SENSORIMOTOR CORTICAL ACTIVITIES 827 



mvm 



^^YV^<,V^^w•^YvAp^^1MW^ 



^, IIWHUUI — 



I 3ec. 



500 ;i». 



FIG. 6. Pattern of firing of units within 

 the motor cortex during the performance 

 of a conditioned avoidance movement. In 

 each record: A, microelectrode in the 

 motor corte.x; B, surface record from the 

 motor area; C, surface record from occipi- 

 tal area; D, electromyogram from the arm 

 performing the avoidance act (CR). This 

 act involved the opening of a switch, indi- 

 cated by an upward deflection in trace D, 

 thereby interrupting a stimulating circuit 

 which would otherwise have been acti- 

 vated foilouing presentation of the condi- 

 tioning stimulus (repetitive photic stimula- 

 tion applied during the time indicated 

 by the line at the bottom of the record). 

 Four different patterns of unitary activity 

 are shown. Top to bottom: acceleration of 

 unit discharge preceding and outlasting 

 the movement; arrest of unit discharge 

 at commencement of conditioning stimu- 

 lus, with return of discharge immediately 

 following the CR ; a brief burst of discharge 

 at the onset of the CS with no further 

 detectable participation of this unit in the 

 response; and arrest of firing both on 

 presentation of the CS and for some time 

 before the performance of the CR, with 

 return of discharge following the move- 

 ment. No changes were observed in the 

 pattern of firing of 25 to 30 per cent of 

 units studied in the motor area containing 

 the arm representation. [From Jasper et al. 

 (2-2.).] 



movement itself as if it were the consequence of it. 

 Changes in firing in anticipation of movement were 

 infrequent and not important. 



Parietal Liihe htfluences on Motor Activity 



The behavior of cells in the motor corte.x, which 

 show changes in firing rate before the movement, 

 recalls the concept of the existence of a plan of antici- 

 pation as postulated by Liepman (272, 273). How- 

 ever, the difKculty here is in translating into neuro- 

 physiological mechanisms what is as yet only a 

 psychological concept. Similar difficulties which 

 also arise in the case of the body scheme do not, 

 howe\er, invalidate either of these concepts. There is 

 much supporting evidence as to the role of the 

 cortex in building up and continuously remodeling 

 the image by which the subject is aware of his position 

 in space and his postural attitude (cf. 113, 292). 

 Certain types of agnosia and apraxia which follow 

 lesions of the parietal lobe ha\e led to localization in 



these regions of mechanisms involved in the formu- 

 lation of the body image (cf. 113). As discussed by 

 Shilder (395), this body scheme is based upon and 

 built up through the sen.sory input but is subject to 

 variations induced by emotional factors. 



According to Denny-Brown & Chambers (119), 

 the exploratory behavior of the monkey, in addition 

 to its orientation in space, also depends mainly on 

 an intact parietal cortex. Defects in behavior follow 

 lesions in different areas of this cortex. Visual avoid- 

 ance reactions in primates, including man, appear 

 following posterior parietal ablation, while antero- 

 lateral ablations release .similar reactions to tactile 

 and nociceptive stimuli. Ablation of area 7 may 

 bring about a more general release of all t\ pes of 

 avoidance reactions. 



Since parietal areas participate, on the one hand, 

 in sensor\- functions by reason of afferent sensory 

 volleys and also respond to stimulation by evoking 

 movements, it is possible that in these areas at least a 

 fairly strict correlation occurs between .sensory input 



