Progressive changes in auditory response patterns to repeated tone during normal wakefulness and paralysis

Abstract

1. In a series of 8 cats, electrodes were implanted in the cochlear nucleus, inferior colliculus, and medial geniculate body, pars principalis and pars magnocellularis. Subsequent studies of auditory habituation were carried out in the normal, waking cat and in the same animal during complete neuromuscular paralysis. During each session 200 presentations of a 1.5 sec duration tone were made at 5 sec intervals. Following a brief rest period, 10 additional tones were presented to test for dishabituation. 2. At each site characteristic tone response patterns of multiple unit fast activity were quantitated with frequency integration. Both onset and sustained response components were analyzed for changes during tone repetitions. 3. During acoustic habituation in the normal, walking cat, the cochlear nucleus generally showed a gradual disappearance of the sharp inhibition 70 msec after tone onset and a progressively more accelerated sustained response. Coincidentally, the inferior colliculus and medial geniculate body, pars principalis and magnocellularis, showed increasing inhibition of both onset and sustained response components. These changes were most prominent during the first 50 trials and are believed to reflect, primarily, habituation of acoustic reflex pathways with gradual disappearance of ear muscle contractions and resultant increases in tone intensity. 4. Acoustic habituation of the same cats during reversible neuromuscular paralysis induced a slight decrease or, less frequently, no progressive change in the cochlear nucleus response; concurrently, the inferior colliculus and medial geniculate body, pars principalis and magnocellularis, showed progressively intense inhibition of both onset and sustained response components. These response alterations, which developed at a rather steady rate for the first 100–150 trials, are believed to reflect central mechanisms of decreasing excitation or increasing inhibition. 5. Habituation sessions repeated on consecutive days resulted in progressive auditory response alterations which often became increasingly enhanced from one day to the next. 6. Background fast activity showed no progressive change at any site during the course of a single habituation session or from one session to the next. 7. Prior conclusions that acoustic habituation does not affect sites peripheral to the medial geniculate body may largely reflect the insensitivity of the stimulation and recording procedures utilized, e.g., clicks or tone bursts and evoked potentials. Although increasingly prominent from the peripheral to central nuclei, the progressive changes recorded in this study during acoustic habituation indicate that modulation of auditory input is possible at all subcortical levels of the classical auditory pathway.