Objective: To explore the feasibility to evaluate objectively auditory temporal resolution using the iso-modulation depth temporal modulation transfer function (TMTF) derived from the amplitude of evoked response to sinusoidally modulated signals.
Methods: Chronic electrodes were implanted in inferior colliculus and auditory cortex of guinea pigs. We recorded the evoked response to sinusoidally modulated tones with modulation frequency being varied from 20 to 400 Hz and modulation depth fixed at 100%. The response amplitude in uV was converted into relative amplitude using the fast Fourier transform (FFT) function provided by Biosig software, and then the iso-modulation depth TMTF was plotted with the relative amplitude changed with modulation frequency. Then we recorded the evoked response to the sinusoidally modulated tones with modulation depth being varied from 100% to 10% and derived the iso-amplitude TMTF comparable to conventional modulation depth threshold TMTF. The derived iso-amplitude TMTF was compared to iso-modulation depth TMTF to determine the validity of iso-modulation depth TMTF.
Results: The iso-modulation-depth TMTF and iso-amplitude TMTF in inferior colliculus and auditory cortex of guinea pigs represented respectively bandpass and lowpass characteristic. The cut-off frequency calculated from the two TMTF methods didn't differ significantly and the cut-off frequency derived from auditory cortex iso-modulation depth TMTF was consistent with behavioral results.
Conclusions: The TMTF plotted with the response amplitude to sinusoidally modulated tones with the modulation depth fixed at 100% and the modulation frequency was a valid method to evaluate objectively auditory temporal resolution.