Changes in spontaneous firing rate and neural synchrony in cat primary auditory cortex after localized tone-induced hearing loss.

Increase in spontaneous neural activity after noise-induced hearing loss has frequently been associated with the phenomenon of tinnitus. Eighteen juvenile and adult cats were exposed for 2 h to a 6 kHz tone with an intensity of 115 dB SPL at the cat's head. Seven non-exposed littermates and seven other normal hearing cats were used as age-matched controls. The trauma cats showed localized hearing losses, as assessed by ABR, ranging from less than 20 to 60 dB. The frequency representation in primary auditory cortex was mapped using an eight-electrode array. Single-unit spontaneous activity was recorded for 15 min. Peak cross-correlation coefficients (R) for unit cluster activity recorded on separate electrodes were calculated. We found elevated spontaneous firing rates in regions with reorganization of the tonotopic map compared to the neurons in the non-reorganized cortical regions in the same animals. A second finding was that in these regions the peak cross-correlation coefficients were also increased relative to the non-reorganized parts. A third finding was that exposed animals showed higher spontaneous activity compared to controls regardless of the presence of cortical reorganization. This may be a correlate of tinnitus in the presence of only minor hearing losses.