Compound action potentials(CAP)and action potential-summatingpotential complex(AP-SP)evoked by click and tone burst respectively were re-corded from the round window in guinea pigs.Correlations between the posi-tive and negative summating potentials(SP+ and SP-)were examined beforeand after impulse sound exposure.The results show that the SP- is related tothe threshold shift of CAP.In comparison to normal condtion,it appears witha high incidence when the 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- maybe suppressed by SP+.Such suppression may be released if OHCs are injured.The dominant SP- as a sign of recruitment may be due to the change in thenonlinear character of IHC following OHC damage.
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