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HANDBOOK OF PriVSrOLOGY 



NEIIROPHYSIOLOGV 11 



of the eves instead of tlie turning; movement of the 

 head. 



The mechanisms for tiirnint; mo\ements (contra- 

 and ipsiversive) again show tonic activity. Tlieir 

 destruction is followed by a tendency to circling 

 movements in the direction opposite to the effect of 

 stimulation, that is in contraversive direction fol- 

 lowing destruction of the ipsiversive structures and 

 in ipsiversive direction following destruction of the 

 contraversive substrate. The perception of sensory 

 stimuli contralateral to the direction of circling fol- 

 lowing operation ma\' be considerably impaired or 

 even abolished [Hess (103, 104)]. This effect cor- 

 responds to the hemianopic impairment of attention 

 and to the results of destruction of the frontal eye 

 fields in human pathology. If the destructon is not 

 too extensive, the horizontal circling movements 

 disappear after a few days. 



The neuronal systems responsible for these move- 

 ments are manifold and, as a result of the length of 

 their fibers, can be found in many areas of the brain. 

 The nucleus caudatus, the nucleus entopeduncularis, 

 the zona incerta and its caudal efferent fibers all 

 contain substrates for contraversive turning move- 

 ments [Hassler (89)]. It is probable that these areas 

 also are connected in part with the efferent pathways 

 originating in the cortical fields responsible for 

 adversive movements. These pathways and their 

 possible interrelation are shown schematically in 

 figure 1 1. In any case, there is in the anterior mesen- 

 cephalon a connection between these neuronal systems 

 and the reticular mechanisms responsible for ipsiver- 

 sive turning movements. The fibers responsible for 

 conjugate deviation of the eyes to the contralateral 

 side seem to cross the mid-line at a higher level. A 

 common 'selecting organ' (Hess) for all turning move- 

 ments to the ipsilateral side seems to e.xist in the 

 mesencephalic reticular formation, as shown in 

 figure 1 1 . 



Cortical Extrapyramidal Areas in Relation to 

 Subcortical Extrapvramidnl Centers 



The functional relationship between the cortical 

 areas and the subcortical extrapyramidal structures 

 requires discussion. In animals stimulation of some 

 areas of the cortex can induce motor reactions even 

 though the pyramidal tract has been severed in the 

 medulla, as Starlinger (245) first showed in dogs. 

 His observations were confirmed later by Prus (214), 

 Rothmann (221), Vogt & Vogt (267), Tower (255) 

 and others. These areas were accordinglv called 



extrapyramidal motor cortical fields. By electrical 

 stimulation, Vogt & Vogt (267) distinguished from 

 area 4, the primary motor field, a secondary field, 

 area 6, where stimulation also produced selective 

 movements which in contrast to those evoked from 

 area 4 are less sharply localized and are accompanied 

 by adversive movements to the contralateral side. 

 Later (268) these workers described the responses to 

 stimulation of many other cortical areas, including a 

 tertiary motor field in the frontal cortex (area 6a/3), 

 various areas producing adversive movements, and a 

 peculiar inhibitory area for rhythmical chewing and 

 licking movements (area 87). Foerster (58) obtained 

 the same results in human subjects. 



Destruction of area 6 as a whole causes "forced 

 grasping.' This first appears as 'coarse grasp reflex' 

 [Seyffarth & Denny-Brown (232)] which can be 

 produced by strong pressure applied to the palm and 

 appears as a reflex contraction of the flexor muscles 

 of the fingers. This effect later turns into a true grasp 

 reflex which is eliminated by complete deafferenta- 

 tion. Finally, the 'instinctive grasp reaction' appears 

 which consists of movements of orientation of the 

 hand produced by contacting any place of the palm. 

 According to Denny-Brown (41 ) the grasp reaction is 

 permanently released only following combined 

 destruction of the cingulate gyrus, the supplementary 

 motor area and area 8. If only parts of these areas 

 are destroyed, the grasp reactions disappear later. 

 Simultaneously with the appearance of the grasp 

 reactions there is a loss of the 'avoiding reaction' (42). 



Bilateral destruction of areas 4 and 6 produces the 

 so-called 'thalamic reflex pattern,' so named because 

 of its similarity to the motor reactions of the de- 

 corticate (thalamic) monkey. In this preparation 

 all postural and locomotor reactions remain unim- 

 paired. However, directed motor activity is dominated 

 by the grasp reactions. If the animal lies on its side 

 the grasp reactions of the extremities lying under- 

 neath are decreased while those of the extremities 

 of the upper side are enhanced. Simultaneously the 

 flexor reflexes are exaggerated and the head is kept 

 elevated [Fulton & Dow (65)]. Tonic neck reflexes 

 are consistently present only if the ventral part of 

 area 4 and 6a, including area 6b«, has also been 

 removed. Additional bilateral destruction of the 

 labyrinth restores the normal posture of the head and 

 decreases the strength of the neck reflexes and all 

 postural reactions including the grasp reflexes. 

 Following head rotation these animals extend the 

 'jaw' limbs and flex the 'skull' limbs and simul- 

 taneouslv show an enhanced grasp reflex. Thus, the 



