Multiple auditory steady state responses (80-101 Hz): effects of ear, gender, handedness, intensity and modulation...

To evaluate how the amplitudes and latencies of auditory steady state responses (ASSRs) to multiple stimuli presented at rates between 80 and 101 Hz vary with the ear of stimulation, the handedness or gender of a subject, and the rate and intensity of the stimuli.

ASSRs were recorded in a group of 56 young adults (27 females, 13 left handed) using several stimulus conditions. In the two main conditions, four sinusoidally amplitude-modulated tones (each uniquely modulated using rates between 80 and 105 Hz) with carrier frequencies of 500, 1000, 2000, and 4000 Hz, were presented concurrently to each ear (eight total). In the first condition the modulation rates for the left ear were slower than those for the right and in the second condition this relationship was reversed. Other conditions evaluated the responses to single stimuli, to multiple stimuli presented in one ear only and to multiple stimuli presented dichotically (four in each ear) with rates that decreased rather than increased with increasing carrier frequency. Stimuli were presented at an intensity of 73 dB SPL except in two conditions wherein the intensity was 53 dB SPL.

At 73 dB SPL, multiple-stimulus ASSRs were significantly reduced (monotic or dichotic) compared with single-stimulus ASSRs, especially at 1000 and 2000 Hz. There were significant differences between monotic and dichotic stimulation. When the stimuli were presented dichotically, the amplitude of the response varied with the relative rates of modulation for the stimuli presented in each ear. ASSRs were larger in the ear with the higher rate when the carrier frequencies were 500 and 1000 Hz and when the modulation rates were <90 Hz. There were no consistent effects of gender or ear of stimulation. There were also no significant effects of handedness.

Presenting multiple stimuli at 73 dB SPL in the same ear decreases the amplitude of the ASSR compared with when the stimuli are presented singly. This is caused by the masking effect of low on higher carrier frequencies and some other inhibitory effect of high on lower frequencies. Dichotic stimulation can increase the amplitude of the response to stimuli modulated more rapidly (and concomitantly decrease the responses to the stimuli modulated more slowly). This effect occurs only for carrier frequencies <2000 Hz and for modulation frequencies <90 Hz. Dichotic stimulation also causes a small but highly significant decrease in the latency of the response compared with monotic stimulation.