Mathematical analysis of VIIIth nerve cap with a linearly-fitted experimental unit response.

The compound action potential (CAP) recorded at the round window has for some time been investigated in order to determine the underlying temporal spike sequence: the firing probability density (FPD) function of the fibres. Charlet de Sauvage et al. [(1980) Hear. Res. 2, 343-346; (1983) J. Acoust. Soc. Am. 73, 616-627] have previously described a close estimate of the far-field unit response (UR) by presenting to the ear a transient electrical stimulation combined with an acoustical masking noise. A linear model of UR, free of noise or extrinsic potentials and based on both experimental data and on the dipole principle of Teas et al. [(1962) J. Acoust. Soc. Am. 34, 1438-1459], was proposed [(1985) Hear. Res. 18, 121-125]. Using the UR model, deconvolutions were performed on CAPs recorded in various conditions of stimulation and of cochlear pathologies in the guinea pig. The principal findings were: (1) a plateau was observed in the amplitude function around 60-70 dB above normal threshold both for click-evoked CAPs and for the corresponding FPDs; (2) FPDs resulting from the deconvolution of high-frequency derived responses showed 2 main peaks with a 0.7 ms interval followed by a second homologous pair with lower amplitude. Convolution computations confirmed the need for the second set with peaks 2 and 3 being only 0.3 ms apart. These findings are interpreted in terms of mechanical excitation pattern.