Mild noise-induced hearing loss at young age affects temporal modulation transfer functions in adult cat primary auditory cortex

Kittens were exposed for 2 h to a 1/3rd octave band of noise centered at 5 kHz and at 120 dB SPL. After the exposure, they were kept in a quiet room for at least 4 weeks, and until they were mature. The noise-exposed cats showed on average 16.5 dB higher ABR thresholds and 13.2 dB higher thresholds at the characteristic frequency (CF) than the control cats for frequencies between 4 and 16 kHz. The frequency-tuning curve bandwidth at 20 dB above threshold was significantly increased compared to controls in the CF region of the hearing loss. In noise-exposed cats, temporal modulation-transfer functions (tMTFs) to amplitude-modulated (AM) noise, but not to periodic click trains, showed a marked increase for modulation frequencies (MFs) below 6 Hz. The vectorstrength in noise-exposed cats increased for all modulation frequencies below 32 Hz for neurons with a CF in the range of the hearing loss. The tMTFs for AMnoise in the noise-exposed group were less band-pass compared to the controls, and in that sense the mild hearing loss could be considered as effectively reducing the central activation in the same way as a reduced sound pressure level. Effects of reduced central inhibition are visible in the broadening of frequency-tuning curves, and in the increased limiting rates for AMnoise.