SENSORY DISCRIMINATION 



'45' 



regions being prominent in most maps so pro- 

 duced. 



In experiments on animals, regions of the cerebral 

 cortex which were thought to be sensory' projection 

 areas were ablated and the effects on sensory- dis- 

 crimination were observed or tested crudely. In the 

 clinic the effects of brain damage on sensory per- 

 ception of patients were observed. 



During the last part of the nineteenth century, 

 considerable progress was also made in the use of 

 electrical stimulating and recording techniques to 

 explore the central nervous system. As Brazier 

 (30) has pointed out, it was known by the end of the 

 nineteenth century that changes in electrical activit\ 

 can be recorded from the cerebral cortex in response 

 to sensory stimulation. Better instrumentation was 

 needed, however, before the full value of electro- 

 physiological methods could be realized. 



There have been a number of significant methodo- 

 logical advances which have marked the study of the 

 senses during the twentieth century. First, and most 

 important, has been the rapid development of new 

 techniques in electrophysiology. Second has been 

 the use of the behavioral testing methods of the com- 

 parative psychologist in studies of sensory function in 

 animals. Third has been tin- refinement of tech 

 niques of experimental surgery. Fourth has been the 

 improvement of instruments for production of 

 sensory stimuli; in many instances, science lias profiled 

 lii 1111 the expansion of electronics in industry, fin- 

 ally, a most important trend which can be seen 

 today is the application of many techniques in a 

 single experiment. Methods of electrophysiology, 

 of neuroanatomy, of psychophysics and of psycho- 

 physiology are often used in a single investigation. 



NKI ROPHYSIOLOGICAL BASIS OF SENSORY 

 DISCRIMINATION 



Tin Primary Sense Modalities 



In his doctrine of specific nerve energies, Johannes 

 Miiller offered an explanation, in anatomical and 

 physiological terms, for discrimination of the pri- 

 mary sensory modalities: vision, hearing, touch, taste 

 and smell. Others extended the theory to account 

 for differences in sense qualities. In examining the 

 present status of neurological theories of sensory 

 discrimination, let us also start with the broader 

 problem of how the sense modalities are differ- 

 entiated and then go on to the more difficult task of 



evaluating the evidence for the neural mechanisms 

 underlying discrimination of attributes of sensation. 4 



The specific nerve energy doctrine assumed that 

 each primary sense modality had its separate nerve 

 supply and separate representation in the central 

 nervous system. An accumulation of evidence from 

 experiments using a variety of anatomical and 

 neurophysiological techniques has shown the es- 

 sential correctness of this basic assumption. Thus, 

 nerve fibers from the retina can be traced to the lateral 

 geniculate bodies in the thalamus and thence to the 

 striate cortex of the occipital lobes. In the auditory- 

 system, similarly, a direct restricted pathway can be 

 traced from the cochlea to a limited region of the 

 cortex of the temporal lobe via the primary cochlear 

 nucleus, superior olivary complex, inferior colliculus 

 and medial geniculate body. Receptors for the skin 

 senses project 10 the parietal lobe, again having 

 p.issed upwards through definite channels in the 

 spinal cord and brain stem. Pathways lor nerve libers 

 from taste and smell receptors have been carefully 

 traced into the central nervous system, but here the 

 evidence for final projection centers is not as clear .is 

 for the visual, hearing and somesthetic systems. 

 Pathways for kinesthetic receptors are parallel and 

 in close relation to those lor the skin senses and ap- 

 parent!) end in the same region of the cerebral 

 cortex. Although it was at one time thought that 

 the vestibular system differed from the other af- 

 ferent s\ stems in that it did not project to the cere- 

 brum but had onk reflex connections via lower 

 brain-stem centers, evidence has been obtained in 

 recent years which indicates an afferent pathway 

 paralleling that of the auditory system and ending in 

 an area of the temporal lulu- which is usually con- 

 sidered 10 be part of the auditory projection region. 

 On the afferent side, at least, the structural organiza- 

 tion for discrimination based upon a place or topo- 

 graphical principle appears to be present in most 

 mammalian species for all of the major senses. 



First scientific evidence for topographical organi- 

 zation of the sensory systems came from clinical 

 studies of the effects of brain lesions produced by 

 disease, accident or surgery, and from experimental 

 ablation studies in lower animals. 5 For the visual 

 and tactual systems, in particular, it was clear that 

 limited lesions of ascending pathways or of relatively 

 small cortical areas result in sensory deficit. For the 



5 Brief historical summaries of this topic are available (14, 

 46, 63, 64, 66, 95, 137, 157, 189, 213). 



