Effect of varying phase between frequency and amplitude modulation on bone conduction auditory steady state responses

Auditory steady state response (ASSR) testing provides a means to objectively estimate hearing levels in newborns and adults for whom behavioral tests prove difficult. When testing these patient groups, it is preferable that clear responses to both air and bone conduction stimuli are obtained in a short amount of time. Much of the literature addressing ASSRs, such as investigations of stimulus and recording parameters, have focused on air conduction ASSRs. The aim of this investigation was to study the amplitudes, latencies, and test times of bone conduction ASSRs elicited using amplitude- (AM), frequency- (FM), and mixed-modulated (MM) stimuli and provide suggestions for optimum recording parameters.

Bone and air conduction multiple ASSRs were recorded from two groups of 20 normal-hearing adults using the Multiple Auditory Steady State Response research system. AM, FM, and MM sinusoidal tones were used (0.5-, 1-, 2-, and 4-kHz carrier frequencies), which were modulated between 78 and 92 Hz. AM depth was 100% and FM depth was 20%. ASSR amplitudes and latencies (calculated using the "preceding cycles" technique) were analyzed for MM phase settings across the cycle from 0° at 45° intervals and compared with AM responses. Optimum phase settings for bone and air conduction ASSRs were calculated using a sinusoidal model based on the amplitude data.

Similar effects of stimulus type and carrier frequency were observed for bone and air conduction ASSRs. AM responses were larger in amplitude compared with FM responses. MM (at all phase settings tested) and AM response latencies increased with decreasing carrier frequency. MM phase setting had a significant (p < 0.01) sinusoidal effect on ASSR amplitudes, compared with AM responses, at 1, 2, and 4 kHz but not 0.5 kHz for air conduction and 1 and 2 kHz but not 0.5 and 4 kHz for bone conduction. Using a sinusoidal function to model this effect, MM phase settings (±95% confidence intervals) of 318° (295 to 350°) and 295° (290 to 310°) are predicted to evoke the largest responses for bone conduction ASSRs at 1 and 2 kHz, respectively. Phase settings of 293° (285 to 310°), 300° (280 to 310°), and 280° (255 to 330°) are predicted for air conduction ASSRs at 1, 2, and 4 kHz, respectively. MM phase setting had little effect on estimated latency. Test times were significantly (p < 0.01) affected by phase setting with both increases and decreases being observed. Test times for ASSRs at 1, 2, and 4 kHz could be significantly reduced if the estimated optimum phase settings are used.

Different stimuli can significantly affect the amplitudes of bone conduction ASSRs. These effects are similar to those observed for air conduction ASSRs. MM stimuli with specific phase settings evoke larger bone conduction ASSRs compared with AM and FM stimuli alone, and calculations show that the time taken to obtain these responses is reduced. Implementation of the suggested optimum settings will promote efficient collection of bone conduction, and indeed air conduction, ASSR data.