Abstract: Click-evoked cortical potentials were recorded in guinea pigs before and after exposure to intensive impulse noise and their computer-averaged amplitudes at different click levels were compared. Post-exposure responses to high-intensity click (peak level 90-120 dB) were systematically augmented, a result quite contradictory to the general rule of the effect of exposure to noise. The maximum post-exposure amplitude might exceed the maximum pre-exposure value by 30-120%. The dosage of exposure seemingly appropriate for this kind of amplitude augmentation was found to be 100-500 wide-spectrum impulses of 150 dB peak SPL and 50 ms duration, or 2000 impulses of 146 dB and 10 ms. A 15 minutes exposure to 105 dB steady white noise also gave marked amplitude augmentation. Temporal response threshold shift 5-20 min after exposure amounted to 16-48 dB. Latency of the maximum response after exposure increased on the average by about 15%. Overdosage of exposure (e.g. 1000 impulses of 150 dB and 50 ms duration) caused a decrease in stead of an increase of the response amplitude and was accompanied by a large threshold shift (up to 68 dB).
No parallel amplitude increment could be found in cochlear potential and in evoked potentials from the medial geniculate body as well as from the cerebellum, a fact indicating that the mechanism underlying post-exposure augmentation of cortical evoked responses was in the cortex itself. This mechanism did not seem to subject to the effect of narcosis, masking and strychninization of the cortex. Possible causes for the occurrence of the post-noise augmentation effect, such as synchronization of electrical activity, increase of excitability and release from certain kind of inhibition, were discussed.