External and middle ear influence on envelope following responses

Area of Science:

• Auditory Neuroscience
• Audiology
• Bioacoustics

Background:

• Between-subject variability in envelope following response (EFR) amplitude hinders clinical application.
• EFR amplitude shows patterns of lower amplitude and higher variability at low (<1.2 kHz) and high (>8 kHz) frequencies compared to mid-frequencies (1-3 kHz).
• This variability is hypothesized to stem from differences in the external and middle ear (EM).

Purpose of the Study:

• To investigate if calibrating stimuli to the cochlea using forward pressure level (FPL) calibration reduces EFR amplitude variability.
• To determine if FPL calibration improves EFR amplitude compared to conventional root-mean-square (RMS) calibration.

Main Methods:

• EFRs were measured in 21 young, normal-hearing adults across four frequency bands (low, mid, high, very high) and four modulation rates.
• Stimuli were calibrated using both conventional coupler RMS and in-ear FPL methods.
• EFRs were analyzed for amplitude and variability across different frequency bands and calibration types.

Main Results:

• FPL calibration resulted in larger EFR amplitudes in the low and very high frequency bands compared to RMS calibration.
• The reduction in between-subject variability of EFR amplitude with FPL calibration was modest and not statistically significant.
• EFRs were measured concurrently from four frequency bands [low (0.091-1.2 kHz), mid (1-3 kHz), high (4-5.4 kHz), and very high (vHigh; 8-9.4 kHz)] with 12 harmonics each.

Conclusions:

• While FPL calibration can enhance EFR amplitude in specific frequency ranges, it does not significantly reduce between-subject variability.
• The limited impact of FPL calibration on variability suggests that sources of EFR amplitude variability may lie beyond external and middle ear transmission.
• Cochlear and/or neural processing are likely dominant contributors to the observed between-subject variability in EFR amplitude.