Short term effects of noise exposure on responses of mouse inferior colliculus neurons
To determine the effects of noise exposure on neural coding of sounds by central auditory neurons, extracellular recordings were obtained from the inferior colliculi of C57BL/6 mice. Exposure intensity and age factors were examined by employing 95 and 110 dB levels of white noise, and two age groups of mice (16-18 day-old and 28-30 day-old). For many neurons, temporal discharge patterns and/or discharge rates were altered by the exposure. In some cases, sustained excitatory responses changed to "onset" responses and vice versa. Statistically significant mean threshold elevations were observed after both intensities of exposure, being more pronounced after 110 dB than 95 dB noise. Also, threshold elevations were more severe at high frequencies than at low frequencies. However, threshold decreases were also observed in several neurons. Q10 values (a measure cf sharpness of tuning) showed a significant mean broadening of tuning curves after exposure. A significant increase in the slopes of discharge rate intensity functions was observed after noise exposure, although individual intensity functions were affected in various ways. Discharge latency was shortened by the 95 dB exposure, while it was prolonged by the 110 dB exposure. Since the response measures analyzed are believed to be involved in neural coding, the data indicate that noise exposure not only induces hearing sensitivity losses, but also alters neural coding processes. A mechanism was proposed that involved the interplay of excitatory and inhibitory processes in the central auditory system. Some evidence of a sensitive age period for susceptibility to noise exposure was also found.