Tone response patterns of the auditory nuclei during normal wakefulness, paralysis, and anesthesia

Abstract

1. In a series of 8 adult cats, electrodes were chronically implanted in the cochlear nucleus, inferior colliculus, and medial geniculate body, pars principalis. Multiple unit responses to 1.5 sec duration tones were subsequently recorded in the normal waking state, during gallamine paralysis and during barbiturate anesthesia. The unit activity was quantitated by frequency integration and recordings during successive tone presentations were subjected to CAT analysis to provide an average response pattern. 2. In the cochlear nucleus an onset burst of increased activity was abruptly depressed after approximately 70 msec. Desynchronized acceleration of unit discharge subsequently appeared at some sites while at others a response occurred which closely followed the frequency of the tone stimulus. With tone cessation a sharp return to the prestimulus activity level occurred. 3. The tone responses of the interior colliculus showed a greater variability and complexity but 3 general patterns could be described. Following an onset burst of increased discharge, the subsequent activity showed either a sustained acceleration (pattern I), a return to prestimulus activity levels (pattern II), or a sustained inhibition (pattern III). With tone cessation offset responses of marked inhibition (pattern I) or acceleration (pattern II) occurred. 4. In the medial geniculate body responses to tone were more difficult to elicit than at the lower levels. Onset burst were rapidly adapting and the subsequent activity either returned to the prestimulus level or showed inhibition. With tone cessation offset bursts of acceleration were frequently present. 5. During neuromuscular paralysis, the characteristic sharp decrease 70 msec after tone onset disappeared at all sites, an effect interpreted as a result of middle ear muscle paralysis. In the cochlear nucleus, responses of desynchronized acceleration shifted to frequency following responses and inhibition in collicular and geniculate responses shifted toward acceleration. Little or no accompanying change in the frequency of background activity occurred. 6. Tone response patterns similar to those elicited during paralysis were recorded during barbiturate anesthesia. Background activity was markedly reduced at the more central auditory sites. 7. It is concluded that by quantitating the fast activity of the auditory nuclei, net effects of tone stimulation can be summarized as distinctive patterns which change progressively as the auditory system is ascended. Alterations in the behavioral state of the animal induce changes in these response patterns and/or in the background activity.