TOUCH AND KINESTHESIS 



405 



FIG. 7. Eight excitatory peripheral receptive skin fields of 

 the cat's foreleg, stiinulation of which activated eight single 

 neurons in the contralateral postcentral cortex. The neurons in 

 question were isolated at the levels indicated (in ii) below the 

 cortical surface in the course of a single microelectrodc penetra- 

 tion made perpendicularly to that surface. The fields are 

 restricted in size and are almost identical in location. [Modified 

 from Mountcastle (181).] 



Some measurements, however, have been made of the 

 fields which project upon neurons of the cerebral cor- 

 tex (181). Several such fields are shown in figure 7, 

 and the graph of figure 8 relates the sizes of the 

 peripheral fields to their location upon the body 

 surface. 



PROJECTION OF PERIPHER.JiL RECEPTIVE FIELDS UPON 



CENTRAL NEURONS. Figure 9 indicates that not all 

 parts of the peripheral receptive field of a thalamic 

 neuron have an equal potency for excitation of that 

 cell. The security of the relation varies from a ma.x- 

 imum usually, though not always, near the center 

 of the field to a minimum at its edge. The synaptic 

 linkages converging upon the central neuron from 

 afferent fibers innervating the edge of the field are 

 apparently .so few as to provoke only minimal activa- 

 tion of the cell, as measured by the early repetitive 

 response (which will be defined below). 



From data of this kind it is possible to reconstruct 

 the pattern of events set in motion in the lemniscal 

 system by a brief mechanical stimulus delivered to the 



skin. Before doing so it is convenient to describe the 

 response properties of single neurons of the system. 



Response Patterns of Neurons of Medial Lemniscal System 



REPETITIVENESS OF RESPONSE TO SINGLE STIMULUS. All 



the evidence at hand from study of first order axons 

 (168, 280) indicates that even very brief mechanical 

 stimuli to hairs or skin surface elicit a short train of 

 impulses in afferent nerve fibers (see fig. 5), even for 

 quickly adapting elements. Such an aff'erent input 

 elicits from the second (15), third (208) and fourth 

 order neurons (183) short high frequency trains of dis- 

 charges, a response pattern which is highly character- 

 istic of the system (fig. 10). Amassian & DeVito (15) 

 have shown that the early repetitive discharge in the 

 cuneate nucleus occurs under different conditions of 

 anesthesia, in the unanesthetized or decerebrate ani- 

 mal, and apart from variation of body temperature 

 from 33 to 41 °C. It is important to emphasize that it 

 occurs also when the afferent volley is made up of a 

 single impulse in each synchronously active fiber. The 

 repetitive discharge therefore is a general property of 

 the postsynaptic cell at the first relay of the system, 

 and indeed of those at each successive relay thereafter. 

 The repetitive response is not absolutely stable even 

 in the deeply anesthetized animal. Here, when exactly 

 the same peripheral stimulus activating a given neuron 

 is repeated at slow intervals some variation in the 

 number of impulses in the early repetitive response 

 does indeed occur. In a population of such responses, 

 many contain a characteristic number of impulses 

 per response (the modal value) while some responses 

 contain more and others fewer impulses (fig. 1 1). The 

 shift in the modal value indicates sensitively the 

 changing parameters of the stimulus, e.g. its intensity, 

 frequency or position (see figs. 9, 10, 12 and 13). 



RESPONSE OF SYSTEM TO SINGLE STIMULUS. Considering 

 the variations in the response of a single neuron when 

 the stimulus shifts across the receptive field it seems 

 possible to reconstruct the events in a population of 

 cells set in motion by a single stimulus, even though 

 it has not yet been possible to record the activity of 

 many single neurons simultaneously. .\ brief, strong 

 peripheral stimulus sets up a burst of impulses in a 

 number of afferent fibers. If the stimulus is brief 

 enough only one impulse occurs in each fiber; if it is 

 strong enough nearly all fibers are activated syn- 

 chronously. The impulses are conducted into the cord 

 and can be assumed to impinge upon a restricted 



