SENSORY DISCRIMINATION 



'455 



size of electrodes used and to their placement in the 

 cochlear nucleus. In the Tasaki & Davis study, the 

 microelectrodes were sufficiently small to record 

 intracellular potentials; those used by Galambos & 

 Davis were larger and could only record extracellular 

 potentials. Furthermore, Tasaki points out that he 

 and Davis probably recorded from the ventral 

 cochlear nucleus where there is an absence of cell 

 bodies with long dendrites and consequently an 

 absence of large external potential fields generated 

 by the cell bodies when their surfaces are uniformly 

 activated. Galambos & Davis, on the other hand, 

 were more likely recording from the dorsal cochlear 

 nucleus where there are many cells with long dendrites 

 which can generate large potential fields around 

 them. In unpublished experiments, Galambos & 

 Tasaki report having observed "suppression of 

 spontaneous discharges with submicroscopic elec- 

 trodes inserted into the dorsal cochlear nucleus of the 

 cat" (204). 



With electrodes in the mitral cell layer of the 

 olfactory bulb of the rabbit, Adrian (4) recorded the 

 activity of single neural units and observed 'spon- 

 taneous' discharge in the absence of intentional 

 stimulation and increase in discharge rate when the 

 concentration of substance stimulating the olfactory 

 receptors was increased. 



Results for all the senses agree that, in addition 

 to an increase in rate of impulse discharge in some 

 units, increase in intensity of end-organ stimulation 

 results in the excitation of more nerve fibers. There 

 is also evidence that some sensory fibers have higher 

 thresholds than others and are therefore activated 

 only by high intensities of stimulation (fig, 142, 1 * > 7- , 

 203). 



Although the simple relationship between stimulus 

 intensity and rate of neural discharge is complicated 

 by such phenomena as spontaneous neural firing 

 and increased rate of discharge in some neurons at 

 the same time that rate is decreased in others, it 

 seems a safe conclusion that if both frequency of dis- 

 charge and increased number of fibers are con- 

 sidered, increase in intensity of end-organ stimulation 

 is represented in the peripheral pathway of each 

 sensory system by increase in total flow of nerve 

 impulses. 



For sensory centers in the brain stem and cere- 

 bral cortex, there is only limited and, for the most 

 part, indirect evidence relating frequency of nerve 

 impulses in individual nerve fibers and spread of 

 response or number of nerve fibers activated as func- 

 tions of peripheral stimulus intensity. It has been 



shown that increase in intensity of peripheral stimula- 

 tion may result in increase in neural discharge in 

 midbrain centers, but, as would be expected, the 

 relation is not always a simple one. As in more 

 peripheral centers and pathways, factors such as 

 spontaneous discharge and its suppression by stimu- 

 lation in some instances and different thresholds of 

 excitation complicate the picture (65, 71, 73, 84, 106, 

 107, 178, 206, 208). 



Studies of responses evoked at the cerebral cortex 

 support the hypothesis that the total number of 

 impulses arriving at higher neural centers is corre- 

 lated with stimulus intensity. At threshold intensities 

 of peripheral stimulation, a small evoked response 

 can be recorded tvpically from a relativelv limited 

 region of the relevant cortical projection area of the 

 deeply anesthetized preparation. As intensitv is in- 

 creased, the amplitude of the evoked response is 

 increased and responses can be obtained from a 

 wider expanse of cortex. Insofar as these changes 

 can be taken as indicators of increased frequency of 

 nerve impulses arriving al the cortex and of increase 

 in number of projection fibers conveying response, 

 1 Inn the evidence from examination of evoked cortical 

 activity supports the hvpothesis stated above (94, 

 123, 210). Investigations using microelectrodes to 

 record responses of individual cortical units also 

 provide evidence that with increase in intensity of 

 peripheral stimulation more units are fired and in 

 many units more impulses per second are elicited 



(".47. 5 6 )- 



Establishing a relationship between intensitv of 

 stimuli applied to sense organs and the total flow of 

 nerve impulses in the neural pathways and centers 

 connected with these end-organs is an essential first 

 step in discovering the neural basis of intensitv dis- 

 crimination, but i( is not sufficient as an explanation. 

 It is also necessary to relate neural activity to be- 

 havioral discrimination. As one example of an at- 

 tempt to do this, Granit (821 has cited the studies of 

 Enroth as evidence for the proposition that bright- 

 ness discrimination in vision is directly related to total 

 flow of nerve impulses in the visual system. Enroth 

 recorded the response of single fibers of the optic 

 nerve when the eye was stimulated by light at dif- 

 ferent flicker rates and different intensities. She 

 counted the number of nerve impulses elicited by 

 the last four flashes of light preceding and immedi- 

 ately following the point at which fusion was reached. 

 When frequency of nerve impulses was plotted 

 against flicker fusion frequency for a large sample of 

 nerve fibers ('on' and 'off' type fibers both being 



