Changes in cat primary auditory cortex after minor-to-moderate pure-tone induced hearing loss.

In this paper we present findings in the primary auditory cortex of cats exposed for 2 h to a 115 dB SPL, 6 kHz tone at 36 days, 56 days or 118 days after birth. We evaluate the effects of age at exposure, amount of hearing loss, and time after induction of trauma on the functional reorganization of the cortical tonotopic map. We found a fairly sharp demarcation in the amount of hearing loss (20-25 dB) that caused cortical reorganization. For localized hearing losses, unmasking of excitatory contributions of neighboring frequency regions was found. For cats showing reorganization of the tonotopic map, the frequency-tuning curve bandwidth at 20 dB above threshold at CF (BW(20dB)) increased with increasing threshold at CF. Threshold at CF, and BW(20dB) increased with time after exposure. Minimum spike latency was initially increased, but subsequently decreased with time after exposure at a rate that was two times faster in cats with reorganized cortex than in cats with normal tonotopic maps, to reach the same asymptotic value. Thresholds at CF were correlated with the peripheral hearing loss at near CF frequencies as estimated from ABR measurements. The correlation between BW(20dB) and CF threshold suggests that part of the reorganization could be due to 'residual' sensitivity of the high frequency neurons to not-affected lower or higher frequencies. However, for CFs above 6 kHz, the BW(20dB) for cats with reorganization of the tonotopic map was significantly lower (on average 0.3 octave, P<0.05) than for cats with normal tonotopic maps. This is not what one would expect in cases of pseudo-plasticity characterized by concurrent shifts in BW(20dB) and CF as a result of residual sensitivity to lower frequencies.