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HANDBOOK OF PHYSIOLOOV 



NEUROPHYSIOLOGY I 



although we do not know ilK-ir hinction, \vc know that 

 there are other nerve fibers which reach the peripheral 

 receptors but are not directly connected with the 

 receptor elements. 



All the actions which focus the sense organs on the 

 stimulus will evoke afferent signals of their own to be 

 related to the signals from the organ itself. Thus the 

 full report which comes to the central nervous svstem 

 will be far more complex and informative than any- 

 thing which could be furnished by any sense organ 

 isolated from the body and controlled only by the 

 electrophysiologist. 



Our primary concern, to be sure, is with the recep- 

 tors and their reaction to the stimulus. How and to 

 what purpose their reaction can be influenced by the 

 central nervous system opens up a different chapter 

 more concerned with the central than the peripheral 

 mechanism. But the receptors are there to decide the 

 line of behavior which the organism should follow; 

 they have to supply all the relevant information as to 

 what is happening from moment to moment, and from 

 this the central nervous svstem selects the items of 

 particular importance. It is essential, therefore, to 

 consider the sense organs not onlv as groups of recep- 

 tors e.xcited by particular physical or chemical events, 

 but as organs capable of presenting a detailed report 

 which will enable the event to be compared with 

 others of the saine class which have occurred before. 

 The description must ije as full as possible, yet it has 

 all to be conveyed l:)y trains of impulses in nerve fibers. 

 Though we can record the impulses there are still a 

 good many problems to l)e settled before we can reach 

 a clear understanding of how the full description of 

 the stimulus is handed on to the brain. 



The eye, for instance, can inform us that there are 

 patches of light on the retina of particular shape, in- 

 tensity and color. We suppose that the shape is sig- 

 naled by the distribution of the nerve fibers which 

 consey the signals and the intensitv by the number of 

 impulses arri\ing at a particular region of the nervous 

 system within a given time. Thomas Young suggested 

 in 1807 that the color may be signaled by particular 

 nervous elements sensitive to particular regions of 

 the spectrum, but in spite of the many fresh data 

 which recent work has given us, we have still to reach 

 agreement as to the way in which the information of 

 color is combined with that of intensity and area. 



Again we are aware that the olfactory organ enables 

 us to distinguish an immense variety of odors. We 

 know that the temporal and spatial pattern of ex- 

 citation in the organ mav varv with the smell and 



that some of the receptors vary considerably in their 

 sensitivitN to different kinds of odor. It seems probable 

 that these different sources of information can be com- 

 bined to give the full range of discrimination, but 

 it is not yet clear how the combination is achieved. 



The receptors in the skin and in the tissues beneath 

 can give a great deal of information about the nature 

 of the object in contact with it and active exploratory 

 movements help us to judge shape, size, hardness, etc. 

 But even a light contact on a passive surface will 

 produce a discharge of impulses in a variety of afferent 

 fibers of different diameter and rate of conduction 

 coming from receptor organs of different structure. 

 Zottcrman's studies of the temperature receptors 

 have shown that these at least form a group with a 

 characteristic structure and behavior. With the 

 receptors for touch, pressure and pain, however, .we 

 are still ignorant of the role of different types of axon 

 and ending in producing sensation which can vary so 

 much in c|uality and in the attention and action which 

 it will arouse. 



With all this to occupy us at the periphery we need 

 not be in too great a hurry to follow the sensory dis- 

 charge into the central nervous system where it will 

 be far less easy to analyze. But there is one problem 

 which deserves mention at the present time because 

 we may be already on the way to its solution, or at all 

 events to its investigation. It is the problem of access 

 to the higher levels of the brain. The sense organs 

 provide a running commentary on a great varietv 

 of environmental circumstances, but the organism 

 has to select the particular reports which have an 

 important bearing on its present and future behavior. 

 The classical method of investigating the sense organs 

 by comparing stimulus and sensation can throw no 

 light on this selectixe treatment, for the subject has 

 to fix all his attention on the one stimulus. He must 

 look for a feeble illinnination or a slight change of 

 color or listen for a faint click or a just detectable 

 change of pitch. When he lectures to a class, however, 

 such stimuli may ha\e no effect at all on his sensory 

 experiences or on his course of action. Indeed this 

 method of research, though it can tell us the effect of 

 a particular sense organ on the attentive mind, cannot 

 be expected to tell us how the other sense organs can 

 be prevented from reaching it. 



This problem of attention is not likely to be settled 

 finally until we know far more about the processes 

 involved in habit formation, in the factors which 

 attach importance to particular stimuli and in those 

 which balance conflicting claims from moment to 



