INHIBITION AND OCCLUSION IN 

 CORTICAL NEURONS* 



A. L. TowE 



Department of Physiology and Biophysics, University of Washington 

 School of Medicine, Seattle, Washington 



The activity of individual cortical neurons has been under scrutiny for nearly 

 a decade. During this time, the general features of this activity have been 

 expounded, and we are now moving into a period of more refined measure- 

 ments and of theory building. From such pursuits will emerge a welter of 

 fine details regarding unit activity and a few general principles that will 

 clarify, to some extent, the functioning of the central nervous system. These 

 principles will rest upon two general processes, as yet poorly understood, 

 termed excitation and inhibition. So far in this symposium the discussion has 

 centered around inhibitory mechanisms in various invertebrates and in several 

 mammalian tissues, especially the spinal cord of the cat. The existence of 

 inhibitory activity in higher synaptic systems, such as the cerebral cortex, 

 remains to be demonstrated. For this discussion we shall confine ourselves 

 to work on the somatosensory system of the monkey (Macaca mulattd). 



If we isolate a single unit in the postcentral gyrus with a microelectrode 

 and then explore the body surface by deflecting hairs, tapping the skin or 

 passing an electric current through it, we can locate a region of the skin which, 

 when so stimulated, will drive the unit to activity. The precise position, size 

 and shape of this region are not, at the moment, of concern. We will only 

 note that the region does not extend indefinitely, but is bounded ; this bounded 

 region is called the unit excitatory receptive field. The various parameters of 

 the cortical unit response change progressively as the point of stimulation is 

 moved from the interior to the edge of this field ; stimulation outside it fails 

 to arouse the unit. However, the boundary is not sharp, but is a tenuous 

 region where the excitatory process provoked by the stimulus and impinging 

 upon the unit becomes too feeble to be detectable. 



In analyzing the interaction effects, we shall confine our attention to those 

 units whose excitatory receptive fields occupy several digits of the hand. When 

 the skin is stimulated electrically, the extent of current spread must be con- 

 trolled. This can be accomplished by stimulating digits rather than broad 



* This work was supported by a grant (B396) from the National Institute of Neuro- 

 logical Diseases and Blindness, Department of Health, Education and Welfare. 



410 



