THE GENERAL PRINCIPLES OF MOTOR INTEGRATION 



791 



is greatly lessened or absent when the normal limbs 

 are uppermost. It is thus 'conditioned" by body con- 

 tact stimulation. 



The "true grasp reflex,' comparable to that seen 

 frequently in man, is a more complex response (21, 

 85J than the traction reaction. The fingers are not 

 spastic in the sense of reaction to passive stretch with- 

 out additional contact (i.e. being pulled upon by 

 tapes) yet counteract a passive stretch on the flexor 

 tendons if this is immediately preceded by a distally 

 moving contact stimulus in a specific area of the palm 

 of the hand or foot (85). The true grasp reflex does 

 not appear for several weeks after removal of one 

 precentral gyrus in the monkey and then only in 

 response to a very firm moving contact. It is unaf- 

 fected by posture. Its presence is soon associated with 

 some measure of ability to flex the wrist or fingers 

 independently of the proximal joints without actual 

 contact with the hand, if motivation is intense. After 

 some months the hand may close in response to 

 simple coarse contact but does not make any orient- 

 ing response to the stimulus. In the course of time the 

 thumb may be approximated to the other fingers in 

 this way but never to the degree possible with intact 

 precentral gyrus. With the appearance of a true 

 grasp reflex, spasticity, especially in the flexors of 

 fingers and toes, lessens considerably. All this se- 

 quence of events has been observed and recorded in 

 human hemiplegia in our clinic by Twitchell (100). 



.After bilateral removal of the precentral gyrus [or 

 indeed, of the whole pre- and postcentral area show- 

 ing antidromic spikes from stimulation of the pyra- 

 mid charted by Woolsey & Chang (105)], the same 

 sequence of events occurs, although a coarse true 

 grasp reflex may then be obtained within a week of 

 the bilateral operation (24, 26). The animal remains 

 very stiff in movement but can regain an astonishing 

 degree of control of purposive movement by vision. 



The type of movement that is completely and 

 permanently abolished by ablation of the precentral 

 gyrus is that we have called the 'instinctive tactile 

 grasping reaction' (21, 24, 26, 85) which is an orien- 

 tation of the hand or foot in space such as to bring a 

 light contact stimulus into the palm (or sole) when 

 very facile grasping then ensues. The instinctive 

 grasp reaction is essentially an exploratory palpa- 

 tion directed vertically into space from the point 

 of contact. It is a stereotactic contactual response, 

 whereas the grasp reflex is a passive contactual 

 reflex which can become stereotactically directed 

 only by means of vision (26). The foci for the in- 

 stinctive grasp reaction are the tips of the fingers and 



their lateral borders, the lips, and the lateral Ijorders 

 of the foot and ankle. Ablation of the hand area of 

 the precentral gyrus abolishes the instinctive grasp 

 reaction in the hand, results in some finger and wrist 

 flexor spasticity, but does not release the traction 

 reaction. Ablation of the oral edge of area 4 releases 

 the traction reaction and proximal spasticity but 

 lea\es the instinctive grasping intact (although limited 

 in spatial extent). Woolsey et al. (106) have shown in 

 the macaque the corresponding representation of 

 proxiinal muscles anteriorly and of distal extremities 

 next to the central sulcus. These differences, we be- 

 lieve, are the explanation of the old controversy re- 

 garding special function of 'area 6' or '4S' in relation 

 to spasticity and of area 4 to paresis (38, 52, 53), 

 for the criteria then used to judge spasticity identified 

 it only in proximal muscles and hence with anterior 

 lesions. 



Following section of the pyramid in the monkey 

 we have noted the same severe loss of use of the oppo- 

 site limbs as was described by Tower (98) with long 

 delay in recovery of movement which is at first chiefly 

 limited to triple flexion. In our own monkeys some 

 slight spastic reaction returned in terms of a soft 

 reaction to passive stretch, with ample but slow ten- 

 don reflexes, and was quite obvious with a partial 

 pyramid lesion. There was slight sustained flexion 

 of the upper limb and extension of the lower; but the 

 traction reaction was not as well developed as after 

 an area 4 lesion, and the animal's ability to convert 

 it to his own ends correspondingly less. A grasp 

 reflex was absent for a long period, then obtained 

 only with heavy moving contact. The difference 

 between precentral and pyramid lesions is therefore 

 chiefly in terms of failure of release of the traction 

 and positive supporting reactions with pyramid 

 lesions, probably owing to integrity of the corticobul- 

 bar fibers. As a result of both types of lesion the 

 motor response to a purely visual stimulus, e.g. reach- 

 ing out for food or support, becomes a simple 'paw- 

 ing' flexion and extension of the limbs (an adaptation 

 of progression) and is very obvious in behavior when 

 these lesions are bilateral in the monkey (26). 



In general terms the exploratory pattern of the 

 tactile placing reaction is comparable to the instinc- 

 tive grasp reaction, although it is possible to disso- 

 ciate the two at the cortical level by types of mid- 

 parietal lesions that interfere with the spatial data 

 that are more particularly required for placing 

 (24, 26). In regard to both these reactions the pre- 

 and postcentral cortex and the corresponding lateral 

 thalamic nuclei appear to be inseparable. The pat- 



