CENTRAL CONTROL OF RECEPTORS AND SENSORV TRANSMISSION SYSTEMS 



755 



Stem reticular formation in unanesthetized animals 

 has an effect on nonhabituated acoustic- or flash- 

 evoked responses that is similar to distraction of atten- 

 tion by extraneous stimulation of the same animals; 

 A) distraction of attention by extraneous sensory stimu- 

 lation may very likely have its effect, as does arousal, 

 by activation of the brain-stem reticular formation; 

 f) animals given barbiturate anesthesia (known to 

 interfere with activity in the brain-stem reticular for- 

 mation) cannot be habituated; d) if animals are 

 habituated prior to being anesthetized, the sensory- 

 evoked potentials change from habituated (reduced) 

 amplitude to the initial prehabituated height but 

 typical habituated responses reappear following re- 

 covery from the anesthetic; e) in habituated animals, 

 a lesion restricted to the pontine or mesencephalic 

 brain-stem reticular formation is followed by perma- 

 nent 'release' from the habituated pattern.'' 



Recently, Fuster (24) has reported that monkeys 

 trained to do difficult tachistoscopic discriminations 

 between two similar objects show improved perform- 

 ance in both their speed of response and percentage 

 of correct choices when the test exposure is preceded 

 by a very brief electrical shock applied to the mesen- 

 cephalic brain-stem reticular formation. More pro- 

 longed stimulation in the same brain-stem location 

 interferes deleteriously with both the reaction time 

 and percentage of correct choices. These findings 

 imply an alteration of visual sensory or possibly judg- 

 mental processes as a result of brain-stem activation. 

 Although there is no way of being certain, in Fuster's 

 experiments, where the effect takes place, it is possible 

 that such changes occur within the first few synapses 

 along the visual pathway. This might be inferred from 

 the experiments of Granit (29) and Hernandez-Peon 

 et al. (39) cited above. More convincing evidence for 

 improvement in the kind of differentiation demanded 

 by tachistoscopic discrimination is reflected in experi- 

 ments by Lindsley (53). He finds that two flashes 

 which are placed close enough together to produce a 



^ .\ note of caution. Much has been learned within the last 

 few years which assigns important functions to the brain-stem 

 reticular formation.' It must be remembered, however, that 

 this region may well contain several functional systems. The 

 studies of Adey, Amassian, Haugen, Moruzzi and their asso- 

 ciates imply that this is the case (2, 8, 34, 47, 61). In the first 

 stages of interpreting the functions of so large and complex a 

 region of the brain, it is natural that somewhat overgcneralized 

 and sweeping conclusions may be alluring. This does not deny 

 the reliability of observations made to date but implies that, 

 when this complex skein of reticular neurons becomes better 

 understood, a greater precision in the localization and charac- 

 terization of its functions may be possible. 



single large-humped electrical wave in the lateral 

 geniculate body are, on stimulation of the brain-stem 

 reticular formation, separated into a two-peaked 

 himip. 



Taken as a whole, all of these behavioral experi- 

 ments reinforce the neurophysiological evidence that 

 the sensory pathways are relatively plastic rather than 

 fi.xed in the transmission of impulses generated by a 

 particular stimulus. Sensory transmission is apparently 

 modifiable in accordance with waking experience. 

 Moreover, the brain-stem reticular formation evi- 

 dently plays an important role in the government of 

 such neuronal plasticity. 



I\'TERPRET.\TIONS 



Remarkable changes take place within sensory 

 circuits when one shifts from the u.se of anesthetized 

 animals to animals without central anesthesia. In the 

 anesthetized state the classical sensory pathways con- 

 vey high amplitude .signals with great reliability and 

 consistency, and there is little activity within the 

 brain-stem reticular formation. Clortical responses to 

 sensory stimuli are greatly amplified and tend to be 

 confined to the classical sensory receiving areas. In 

 the waking brain, without central anesthesia, the 

 classical sensory pathways convey signals that are less 

 reproducible from one moment to the next. Indeed, 

 over a period of some minutes or hours there may be 

 remarkable alterations in the size of evoked responses 

 to a given stimulus. In addition, there are widespread 

 responses elicited throughout extensive cortical and 

 subcortical regions. It seems obvious now that the 

 classical sensory pathways and cortical projection sys- 

 tems, no matter how necessary they might be to per- 

 ception, are not in themselves .sufficient for perception. 



The extralemniscal sensory pathways, coursing 

 through the brain-stem reticular formation and 

 diffusely projecting thalainic nuclei, appear to have a 

 general function of providing an integrative back- 

 ground or context for perception. Their contribution 

 in this respect may be likened usefully to the organi- 

 zational contribution in movement and behavior that 

 is made by the descending extrapyramidal projec- 

 tions. They may be thought to provide a general 

 .sensory awareness and feeling tone comparable to the 

 background of excitability and motor tone generated 

 by the extrapyramidal system. Nonetheless, they may 

 convey more specific sense data too. Haugen & 

 Melzack (34), for example, report persuasive evidence, 

 soine of which appears in figures 13, 14 and 15, that 



