SENSORIMOTOR CORTICAL ACTIVITIES 



799 



COMPARATIVE STUDIES OF EXCITABLE CEREBRAL CORTEX 



Historically, since the earliest experiments of 

 Fritsch & Hitzig (158) a formidable body of data has 

 been gathered on the arrangement of cortical zones 

 from which somatic movements can be elicited by 

 electrical stimulation or by application of drugs (31, 

 32, 186, 223) to the cortical surface. With increasing 

 sophistication of electronic techniques, many of the 

 problems associated with control of stimulus param- 

 eters have been solved, but the probability of cur- 

 rent spread to areas remote from the site of stimula- 

 tion has continued to beset investigations in this 

 area and may be responsible for many seeming in- 

 consistencies. In their experiments, Fritsch & Hitzig 

 (158) and Hitzig (205-207) used bipolar gahanic 

 stimulation. In 1873, Ferrier (143) introduced faradic 

 stimulation as preferable to galvanic stimulation. 

 Although in his hands the movements were less dis- 

 crete than those described by Fritsch & Hitzig and 

 were evoked from wider areas of cortex, refinements 

 in the faradic technique led to its general acceptance. 

 A logical extension of this method has been the appli- 

 cation of square-wave and other pulse trains under 

 carefully controlled conditions (see below). 



Phylelic Aspects of Cerebral Cortical Representation 



The progressively augmented complexitx' in the 

 cortical motor representation of body regions seen in 

 the higher mammals may be assessed by the increasing 

 intricacy of motor patterns elicited by cortical stimu- 

 lation in higher primates, including man, and by the 

 severity and persistence of motor defects after corti- 

 cal resection in the primates as compared with lower 

 mammals. 



It must be emphasized that cortical stimulation 

 with trains of pulses or sine waves has failed to elicit 

 anything but fragments of skilled movements. The 

 relatively crude nature of these responses may well 

 arise from the inability of artificial stimulation to 

 simulate the natural patterns of cortical excitation. 

 The progressive incorporation and elaboration of 

 motor functions at the cerebral cortex may be corre- 

 lated, too, with the relative and absolute increase in 

 the size of the responsive areas in man and higher 

 apes, and with the increasing differentiation of the 

 microscopic arrangements of the motor cortex, both 

 in the volume of the dendritic field of individual 

 cells and in the laminar organization of the cortex as 

 a whole. 



In reptiles, such as the alligator, turtle and lizard. 



the formation of the motor corte.x is foreshadowed 

 (29, 30, 222). In these forms, the axons of the stimu- 

 lable cortical cells, do not form a corticospinal tract; 

 this makes its first appearance in mammals (226). 



In the monotremes, Martin (291) found a large, 

 ill-defined responsive area on the anterior half of the 

 lateral surface of the hemisphere of the platypus. 

 There was a large face area with forelimb responses 

 from the same region at a higher threshold. No hind- 

 limb movements were evoked. The marsupials also 

 showed a poor separation of face and forelimb repre- 

 sentations. Hind-limb representations tended to be 

 inconstant and movements diffuse (i, 145). Even in 

 the higher marsupials, such as the kangaroo, hind- 

 limb responses were not elicited in some animals 

 (437). Huber (212) suggests that, in the evolution of 

 their motor cortex, the marsupials have not reached 

 the stage where their hind-limb representations are 

 stably represented in the cortex, and that the response 

 variations noted may reflect individual variations in 

 the degree of representation. Since in the marsupial, 

 at least, maps of the excitable areas indicate move- 

 ments from regions forming parts of the somatic 

 sensory cortex, the possibility must also be considered 

 that movements are evoked in response to a sensation 

 (5) rather than as part of a primary motor response. 



Turning to the placental mammals, it would not 

 seem fruitful to re\iew extensively at this point the 

 numerous studies of cortical motor functions in lower 

 mammals since, at best, the knowledge so gained 

 gives but a fragmentary picture of the motor organiza- 

 tion in the total gamut of placental mammals. 



Among the smaller mammals, including insec- 

 tivores, bats and rodents, the excitable cortical areas 

 lie close to the anterior pole of the hemisphere. Rep- 

 resentation of facial musculature is more extensive 

 than of other body regions in these animals, with 

 representations of the snout muscles predominating 

 over other facial zones in forms, such as the hedgehog, 

 where the snout plays a major functional role (288, 

 437). Limb responses tend to be meager, with overlap 

 between forelimb and hind-limb areas. Hind-limb 

 responses are elicited at higher thresholds than those 

 from the forelimb and have not infrequently been 

 reported as absent. In the ungulates, limb representa- 

 tions have likewise been diHicult to evoke, even with 

 stimulation under local anesthesia (28, 397). 



In the carni\ores, the excitable areas are grouped 

 around the cruciate sulcus. In these animals a greater 

 complexity of cortically induced movement parallels 

 increasing histological differentiation. Facial and 

 forelimb areas are clearly separated and contralateral 



