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.