Bone Conduction Auditory Steady State Response: Investigations into Reducing Artifact

Abstract

Auditory steady state responses (ASSRs) can be reliably recorded in sleeping neonates and can be used to estimate hearing thresholds in individuals with normal or impaired hearing. However, artifactual responses can contaminate recordings, particularly when recording bone conduction ASSRs. The aim of this investigation was to study the presence of electromagnetic artifact in bone conduction ASSRs and to investigate methods of reducing the amplitude of the artifact. Using a simulation of a patient, ASSR recordings were obtained to multiple frequency bone-conducted stimuli using the research MASTER system. ASSR stimuli were sinusoidal tones with the carrier frequencies 500, 1000, 2000, and 4000 Hz that were 100% amplitude modulated at 83, 87, 91, and 95 Hz, respectively. After the assessment of different equipment layouts, the effects of (1) altering the analog-to-digital conversion rates and (2) screening the transducer cable and B-71 transducer and connecting the screens to ground were investigated. For ASSR recordings, artifacts were observed at the modulation rates of the 1000-, 2000-, and 4000-Hz carrier frequencies. As expected, the artifact was proportional to stimulus intensity and was observed when using sampling rates of 1000 and 2000 Hz but not 1250 and 1600 Hz. Altering the electrode lead/transducer cable layout and orientation of the B-71 transducer significantly affected the amplitude of the artifact. Screening of the B-71 transducer and cable and connecting the screens to ground reduced the amplitude of the artifact to a level that was not significantly above background noise. A mu-metal screen had a significantly greater effect than aluminum or copper. If stimulus artifact is present in the recorded EEG, for certain choices of carrier frequency and sampling rate it is possible to record artifactual signals at the same modulation rates as those used to elicit ASSRs. This is problematic as the artifact may then be misinterpreted as an ASSR response. The results from this study have demonstrated that careful attention to equipment layout, choice of sampling rate, using grounded screens to screen the B-71 transducer and transducer cable can all be effective in reducing the level of stimulus artifact.