Passive exposure of adult cats to moderate-level tone pip ensembles differentially decreases AI and AII responsiveness in the exposure frequency range

Abstract

Passive exposure of adult animals to a random ensemble of tone pips band limited between 4 and 20 kHz has been shown to suppress neural activity in primary auditory cortex (AI) to sounds in the exposure frequency range. In the long-term (>3 months), the suppressed neurons can be reactivated by frequencies above and below the exposure range, i.e., tonotopic map reorganization occurs. The suppression can be at least partially reversed after a long period of quiet recovery, as the moderate-level exposure does not impair peripheral hearing. Here we exposed adult cats, for 7-13 weeks without interruption, to two different moderate-level tone pip ensembles, in separate experiments. One exposure stimulus consisted of an octave-wide 2-4 kHz band, which overlaps substantially with the cat vocalization range; the other consisted of a pair of third-octave bands centered at 4 and 16 kHz. We again report a decrease in AI responsiveness in the exposure frequency range, irrespective of the exposure stimulus bandwidth or center frequency, and a slow, partial recovery over a 12-week post-exposure window. In contrast to our previous studies, the suppression in both of the present experiments extended well beyond the exposure frequency range. In particular, following the 4 and 16 kHz experimental acoustic environment, AI activity was strongly suppressed not only in response to frequencies close to the two exposure bands, but also in response to frequencies between the bands, i.e., the results resembled those to a single broadband stimulus spanning the 3-18 kHz range. On the other hand, responses in secondary auditory cortex (AII) were suppressed predominantly around 4 and 16 kHz, with little or no suppression in between.