TOUCH AND KINESTHESIS 



425 



portant finding of all workers that the removal of the 

 postcentral region does not necessarily lead to any 

 early or permanent deficits in somesthetic discrimina- 

 tions indicates strictly only that the postcentral region 

 is not the sole cortical recipient and distributor of all 

 the corticopetal tactile activity. This, of course, is also 

 apparent from the electrophysiological considerations. 

 Whether, however, such findings indicate in any way 

 that the postcentral cortex is not relevant for processes 

 determining the somesthetic discrimination capacity 

 of the animal is another question. The anatomical and 

 electrophysiological evidence leaves hardly any douijt 

 that it must be relevant. The experimental psycho- 

 physical data are indicative that the postcentral region 

 need not always be essential. It seems reasonable to 

 consider the possibility that the answer may lie in the 

 nature of the somesthetic task which the animal is 

 trained to perform. If the somesthetic cues used in the 

 experiments differed crudely from each other (and 

 certainly most of them did so), it is possible that any 

 somatic sensory inflow which reaches the cortex after 

 removal of the postcentral region still contains enough 

 information to enable the animal to perform the task. 

 It seems probable that the unique role of the first 

 somatic field will become apparent if the animal is 

 asked to perform a task requiring the highly detailed 

 and complex information which, as it appears, is 

 available only to the primary receiving field. If this 

 were so the postcentral cortex would be necessary for 

 any tactile or kinesthetic discrimination task of suffi- 

 cient complexity. Whether it alone is ever sufficient 

 for learning of such a discrimination is yet 10 be de- 

 termined. The chances are that the answer to this 

 question will depend on what the animal is asked to 

 do with the information it has available. 



CONCLUDING REM.ARKS 



It may be useful to discuss at the conclusion of this 

 chapter experimental work which appears necessary 

 for the clarification of some ideas regarding tactile and 

 kinesthetic sensations. 



In the section dealing with the neural events in the 

 peripheral fibers it has been pointed out that a re\ival 

 in some form of the basic concepts of Head may be- 

 come advisable. What is well established is that 

 specific tactile receptors exist; what can be deduced 

 from some observations but what is bv no means vet 

 demonstrated is that generalized receptors, presum- 

 ably responding to all modes of cutaneous stimulation, 

 may exist as well. It seems futile to denv or ignore 



the convincing evidence regarding the specificity of 

 some tactile receptors; it is probably too much to 

 expect, on the other hand, that all experimental find- 

 ings will yet become understandable within the frame- 

 work of von Frey's concepts. It seems probable that 

 the clouds over the classic concepts are real and that 

 further work will establish the existence of generalized 

 (in addition to the specific) receptors which will 

 probably reopen the question of the existence of 

 epicritic and protopathic sensibility. 



Assuming that this development will take place the 

 problem will be to determine to what extent the 

 medial lemniscal and the spinothalamic systems are, 

 respectively, activated by these two types of receptors. 

 As far as the tactile activity is concerned there is 

 hardly any doubt that specific receptors activate the 

 medial lemniscal system. The available evidence indi- 

 cates that this system could represent the tactile (and 

 kinesthetic) epicritic system. The fundamental ques- 

 tion as to whether this system can be activated as well 

 by nociceptive and thermal stimuli must remain un- 

 answered at the moment. In anesthetized animals 

 only mechanical stimuli activate the medial lemniscal 

 system. It is, however, not known whether this repre- 

 sents the true state of affairs or whether such findings 

 are caused by anesthesia. The spinothalamic system 

 could be the obvious representative of the protopathic 

 system if the latter should exist. What little is known 

 about its tactile activity is compatible with the idea 

 that generalized receptors activate it. It is this system 

 which — in sharp contrast to the medial lemni.scal 

 system — seems to distriijute, together with other tracts 

 arising in the posterior horns the sensory somatic 

 activity throughout the brain stem. An extensi\'e inter- 

 locking between the medial lemniscal and the spino- 

 thalamic systems occurs in two places. The first is the 

 synaptic region of the posterior horns where the 

 lemniscal activity plays upon the cells of origin of the 

 spinothalamic system; the .second is the ventrobasal 

 complex of the thalamus where the spinothalamic 

 activity in turn interacts with the medial lemniscal 

 system. However, nothing at all is known as to the 

 meaning of these interactions. From clinical studies it 

 is clear that sensations of pain, temperature and 

 tickle, and those accompanying sexual excitement de- 

 pend upon the integrity of the spinothalamic .system. 

 Although this knowledge has been gained on large 

 human material, very little is known for certain about 

 the functional organization of the system, not even 

 whether or not its fibers are modality specific. It is 

 obvious that a number of basic problems of sensation 

 could be profitably explored in man in connection 



