CHANGES OF CORTICAL FM-EVOKED RESPONSES IN GUINEA PIGS AFTER EXPOSURE TO INTENSIVE NOISE

After exposure either to a steady intensive noise (white noise, repetitive clicks, or narrow band noise, all of 110 dB SPL and 15 minute duration) or to impulse noise (300 impulses of 150 dB peak SPL and 50 ms impulse duretion) amplitudes of the evoked potentials recorded from the primary auditory cortex of guinea pigs in response to frequency modulation of pure tones were found to be systematically augmented while the response thresholds (△F_m) markedly lowered for most carrier frequencies and in most conditions. For all threshold lowering cases, the averaged △F_m shift was about 1/2 of their pre-exposure values, with some extreme examples where post-exposure △F_m could be as small as only 1/5 of the pre-exposure ones or even lower. Post-exposure amplitude augmentation of evoked responses can be considered as a sign of certain cortical facilitation induced by noise. Lowering of △F_m tends to suggest that such cortical facilitation implies some temporary functional change in the sense of improving auditory discrimination.