1572 



IIVXUBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



lhat 'attention' or the process of exclusion of cert. lin 

 sensory messages in favor of others may begin at var- 

 ious levels of the sensory input as well as at the thal- 

 amic and cortical levels. Bv decortication, or by le- 

 sions or transections at critical points in the nervous 

 -wem it should be possible to determine whether 

 the reticular formation, the cortex or both are es- 

 sential to ihis particular form of sensory inhibition. 



Jasper et <d. (134)) in examining the unit firing 

 patterns in the motor cortex of monkeys during con- 

 ditioned avoidance responses, found inhibition of unit 

 discharge was more common during the CS-CR in- 

 terval but observed increased firing during the CR 

 itself. To an unreinforced differential stimulus there 

 was decreased unit firing which they interpreted as 

 habituation rather than active inhibition. During the 

 light-shock avoidance conditioning they found in 

 the parietal and occipital cortex more evidence that 

 the units would respond to the temporal patterning 

 of the CS Hashing light and that these were comingled 

 with secondarv evoked responses to the somatic un- 

 conditioned stimulus. Occipital evoked potentials 

 were strong through the CS-CR interval but were 

 reduced sharply just preceding the CR, which they 

 interpret as related to a shift in attention, as previously 

 ill scribed by Jouvet & Hernandez-Peon (136). In 

 tentative conclusions concerning these preliminary 

 experiments, Jasper and colleagues propose that 

 "specific sensory or motor systems "I the brain are not 

 primarily involved in the neuronal circuits critical 

 for the conditioned response" and that "search should 

 be directed to non-sensor\ -motor cortex and prob- 

 ably to centrally situated neuronal systems of the 

 brain stem which receive convergent patterns of im- 

 pulses from in.iiiv sources." In this respecl the work 

 11I Buser, Albe-Fessard and others mentioned above 

 would seem to prov tde clues concerning convergence 

 areas, Jasper and colleagues have emphasized hab- 

 ituation and conditioning as contrasting and antagon- 

 istic processes, pointing out that conditioning involves 

 elimination of all irrelevant stimulus-response re- 

 actions which are not reinforced. This they call ha- 

 bituation Before the CS-CR pattern is establshed bv 

 appropriate reinforcement, thev imply that attention 



11I .1 specific nature must be lueused upon the correct 



sequent e 



With regard to alerting responses and habituation, 



Jasper et al. (134) observed that cortical cells might 

 tin- quite activel) during drowsiness 01 sleep, some 

 firing iinlv during bursts of slow waves would cease 



to lire when the overall BEG indie. mil activation or 

 arousal patterns Still other units would fire verj 



actively when the animal was awakened or aroused, 

 but would decrease their rate of firing during con- 

 tinued alert or excited states. The habituation of 

 such arousal responses has been studied by Sharpless 

 & Jasper (213), using the EEGas an indicator as well 

 as behavior. 



Sharpless & Jasper discuss the nature of habitua- 

 tion, and describe it as differing from sensory adapta- 

 tion and nerve accommodation in its temporal char- 

 acteristics. Like learning, it seems to be represented 

 in the central nervous system by some form of plas- 

 ticity. It may develop even with long intervals be- 

 tween stimulus presentations and may persist for 

 hours or days. Habituation to a specific stimulus 

 quality or intensity will give way to a unique or novel 

 one, or to one differing in intensity only. It seems to be 

 characterized by the fact that a particular reaction 

 or response to a specific stimulus repeated at intervals 

 over a period of time will eventually fail to occur un- 

 less reinforced in some manner bv a painful, rewarding 

 or otherwise effective stimulus for that response. In 

 this respect it mav resemble extinction of a condi- 

 tioned response, but in other respects it is different. 

 It may be restored immediately upon a shift to another 

 stimulus mode or following a change in intensity in 

 the same sense mode, even though the original stim- 

 ulus is again employed. These authors liken this nega- 

 tive adaptation or habituation to a decrease of alert- 

 ness or attention such as is found in everyday tasks 

 like those described at the start of this chapter, where 

 repetition, monotonv, boredom and the like are in- 

 volved. It is similar also to the disappearance of the 

 'orienting reflex' of Pavlov or the "startle response' of 

 Landis & Hunt ( 1 ",->) which succumb to repetition. 



Using the arousal reaction of the EECi (shift from 

 high-voltage, synchronized slow waves of sleep to low- 

 voltage, desynchronized last waves ol wakefulness 



a criterion response which disappears upon jo iii [fj 

 repetitions ol an arousal tone (habituation), Sharpless 

 & Jasper studied a great v.nieiv ol characteristics of 

 this phenomenon. Thev conclude that habituation of 

 the arousal reaction is specific to the quality, niodalitv 

 or pattern of .1 given stimulus. Two tvpes ol arous.il 



reaction were differentiated, a longer Listing one more 



susceptible to habituation and a shorter lasting one 

 less susceptible. The tunic slow and persistent arousal 

 response is believed to be mediated bv the mesence 

 phalic portion of the ARAS; the taster more differ- 

 entiated arousal response appeared to be dependem 

 upon the DTPS, for specific nonpatterned stimuli the 

 habituation of the activation or arousal response could 

 be detected in the cortex, thalamus .mil mn nlar for- 



