Changes of positive and negative summating potential before and after impulse sound exposure in guinea pig cochlea

Compound action potential (CAP) and action potential-summating potential complex response (AP-SP) evoked by click and tone burst respectively were recorded from chronic electrode at round window in guinea pig. Corelations of positive summating potential (SP^-) and negative summating potential (SP^-) with recovery of them were observed before and after impulse noise exposure. The results show that the appearence of SP^- is related to CAP thresthold shift. The appearence of SP^- is more frequently than is in normal condition when auditory thresthold shift is equivatent to or over 30 dB. The lower the amplitude of SP⁺ is, the higher the amplitude of SP^-, and vice versa. It suggests that SP^- may be suppressed by SP⁺ in normal hearing. The suppression may be released if OHCs were injured. Dominent SP as the sign of recruitment may be induced by the change of nonlinear character of IHC following the damage of OHC.