Effects of passive, moderate-level sound exposure on the mature auditory cortex: spectral edges, spectrotemporal density, and real-world noise.

Persistent, passive exposure of adult cats to bandlimited tone pip ensembles or sharply-filtered white noise at moderate levels (∼70 dB SPL) leads to a long-term suppression of spontaneous and sound-evoked activity in the region(s) of primary auditory cortex (AI) normally tuned to the exposure spectrum, and to an enhancement of activity in one or more neighboring regions of AI, all in the apparent absence of hearing loss. Here, we first examined the effects of passive exposure to a more structured, real-world noise, consisting of a mix of power tool and construction sounds. This "factory noise" had less pronounced effects on adult cat AI than our previous random tone pip ensembles and white noise, and these effects appeared limited to the region of AI tuned to frequencies near the sharp factory noise cutoff at 16 kHz. To further investigate the role of sharp spectral edges in passive exposure-induced cortical plasticity, a second group of adult cats was exposed to a tone pip ensemble with a flat spectrum between 2 and 4 kHz and shallow cutoff slopes (12 dB/oct) on either side. Compared to our previous ensemble with the same power in the 2-4 kHz band but very steep slopes, exposure to the overall more intense, sloped stimulus had much weaker effects on AI. Finally, we explored the issue of exposure stimulus spectrotemporal density and found that low aggregate tone pip presentation rates of about one per second sufficed to induce changes in the adult AI similar to those characteristic of our previous, much denser exposures. These results are discussed in light of the putative mechanisms underlying exposure-induced auditory cortical plasticity, and the potential adverse consequences of working or living in moderately noisy environments.