• Astudillo, Juan Pablo Faundez
• Undurraga Lucero, Jaime Andres
• Van Yper, Lindsey

Abstract

Background. Binaural hearing and, in particular, interaural timing differences (ITDs) are crucial for sound source localisation, sound segregation, and speech perception in noise. Despite its importance, there is currently no clinical tool to objectively assess ITD processing. While recent studies have shown that electroencephalography (EEG) measures – such as the ‘acoustic change complex’ (ACC, Ross et al., 2007) and the ‘interaural phase modulation-following response’ (IPMFR; Haywood, et al., 2015; Undurraga et al., 2016) - can be used to assess ITD processing in a laboratory setting, the question remains whether they can be used in the clinic. This study is the first step towards the development of a clinical tool to objectively assess ITD processing. To this end, we assess the effects of stimulus and analysis parameters. More specifically, we examined the effect of intensity, interaural level differences (ILDs), and interaural phase modulations; as well as referencing and noise reduction techniques. <br/>Methods. Twenty normal-hearing adults participated in our study. The stimulus comprised of a 500 Hz carrier tone, 100% amplitude modulated at a rate of 40.4 Hz. IPDs were conveyed in the temporal fine structure and periodically switched from +90° (right-leading) to –90° (left-leading), and from 0° (diotic) to 180° (out-of-phase). Changes in IPD were presented at a rate of either 0.6 Hz or 6.7 Hz to elicit ACC or IPM-FR, respectively. Stimuli were presented at intensity levels of 65, 70, and 75 dB(A), and with ILDs of 0, 5, and 10 dB(A). The EEG data was analysed in two ways: one that resembles a laboratory set-up (i.e. multichannel recording referenced to Cz, using spatial filtering techniques), and one that resembles a clinical setup (i.e. two-channel recording referenced to Fpz without spatial filtering). <br/><br/>Results. ACC and IPM-FR could be obtained in all participants. ACC amplitudes did not change as a function of intensity for any of the IPM conditions (ANOVA; +/-90°, p = 0.056; 0°/180°, p= 0.087), however, smaller responses were observed when introducing ILDs, however, there are no statistically significant differences across conditions (ANOVA; +/-90°, p = 0.2; 0°/180°, p= 0.55). Unlike the ACC amplitudes, IPM-FR amplitudes did change with intensity: larger amplitude were found for 70 and 75 compared to 65 dB(A) in both IPM conditions (ANOVA; +/-90°, p &lt; 0.05; 0°/180°, p &lt; 0.05). Introducing +/-10 dB ILDs resulted in smaller IPM-FRs only in the 0°/180° condition (ANOVA; p &lt; 0.05). Both ACC and IPM-FR amplitudes were affected by the EEG data analysis strategies with smaller responses obtained when referenced to Fpz. While noise reduction techniques did not change the response amplitude, using noise reduction did result in better signalto- noise ratios. <br/><br/>Conclusions. ACC and IPM-FR represent objective neural measures of ITD processing. Amplitudes of IPM-FRs and ACCs are affected by interaural asymmetries of 10 dB(A) depending on the IPM, and are reduced when referenced to Fpz. Both techniques are promising to clinically assess ITD processing, but stimuli and analysis parameters should be carefully considered.

Topics

• impedance spectroscopy
• phase