SENSORIMOTOR CORTICAL ACTIVITIES 



805 



pletely masked by other stronger movements. For 

 example, fanning of the toes occurred at an intensity 

 of i.o v., while with 1.4 v. no fanning of the toes oc- 

 curred, but violent flexion of the knee and hip was 

 elicited. 



Murphy & Gellhorn (341 j found by direct measure- 

 ment that spread of current was probably a negligible 

 factor in spread of additional movements at higher 

 stimulus intensities. They incised the cortex between 

 two points representing shoulder and thumb areas 

 and inserted an insulating sheet into the incision. The 

 original responses from these two points still occurred. 

 Moreover, differential responses elicited from the 

 'shoulder" point, with high and low frequencies pro- 

 ducing separate shoulder and thumb movements 

 respectively, were still present after the incision. The 

 same changes of motor response with varying fre- 

 quencies persisted on stimulation of white matter 

 after removal of the overlying cortex, suggesting that 

 integrative processes at subcortical or spinal levels or 

 both may be important in producing these variations 

 in motor response dependent on either pulse frequency 

 or duration. 



The part played by cells in different layers of the 

 cortex in the initiation and maintenance of movement 

 remains obscure, and unit studies bearing on this 

 problem will be discussed below. It may be pointed 

 out from thermocoagulation and depth studies that, 

 while motor responses may arise from pyramidal 

 cells in the deeper layers of the cortex, there is some 

 evidence that superficial cellular layers are also in- 

 volved (53, 124). Dusser de Barenne & Marshall (128) 

 showed a definite lowering of the threshold to faradic 

 stimulation when i per cent procaine was painted in 

 a circle around the point stimulated and suggested 

 that there was a release of this point from inhibiting 

 influences of surrounding cortex. 



It has long been recognized that motor responses 

 from the cerebral cortex may involve inhibitory as 

 well as excitatory effects (70). Bosma & Gellhorn 

 (52) noted decreased firing in the electromyogram 

 during cortical stimulation and the direct visual ob- 

 servations of Clark & Ward (92), Cooper & Denny- 

 Brown (loi), Sherrington (394) and Cure & 

 Rasmussen (106) revealed that subliminal stimuli, 

 insufficient to produce an active mosement, elicited 

 an obvious relaxation of the tonically contracted ex- 

 tremity. Raising the stimulus considerably above 

 threshold produced stronger and additional move- 

 ments. Instances of simultaneous contraction of an- 

 tagonistic muscles were not observed either during 



cortical stimulations (192) or when the stimulus was 

 applied to the white matter (191). 



Nature of Cortical Representations 



Turning to the question of the nature of the cortical 

 inotor representation of body regions in the cerebral 

 cortex, one is immediately faced with the problem as 

 to whether individual muscles achieve a representa- 

 tion in the precentral strip, or whether the representa- 

 tion is of groups of muscles concerned in patterned 

 activity necessary to the performance of movement. 

 The issue has been vigorously debated and admits 

 of no easy solution. Moreover, studies in unanesthe- 

 tized animals indicate the possibility of evoked motor 

 responses from widespread areas of the cerebral 

 cortex (276, 277). 



Walshe (462-465) has argued that whenever a re- 

 action is evoked from stimulation of the motor area, 

 it is an organized pattern of response involving re- 

 ciprocal innervation of opposing muscle groups and 

 not merely the reaction of a single muscle or part of a 

 muscle. The opposite point of view, namely that in- 

 dividual muscles may be activated by appi'opriate 

 stimuli, has been vigorously championed by Fulton 

 (161 ) and others. Chang et al. (88) isolated eight mus- 

 cles and showed that, at certain cortical points, thresh- 

 old stimuli evoked "solitary responses", i.e. an isolated 

 reaction of a single muscle. In some of the larger 

 muscles, separate groups of fasciculi within a par- 

 ticular muscle mass were indi\idually stimulable, and 

 they concluded in consequence that at some level 

 "muscles must be represented as muscles" so that 

 they can be manipulated and organized into inove- 

 ments. 



At least a partial solution to this intellectual im- 

 passe has been offered by Liddell & Phillips (269- 

 271), who noted the change in the baboon's cortex 

 from the map of few effects and extensive responsive 

 areas elicited by single shocks to the map of many 

 effects and narrow areas delineated by repetitive 

 stimulation at 50 per sec. The latter grows into the 

 traditional map of 'motor points.' They agree with 

 Walshe that Jacksonian fits have their characteristic 

 form of onset because the movements concerned are 

 tho.se that have the widest fields of low threshold. In 

 further studies of the motor cortex in the baboon, 

 with single shocks i to 5 ma in strength and 5.0 msec, 

 in duration, they elicited flick movements of thumb, 

 index and minimus, of the hallux and other toes, of 

 the tongue and angle of the mouth, centered on the 

 middle, medial and lateral thirds, respectively, of the 



