We have recently demonstrated that persistent, passive exposure of adult cats to bandlimited tone pip ensembles at moderate intensities (∼70 dB SPL) leads to a long-term suppression of neural activity in auditory cortex, in the absence of hearing loss. With wideband ensembles (4-20 kHz), the suppression is limited to the exposure frequency range; with narrowband ensembles (2-4 kHz, or third-octave bands centered at 4 and 16 kHz), suppression occurs not only within but also well beyond the exposure range, at least in primary auditory cortex (AI). (In secondary cortex (AII) suppression remains limited mostly to the exposure range even for narrowband ensembles.) We report here on two additional experiments. First, we demonstrate suppression in both AI and AII upon exposure to 4-20 kHz bandlimited noise, thus generalizing our previous results obtained with tonal ensembles. However, we found a somewhat different suppression pattern with noise. Whereas 4-20 kHz tone exposure produced relatively uniform suppression over the 4-20 kHz range, save for a small local minimum at ∼10 kHz, 4-20 kHz noise produced maximal suppression over ∼4-10 kHz, which then progressively weakened with frequency up to 20 kHz. Second, we outline the time course of the emergence of response suppression in AI, using the above-mentioned pair of third-octave bands as the exposure stimulus. Suppression emerged relatively rapidly, within a week of exposure onset, and was initially confined to frequencies close to the 4 and 16 kHz stimulus bands. Over the course of several more weeks, the suppression broadened to cover the entire 4-16 kHz range. We discuss these new findings with reference to the putative mechanisms underlying exposure-induced auditory cortical plasticity.
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