Changes of summating potentials in the guinea pig cochlea after impulse sound exposure

Compound action potentials (CAP) and action potential-summating potential complex (AP-SP) evoked by click and tone burst respectively were recorded from the round window in guinea pigs. Correlations between the positive and negative summating potentials (SP and SP-) were examined befroe and after impulse sound exposure. The results show that the SP- is related to CAP threshold shift. In comparison to normal condition, it appears with high incidence when auditory threshold shift reaches 30 dB or more. SP- increases in amplitude when SP decreases. The smaller the amplitude of SP , the larger that of SP-. This suggests that in normal hearing condition SP- may be suppressed by SP . Such suppression may be released if OHC are injured. The dominant SP- as a sign of recruitment may be due to the change in the nonlinear character of IHC following OHC damage.