Reliability of Measures of N1 Peak Amplitude of the Compound Action Potential in Younger and Older Adults.

Purpose: Human auditory nerve (AN) activity estimated from the amplitude of the first prominent negative peak (N1) of the compound action potential (CAP) is typically quantified using either a peak-to-peak measurement or a baseline-corrected measurement. However, the reliability of these 2 common measurement techniques has not been evaluated but is often assumed to be relatively poor, especially for older adults. To address this question, the current study (a) compared test-retest reliability of these 2 methods and (b) tested the extent to which measurement type affected the relationship between N1 amplitude and experimental factors related to the stimulus (higher and lower intensity levels) and participants (younger and older adults). Method: Click-evoked CAPs were recorded in 24 younger (aged 18-30 years) and 20 older (aged 55-85 years) adults with clinically normal audiograms up to 3000 Hz. N1 peak amplitudes were estimated from peak-to-peak measurements (from N1 to P1) and baseline-corrected measurements for 2 stimulus levels (80 and 110 dB pSPL). Baseline-corrected measurements were made with 4 baseline windows. Each stimulus level was presented twice, and test-retest reliability of these 2 measures was assessed using the intraclass correlation coefficient. Linear mixed models were used to evaluate the extent to which age group and click level uniquely predicted N1 amplitude and whether the predictive relationships differed between N1 measurement techniques. Results: Both peak-to-peak and baseline-corrected measurements of N1 amplitude were found to have good-to-excellent reliability, with intraclass correlation coefficient values > 0.60. As expected, N1 amplitudes were significantly larger for younger participants compared with older participants for both measurement types and were significantly larger in response to clicks presented at 110 dB pSPL than at 80 dB pSPL for both measurement types. Furthermore, the choice of baseline window had no significant effect on N1 amplitudes using the baseline-corrected method. Conclusions: Our results suggest that measurements of AN activity can be robustly and reliably recorded in both younger and older adults using either peak-to-peak or baseline-corrected measurements of the N1 of the CAP. Peak-to-peak measurements yield larger N1 response amplitudes and are the default measurement type for many clinical systems, whereas baseline-corrected measurements are computationally simpler. Furthermore, the relationships between AN activity and stimulus- and participant-related variables were not affected by measurement technique, which suggests that these relationships can be compared across studies using different techniques for measuring the CAP N1.