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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



play an analogously important, but as yet undefined, 

 role in the perception of each of these modalities. 



It is generally accepted that the classical ascending 

 sensory pathways connect directly or indirectly with 

 the brain-stem reticular formation (78). Rasmussen 

 reports that the centrifugal auditory projection sys- 

 tem also sends branches into the same general region 

 (personal communication). It may be by virtue of 

 such collateral connections to the brain-stem reticular 

 formation that most sensory pathways and certain 

 zones of the cerebral cortex have indirect reciprocal 

 relations with the cerebellum (6, 32, 74, 75). Each of 

 the cerebral and cerebellar cortical areas which is 

 capable of exercising an eflfect upon impulses trans- 

 mitted along the classical ascending sensory pathways 

 possesses projections to the brain-stem reticular for- 

 mation. Moreover, stimulation of the brain-stem re- 

 ticular formation is known to evoke very notable 

 sensorv control effects. This much is highly suggestive. 

 But whether the descending sensory projections which 

 parallel the classical ascending sensory pathways and 

 the cerebral and cereljellar projections into the brain 

 stem each have an influential access to the ascending 

 sensory transmission systems must await definitive 

 experimental proof. 



To these considerations should be added the fact 

 that there is really no physiological boundary between 

 central sensory and motor mechanisms. Each central 

 pattern for the initiation of movement has its neuronal 

 repercussions upon central sensory patterns, and each 

 performed movement introduces alterations in sensory 

 input patterns. In this way sensory and motor systems 

 are inextricably bound together both internally and 

 externally. 



Transactional Mechanisms Relating to 

 Sensory Control Systems 



It is now possible to identify six extensive, mutually 

 interacting systems: a) the classical ('lemniscal') 

 ascending sensory pathways projecting finally upon 

 the classical sensory receiving areas of the cortex, b~) 

 the parallel ('extralemniscal') ascending sensory 

 pathways which reach more widespread regions of 

 the cortex by way of the brain-stem reticular forma- 

 tion, f) the classical ('pyramidal') descending motor 

 pathways projecting directly from cortex to lower 

 motoneuron aggregations, ctj the parallel ('extra- 

 pyramidal') motor pathways which descend to the 

 motor nuclei indirectly by way of the basal ganglia 

 and the brain-stem reticular formation, e) the brain- 

 stem reticular formation which is known to exert 



modifying influences upward upon both the cerebral 

 and cerebellar hemispheres and downward upon both 

 sensory and motor synaptic relays, and /) the cen- 

 trifugal sensory control mechanisms which may in- 

 volve fibers coursing in reverse direction parallel to 

 the classical ascending sensory pathways and which 

 may also implicate projections from cerebral and 

 cerebellar loci through the brain-stem reticular for- 

 mation. 



The interdependence of these six systems is obvious. 

 Evidently they are all knit together by the brain-stem 

 reticular formation which could not be efTectively 

 studied in animals with central anesthesia. Because 

 of this experimental limitation, antecedent concep- 

 tions had to deal with relatively independent sensory 

 and motor systems which were more stable, imperious 

 and reliable in their handling of signals than is the 

 case in the unanesthetized brain. 



Since collaterals from the classical ascending 

 pathways influence the reticular formation and the 

 reticular formation in turn modifies the initiation and 

 transmission of impulses along the classical sensory 

 pathways, since both of these systems interact with 

 each other again in the sensory receiving cortex, 

 since the reticular formation by way of the diffusely 

 projecting thalamic nuclei modifies activity generally 

 throughout the cortex and the cortex in turn modifies 

 activity within the reticular formation, since the cere- 

 bellum is similarly linked both ways with the brain- 

 stem reticular formation, etc., one can begin to 

 visualize the extent of abstraction imposed by the 

 experimental isolation of only a few elements of this 

 entire complex. Moreover, it is not possible to de- 

 fine how such a 'transactional mechanism' (55) might 

 operate on the basis of any single experimental ap- 

 proach. By adding evidence from studies that in- 

 corporate both neurophysiological and behavioral 

 techniques, it is possible to add a new dimension 

 to the conception of the mechanisms involved in 

 the central control of sensorv transmission. 



SENSORY ATTENTION, HABITU.ATION AND CONDITIONING 



Auditory Habituation 



By means of electrodes implanted within the dorsal 

 cochlear nucleus, Galambos, Hernandez-Peon and 

 their associates have recorded potentials elicited by 

 acoustic stimulation during the course of behavioral 

 studies on unanesthetized cats. Responses to the same 

 tone pip show modest fluctuations in amplitude and 



