Exposure to high intensity sound damages the inner ear, leading to hearing loss and tinnitus. Previous studies have found that such acoustic trauma also changes neural activity in the auditory cortex. For example, the acoustic trauma increases spontaneous activity and changes the tonotopic map in the auditory cortex. However, for all these studies on changes in neural activities following acoustic trauma, the effect of acoustic trauma on the synchrony of steady state activity in the auditory cortex has not been fully understood. In this study, we targeted the rat auditory cortex, and investigated how the acoustic trauma changes phase synchrony of the steady state activity.
Adult male Wistar rats at postnatal week 9, with a body weight of 270 g, were used. A microelectrode array with a grid of 96 recording sites recorded local field potentials (LFP) from the fourth layer of the auditory cortex under isoflurane anesthesia. We focused on phase locking values (PLV) of LFP to quantify the phase synchrony. As the sound stimuli, 9 pure tones (4, 6.3, 10.1, 12.7, 16, 20.2, 25.4 and 32 kHz, 60 dB SPL) were presented for 30 seconds, each of which was interleaved with a silent block of 30 seconds. After the first recording, the rat was exposed to intense pure tone (16 kHz, over 110 dB SPL) for 1 hour. Then, second recording was made using the same sound stimuli within an hour after the exposure. From the recorded LFPs, we calculated PLVs in 5 bands (theta, 4 - 8 Hz; alpha, 8 - 14 Hz; beta, 14 - 30 Hz; low gamma, 30 - 40 Hz; high gamma, 60 - 80 Hz). Then, a difference of mean of PLV in the stimulus block with respect to the mean of PLV in the adjacent silent block was calculated as ΔPLV.
Consequently, we found that ΔPLV in all bands tended to decrease after the intense sound exposure. This trend was more evident in the case of ΔPLV for 16-kHz pure tone, and ΔPLV in the alpha and beta bands. These results suggest the acoustic trauma immediately changes the phase synchrony in the steady state activity in auditory cortex.