The effect of varying stimulus phase between frequency and amplitude modulation on auditory steady-state responses in neonates.

OBJECTIVE: To compare auditory steady-state responses (ASSRs) to air-conducted amplitude, frequency, and mixed modulated stimuli (AM, FM, and MM, respectively) in neonates. DESIGN: Multiple ASSRs to AM, FM, and MM to 0.5, 1, 2, and 4 kHz tones modulated between 0.078 and 0.092 kHz were recorded and compared. MM phase settings across the cycle at 45° intervals were used and optimum phase settings were predicted using a sinusoidal model. STUDY SAMPLE: Twenty neonates with click ABR thresholds of ≤ 40 dB nHL. RESULTS: ASSR amplitudes were significantly larger to AM than FM stimuli. MM phase setting had a significant effect on amplitude at 1, 2, and 4 kHz but not 0.5 kHz. MM phase settings (± 95% confidence intervals) of 276° (± 9.5°) and 270° (± 19.1°) were predicted for 1 and 2 kHz, respectively. The 0.5 and 4 kHz data were not sufficient to model any effect of phase. MM and AM response latencies increased with decreasing carrier frequency. Some MM response latencies were significantly different from AM response latencies, however no consistent trend was apparent. Test times were significantly affected by phase setting. CONCLUSIONS: MM phase settings have a significant effect on ASSR response amplitude and latencies in neonates.