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HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY II 



of the nucleus ventralis lateralis through an action 

 which has been interpreted as inhibitory in nature. 

 On the other hand, inhibition of cortically-induced 

 movements has been reported from simultaneous 

 stimulation of the motor cortex and the nucleus 

 ventralis lateralis (26). This action, however, is not 

 mediated at the cortical level. 



The evidence cited concerning the behavior of 

 cortical units following stimulation of the nucleus 

 ventralis lateralis thalami may support the hypothesis 

 of the coexistence at the cortical level of a damping 

 cerebellar influence as well as an independent effect 

 of a facilitatory nature. The same paleocerebellar 

 stimulation produces opposite effects on motor 

 responses depending on the background of activity 

 artificially induced in the motor cortex as with strych- 

 nine (332). There remains the basic difficulty of 

 ascribing these influences to actions exerted partially, 

 at least, at the cortical level. 



The question as to whether cerebellar impufses 

 reaching the cortex after relaying in the thalamus 

 may convey some particular sensory attribute is in no 

 way solved by the experiments discussed above. Since 

 weight discrimination is permanently lost in the 

 monkey only if, in addition to the medial leminiscus, 

 the dentatorubrothalamic tract is also interrupted, 

 whereas section of either alone does not produce 

 permanent deficit (398), it might be inferred that 

 these impulses may carry information of a precise 

 sensory modality. Yet, cerebellar influences seem 

 capable of modifying evoked potentials in the sensory 

 cortex which probably provide the background for 

 an appropriate motor behavior. It has been found 

 that acoustic (406), as well as somatosensory and 

 visual (Snider & Sato, personal communication) 

 evoked potentials in the cortex, are greatly modified 

 by a preceding cerebellar volley. 



The cerebellocerebral connections are paralleled 

 by the corticopontocerebellar paths (see 219; Chap- 

 ter LI by Brookhart). Just as cerebellocerebral paths 

 are not limited in their projections to motor areas 

 but involve sensory cortex as well, so also descending 

 pontocerebellar paths arise widely in the cortex out- 

 side the motor areas. The functional role of these very 

 powerful streams remains largely unknown. It has 

 been suggested that those which arise from the 

 parietal lobe, at least, may be involved in the regula- 

 tion of sensory inputs (354); but in the ab.sence ot 

 experimental evidence, other hypotheses cannot be 

 discarded. Thus, Ruch (384) considers that the role 

 of the cerebellum in movement is to compare the 

 action initiated at the cortical le\-el with the imaiie of 



the resulting motor performance transmitted to the 

 cerebellum for the periphery. From this comparison, 

 the cerebellum would initiate appropriate corrections 

 to minimize discrepancies. 



The experiments of Fulton and his colleagues (24, 

 163) bear directly on the problem of reciprocal cere- 

 bral-cerebellar interconnections in relation to move- 

 ment. The tremor of decerebellated cats was abolished 

 by decortication, and in the baboon and macaque 

 by resection of areas 4 and 6. The ablation of area 6 

 alone increased the tremor. If the fact that voluntary 

 movements are reduced in range and quality by 

 cortical resection alone is neglected, the tremor, which 

 is related to 'voluntary' movements, and which can 

 be taken as a sign of disorganized function, is abol- 

 ished by suppressing this function. Even if there is no 

 doubt that the corticocerebellar interrelationships 

 provide the basis of an exceedingly important regu- 

 latory system (405), the problem seems, however, to 

 be of a greater complexity. Combined cerebellar and 

 corticomotor ablations produce greater deficits than 

 would be expected from summation of the effects of 

 the two ablations performed separately, according 

 to the observations of Luciani (280) confirmed by 

 Carrea & Mettler (81). In addition, the importance 

 of other systems has also been suggested by the finding 

 that lesions of the contralateral ansa lenticularis 

 greatly reduce the cerebellar tremor and ataxia fol- 

 lowing cerebellar cortical and nuclear lesions (79). 

 These disturbances have been tentatively interpreted 

 as at least partially due to intact outflows from the 

 globus pallidus. 



Intra- and Interareal Connections of 

 Ipsilateral Hemisphere 



The functional role of intrahemispheric connec- 

 tions is far from clear. This problem is not exclusively 

 confined to the so-called intercortical and intra- 

 cortical association fibers. It is also a matter of the 

 size of the cortical field and the nature of the influ- 

 ences which may be exerted both by the numerous 

 recurrent axon collaterals of cortical cells and by the 

 horizontal cells in the first layer (371). Relevant to 

 the same problem are also the questions of the recep- 

 tive field of the individual cortical neuron, as may be 

 indicated bv the size of its dendritic arborization, 

 and the extent of the field of distribution of each 

 afferent fiber to a number of adjacent neurons (cf. 

 396). Studies relevant to these problems in normal 

 and isolated cortex are discussed in the recent l)ook 

 by Burns (74). 



