Changes in spontaneous neural activity in the dorsal cochlear nucleus following exposure to intense sound: relation to threshold shift.

Previous studies have shown that the dorsal cochlear nucleus exhibits increased spontaneous activity after exposure to intense sound. Such increases were apparent 1-2 months after the exposure and were generally proportional to the shift in response thresholds induced by the same exposure. The purpose of the present study was to determine whether this sound-induced increase in spontaneous activity is an early event which can be observed shortly after exposure. As in previous studies, anesthetized hamsters ranging in postnatal age from 60-70 days were exposed to a 10-kHz tone at levels between 125 and 130 dB SPL for a period of 4 h. Control animals were similarly anesthetized but were not exposed to the intense tone. Exposed animals were examined in two groups, one at 30 days after exposure, the other at 2 days after exposure. Time of exposure was adjusted so that all animals were between 90 and 100 days of age when spontaneous activity was studied electrophysiologically. The results showed that the increases in spontaneous activity, which were evident at 30 days after exposure, were not observed in animals studied 2 days after exposure. This result contrasted with the effect of the intense tone exposure on neural response thresholds. That is, the shifts in response thresholds seen 2 days after exposure were similar to those observed in animals studied 30 days after exposure. These results indicate that changes in spontaneous activity reflect a more slowly developing phenomenon and occur secondarily after induction of threshold shift.