CENTRAL CONTROL OF RECEPTORS AND SENSORY TRANSMISSION SYSTEMS 



757 



synapses. There is good e\idence, too, that this 

 mechanism is discharged by way of the brain-stem 

 reticular formation; the action might originate there 

 or perhaps elsewhere within the cerebral and cerebel- 

 lar hemispheres, but it undoubtedly funnels through 

 and may be significantly modified by the reticular 

 formation. The end effect of this mechanism may be 

 facilitatory or inhibitory, but in many central relays 

 it appears to be predominantly inhibitory. The 

 mechanism clearly depends upon an active process; 

 its effect can be interrupted by transection of the 

 neuraxis above the level of testing, by deep anesthesia 

 and, more specifically, by the placement of a lesion 

 in the central core of the brain-stem reticular forma- 

 tion. The dynamic operation of this mechanism ap- 

 pears to be responsible during wakefulness for 

 fluctuations in the amplitude of sensory-evoked 

 responses. 



Beha\ioral studies, too, indicate that this mecha- 



FiG. 14. Evoked potentials following tooth pulp stimulation 

 recorded from three loci at the same mesencephalic level. ^tI. 

 Recorded from the portion of the central tegmental fasciculus 

 ascending alongside the periaqueductal grey. B. Recorded from 

 the pathway within the intermediolateral portion of the central 

 grey. C, Small early response followed, after a long latency, by 

 a second longer discharge, recorded from the region of the 

 decussated brachium conjuncti\-um. Note differences in the 

 time scale. Note also that, although all three of these individual 

 loci may be considered parts subsumed within the general 

 regional designation of the brain-stem reticular formation and 

 each is a bilaterally represented pathway, they are nonetheless 

 distinguished from one another by differences in latency, am- 

 plitude and duration of response. [From Kerr el al. (47).] 



nism plays a dynamic role during wakefulness. Here 

 its operational effect is usually a reduction of sensory 

 signals, an effect that is active in inverse relation to 

 the degree of attention or interest enlisted by that 

 particular stimulus. The mechanism seems to be less 

 active (to inhibit less) when a stimulus is novel or 

 when a stimulus is given special significance, as by its 

 association with an important unconditioned stimu- 

 lus. The mechanism appears to be more active (to 

 inhibit more) in relation to signals arising from stimuli 

 to which habituation has been developed and other 

 stimuli, even though not rendered ineffective by 

 habituation, from which attention has been with- 

 drawn. 



Briefly, this sensory control mechanism appears to 

 provide the perceptual processes with an active or- 

 ganizing principle, including an element of purpose, 

 which tends to select and modify sensory messages 

 within the earliest stages of their trajectory. If overt 

 behavior may be assumed to provide a cogent index 

 for the interpretation of telos, then this sensory control 

 mechanism is designed to diminish the engagement 

 of higher centers with those signals that have the least 

 significance to the individual. 



A mechanism operating in this way requires that 

 incoming signals be identified and given significance. 

 How might this identification and attachment of value 

 come about? Only partial answers can be provided at 

 this time. Continuous electrographic recordings from 

 multiple sites indicate that, when a behaving animal 

 encounters a new situation, at first a very large terri- 

 tory of the brain is drawn into a novel activity. As the 

 experience is repeated many times, there develops a 

 significant economy in terms of the extent of brain 

 involvement. Perhaps recognizable signals can eventu- 

 ally be reduced to a quite small number of impulses, 

 representing minuscule abstractions of reality. Perhaps 

 recognizable identity can be established e\en before 

 the sensory-evoked impulses have time to ascend ail 

 the way to cortex and back. Something of a parallel 

 sort appears to take place within motor circuits as one 

 proceeds from the execution of a complex novel move- 

 ment to that same movement when it is established 

 as an ingrained motor habit. There is evidently an 

 analogous economization and automatization of 

 neuronal activity in relation to the habituated act as 

 finally executed. 



The attachment of value to such identified signals 

 could presumably come about quite naturally through 

 the activation, pari passu, of certain portions of the 

 brain's primary reinforcement systems (see Chapter 

 LXII l)y Stellar in this work). .\ number of the struc- 



