754 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



been established, Hernandez-Peon et al. (38) have 

 tested the effects on auditory-evoked potentials of 

 distraction by visual and olfactory stimuli. As may be 

 seen in figure 12, when mice in ajar are placed before 

 the experimental animals, or when fish odors are 

 blown into the cage through a tube, the formerly high 

 amplitude auditory responses are immediately re- 

 duced in amplitude. The effect is as if the cats had 

 suddenly shifted from a naive to an 'habituated' state 

 with reference to the auditory test-signal responses. 

 But when the mice are removed or the odor blowing 

 stopped, and after the cats are apparently relaxed 

 once more, the evoked auditory potentials return 

 again to their initial high level of amplitude. The 

 duration of the reduced auditory potentials corre- 

 sponds closely with the period when the animals are 

 distracted by the nonacoustic signals. 



Visual Responses 



Using electrodes implanted in the brain-stem reticu- 

 lar formation, optic tract, lateral geniculate body and 

 optic radiation, Hernandez-Peon et al. (39) were able 

 to analyze the effects of intercurrent brain-stem stimu- 

 lation on the relay of flash-evoked responses through 

 the retina and lateral geniculate body. When light 

 flashes are reiterated over an extended period of time. 



the flash-evoked responses at each point along the 

 visual pathway tend to diminish in amplitude. This 

 suggests that there is a mechanism of habituation 

 operating in the visual system. .Stimulation of the 

 brain-stem reticular formation or behavioral distrac- 

 tion by nonvisual stimuli is associated with a reduc- 

 tion in amplitude of the nonhabituatcd photic re- 

 sponse. 



Behavior and Xeurophyswlogy 



Although these studies are quite recent, and only 

 a few aspects of a potentially very rich field have been 

 touched upon, certain features merit special comment. 

 It is evident that the activity taking place along at 

 least the auditory and visual pathways, and possibly 

 the olfactory and somesthetic sensory systems as well, 

 is vulnerable to systematic intervention in accordance 

 with previous experience (habituation) and shift of 

 attention (distraction) (11, 45 and other pertinent 

 chapters in 19). It is inferred, but not yet firmly estab- 

 lished, that these dynamic changes in activity within 

 the sensory paths are accomplished by some mecha- 

 nism involving the brain-stem reticular formation. 

 The evidence is as follows: «) activation of the i)rain- 



FIG. 12. Modification of click-evoked 

 responses in the cochlear nucleus during 

 'attention' in the unanesthetized cat. Re- 

 cording from implanted electrodes in 

 the dorsal cochlear nucleus in an ani- 

 mal prior to habituation to click signals 

 delivered every second. Photographs are 

 simultaneous vi'ith potential recordings 

 opposite. Top and bottom : Cat is relaxed; 

 the click responses are large. Middle: 

 While the cat is visually attentive to 

 mice in a jar, the click responses are 

 diminished in amplitude. [From Her- 

 nandez-Peon el al. (38).] 



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