CENTRAL CONTROL OF RECEPTORS AND SENSORY TRANSMISSION SYSTEMS 



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FIG. II. Modification of amplitude of click-evoked responses 

 in dorsal cochlear nucleus according to experience of the 

 animal. Unanesthetized cat with recording electrodes im- 

 planted in the dorsal cochlear nucleus. Amplitude of responses 

 elicited by clicks repeated every second gradually declines over 

 many trials. The animal exhibits behavioral as well as electro- 

 physiological evidence of habituation to the click stimuli. After 

 habituation, if weak electric shocks are applied to the foreleg 

 of the same animal in temporal association with the clicks, the 

 click -evoked responses gradually become increased in ampli- 

 tude. The dehabituation' can occur within only a few trials if 

 sufficiently powerful shock stimuli are applied, as shown by 

 Galambos et al. (27). The dehabituation' is plastic in the sense 

 that the amplitude of the click -evoked responses declines once 

 more after the shock stimuli are discontinued. This kind of 

 modification of sensory-evoked responses has been taken as an 

 objective evidence of conditioning. [Modified from Jouvet & 

 Hernandez-Peon (45).] 



undergo periods of waxing and waning. If the same 

 tone signal is repeated many times, the amplitude of 

 the evoked dorsal cochlear nucleus responses tends 

 gradually to become reduced to a new lower level, 

 although the fluctuations still persist (27, 37, 38, 45). 

 The authors refer to this as auditory 'adaptation' or 

 'habituation.' If the tone is shifted up or down in 

 pitch, the ev'oked potentials return to a higher ampli- 

 tude once more, but rehabituation can be established 

 to the new tone signal. After habituation to a particu- 

 lar tone has been thoroughly established and the tone 

 is then associated with a nearly simultaneous signal, 

 such as an electric shock to the foreleg or across the 

 chest, a high amplitude cochlear response will re- 



appear. This has been referred to as 'dehabituation' 

 (35). After discontinuation of the electric shock, a slow 

 rehabituation to the auditory signal takes place (27, 

 45), as is shown in figure 1 1 . 



Auditory Conditioning 



These fluctuations in amplitude of the responses 

 recorded within the first central relay stages along the 

 auditory pathway may be reflected by roughly parallel 

 shifts in the animal's behavior. When first introduced 

 to the test-tone signals, an animal attentively alerts to 

 each tone pip. As electrographic evidence of habitua- 

 tion occurs, the animal shows less behavioral evidence 

 of devoting attention to the acoustic signals. When 

 habituated and then given an unconditioned electric 

 shock in association with the tone signals, the animal 

 behaves as if it has suddenly acquired an increased 

 interest in the associated tone. Growth in behavioral 

 evidence of attention usually takes place a few trials 

 in ad\ance of the growth in amplitude of the evoked 

 dorsal cochlear response, but the modified 

 (conditioned) cochlear response lasts approximately 

 as long, during e.\tinction trials, as the overtly ex- 

 pressed attention. The electrophysiological plasticity 

 in response of the nervous system has with some justi- 

 fication been taken as an objective indication of con- 

 ditioning. 



Using electrical shocks applied across the chests of 

 cats, Galambos et al. (27) have found that only ten 

 or twentv such unconditioned stimuli, applied in asso- 

 ciation with clicks to which the cats had previously 

 become thoroughly habituated, were sufficient to 

 cause electrographic as well as behavioral evidence of 

 conditioning. Simultaneous recordings made in the 

 cochlear nucleus, auditory cortex, hippocampus, 

 septal area and head of the caudate nucleus show that 

 electrographic changes associated with this kind of 

 conditioning may occur at several difTerent levels 

 along the auditory pathway and in regions other than 

 the classical auditory system. Cycles of associated be- 

 havioral and electrophysiological evidence for condi- 

 tioning and extinction can apparently be repeated 

 indefinitely. [Galambos et al. (27) may be consulted 

 for additional evidence and commentary on condi- 

 tioning in relation to modifications of electrical 

 activity in the brain. This subject is also discussed by 

 Galambos in Chapter LXI of this work.] 



Shifts of Attention 



Recording from electrodes implanted in the dorsal 

 cochlear nucleus of the cat before habituation had 



