2.3 | ASSR measurement
ASSRs were recorded using a Bio-Logic MASTER II (Navigator Pro,
Bio-Logic, San Carlos, CA) system. All participants were at rest, in the
supine position. They were asked to relax and to close their eyes. The
two active electrodes were placed at the vertex (active), and at the
ipsilateral mastoid (reference), whereas the ground electrode is placed
on the forehead. All electrode impedances were less than 5 kΩ.
Air-conducted stimuli were presented to both ears through ER-3A
earphones (Etymotic Research Inc., Elk Grove Village, IL). Four carrier
frequencies were tested (0.5 k, 1 k, 2 k, and 4 kHz). These frequencies
were 100% amplitude-modulated and 20% frequency modulated at
modulation frequencies of 82, 87, 91, and 96 Hz (left side) and 84, 89,
94, and 99 Hz (right side), respectively, at intensities of ≤ 80 dB HL.
And in the intensity of ≥ 90 dB HL, it was modulated at modulation
frequency of 67 Hz (left side) and 69 Hz (right side). The stimulus
intensity was started from 60 dB HL to the maximum presentation level
(115 dB HL). The thresholds were determined at 5-dB precision. During
the recording, 16–32 sweeps were made in each block, and data were
averaged in the time domain and then subjected to a fast Fourier
transform analysis. An F-ratio with a p value smaller than 0.05
was assumed meaningful for ASSR data analysis.