Patterns of habituation to electrical stimulation of reticular and auditory pathways

Patterns of habituation of the orienting response to electrical stimulation of midbrain, thalamus, and cortical components of the primary auditory pathway and the ascending reticular system were studied in awake, freely mobile cats. Individual orienting responses to high-frequency stimulation of structures in the auditory pathway could not be distinguished from orienting responses to stimulation of reticular structures. However, with repeated presentations of the stimulus, the orienting response to auditory pathway stimulation habituated significantly faster than did the orienting response to stimulation of structures in the ascending reticular system. Within the auditory pathway, orienting to stimulation of the inferior colliculus, the medial geniculate body, or primary auditory cortex did not result in distinguishable patterns of habituation. Stimulation of parietal cortex resulted in habituation that occurred significantly slower than habituation to auditory cortex stimulation, and significantly faster than habituation to stimulation of the midbrain reticular formation or the centre median nucleus of the thalamus. These results are discussed with regard to a hypothetical arousal system that includes the brain stem reticular formation, the intralaminar nuclei of the thalamus, and parts of the parietal cortex.