ASSR Postprocessing

The EEG recordings were analyzed per block (epoch) of 65,536 samples, corresponding to 2.18 s each. Only recordings with 162 blocks and an artifact rejection level of ±30 μV were used to ensure consistent ASSR detection. On this basis, 93.75% of the total data collected was used (only 15 of 240 recordings were discarded). Table 2 shows the number of recordings included in the analysis for each condition. The ASSR is typically analyzed in the frequency domain by epochs with a duration relative to the periodicity of the ASSR. The EEG spectrum is then composed by the EEG noise produced and the ASSR, which is found at the frequency bin of the stimulus repetition rate and its harmonics (Picton et al., 2003; Rance, 2008). Here, the ASSR data were analyzed offline with the weighted averaging method (John et al., 2001) and an F-ratio test with a strict error rate of 1% (Dobie & Wilson, 1996). ASSR detection was individually evaluated for each of the first four response harmonics, and without making use of the multiharmonic detector of the standard Eclipse (Cebulla et al., 2006). The multiharmonic detector uses the amplitude and phase of the fundamental frequency and higher harmonics (12 harmonics in the commercial Eclipse system) of the ASSR to determine whether the response is present. Although a multiharmonic detector is more sensitive than the F-ratio test, this was used to analyze the detection rates of the individual ASSR harmonics (Dobie & Wilson, 1996). The noise-corrected ASSR amplitude (in dB reference to 1 nV, hereafter, referred to as ASSR level) for the different acoustic conditions tested (Dobie & Wilson, 1996) was the primary variable of interest in this study. The ASSR level is then calculated by subtracting the estimated noise power from the response power. The response power was estimated at the frequency bin of the repetition rate, while the noise power was estimated by averaging the noise power across 20 evenly distributed frequency bins around the response bin. The frequency bins of the harmonics of 50 Hz line noise (e.g., 50, 100, 200, . . . Hz) were excluded from the noise power calculation to avoid the interference from the AC power supply noise. The frequency bins corresponding to any other stimuli repetition rate harmonics were also removed from the noise power calculation. The ASSR and noise amplitudes were estimated for each harmonic. With 20 harmonics used for the noise estimate and a 1% error rate, the critical value for the F-test is F(2, 40)_0.99 = 5.18, corresponding to a signal-to-noise ratio of 7.1 dB. The detection rate was calculated based on the total number of recordings included in the analysis for each condition listed in Table 2. The ASSR was analyzed for each individual harmonic to determine whether there could be a correlation between the ASSR level and the stimulus modulation for each individual harmonic. It is noteworthy, however, that if any harmonic was detected, the stimulus was heard.

Number of Measurements per Condition Included in the Analysis After Postprocessing With an Artifact Rejection Level of ±30 µV.

Note. The tested acoustic conditions correspond to the reference anechoic (REF), and three simulated rooms which mimic an audiometric testing booth (ATB), a room recommended by the British Society of Audiology (BSA) for sound-field audiometry for pediatric assessment, and a standardized listening room (IEC).