

 ablation of the cortical projection areas although 

 thresholds, at least in weight-lifting tests, may be 

 slightly raised (188). 



Effects of transecting afferent pathways at sub- 

 cortical levels or of destroying subcortical centers of 

 the sensors systems base been studied in only a vers 

 few experiments. Experiments in the rat show a loss 

 in capacits to make brightness discriminations after 

 ablation of the pretectile region of the thalamus and 

 the optic tectum in addition to bilateral ablation of 

 the striate cortex (119). R.iab & Ades (171) base 

 reported that, although the difference thresholds are 

 increased, the cat can make discriminations of in- 

 tensity of pure tones after bilateral ablation of the 

 inferior colliculi as well as of the audilors cortex. 

 The experiments of Sjoqvist & Weinstein (193) show 

 that kinesthetic discrimination of differences in 

 intensity (weight-lifting test) can be made after 



bilateral section of the medial lemniscus and spino- 



10 In in. ins c.irls and even in some more recent studies, .ill of 

 tin- visual projection areas is they would be defined at present 

 were QOt ablated; or, from tin- anatomical data reported, it is 

 impossible In es .ilu.iir tin- completeness of the ablations. In the 

 studies cited there sv.is an attempt at anatomical control, and 

 ill sum. 1 .isrs at least in e.u h stu,l\ the v isu.il em te\ appears to 



have been complete!) removed or the radiations toil transected 



