Electrophysiological correlates of azimuth and elevation cues for sound localization in human middle latency auditory evoked potentials

A cortical biomarker of audibility and processing efficacy in children with single-sided deafness using a cochlear implant

The goals of the current study were to evaluate audibility and cortical speech processing, and to provide insight into binaural processing in children with single-sided deafness (CHwSSD) using a cochlear implant (CI). The P1 potential to acoustically-presented speech stimuli (/m/, /g/, /t/) was recorded during monaural [Normal hearing (NH), CI], and bilateral (BIL, NH + CI) listening conditions within a clinical setting in 22 CHwSSD (mean age at CI/testing 4.7, 5.7 years). Robust P1 potentials were elicited in all children in the NH and BIL conditions. In the CI condition: (1) P1 prevalence was reduced yet was elicited in all but one child to at least one stimulus; (2) P1 latency was prolonged and amplitude was reduced, consequently leading to absence of binaural processing manifestations; (3) Correlation between P1 latency and age at CI/testing was weak and not significant; (4) P1 prevalence for /m/ was reduced and associated with CI manufacturer and duration of CI use. Results indicate that recording CAEPs to speech stimuli in clinical settings is feasible and valuable for the management of CHwSSD. While CAEPs provided evidence for effective audibility, a substantial mismatch in timing and synchrony of early-stage cortical processing between the CI and NH ear remains a barrier for the development of binaural interaction components.

Localization by unilateral BAHA users

OBJECTIVES: Patients with unilateral hearing loss report difficulty hearing conversation on their impaired side, localizing sound, and understanding of speech in background noise. The bone-anchored cochlear stimulator (BAHA) (Entific, Gothenburg, Sweden) has been shown to improve performance in persons with unilateral severe-profound sensorineural loss (USNHL). The purpose of this study is to evaluate the effectiveness of BAHA in sound localization for USNHL listeners.

STUDY DESIGN: Prospective study of 12 USNHL subjects, 9 of whom received implants on the poorer hearing side. A control group of 10 normal hearing subjects were assessed for comparison. Localization with and without BAHA was assessed using an array of 8 speakers at head level separated by 45 degrees. Error analysis matrix was generated to evaluate the confusions, accuracy in response, and laterality judgment.

RESULTS: The average accuracy of speaker localization was 16% in the unaided condition, with no improvement with BAHA use. Laterality judgment was poorer than 43% in both aided and nonaided conditions.

CONCLUSIONS: Patients with UNSNHL had poor sound localization and laterality judgment abilities that did not improve with BAHA use.

The influence of externalization and spatial cues on the generation of auditory brainstem responses and middle latency responses

The effect of externalization and spatial cues on the generation of auditory brainstem responses (ABRs) and middle latency responses (MLRs) was investigated in this study. Most previous evoked potential studies used click stimuli with variations of interaural time (ITDs) and interaural level differences (ILDs) which merely led to a lateralization of sound inside the subject's head. In contrast, in the present study potentials were elicited by a virtual acoustics stimulus paradigm with 'natural' spatial cues and compared to responses to a diotic, non-externalized reference stimulus. Spatial sound directions were situated on the horizontal plane (corresponding to variations in ITD, ILD, and spectral cues) or the midsagittal plane (variation of spectral cues only). An optimized chirp was used which had proven to be advantageous over the click since it compensates for basilar membrane dispersion. ABRs and MLRs were recorded from 32 scalp electrodes and both binaural potentials (B) and binaural difference potentials (BD, i.e., the difference between binaural and summed monaural responses) were investigated. The amplitudes of B and BD to spatial stimuli were not higher than those to the diotic reference. ABR amplitudes decreased and latencies increased with increasing laterality of the sound source. A rotating dipole source exhibited characteristic patterns in dependence on the stimulus laterality. For the MLR data, stimulus laterality was reflected in the latency of component N(a). In addition, dipole source analysis revealed a systematic magnitude increase for the dipole contralateral to the azimuthal position of the sound source. For the variation of elevation, the right dipole source showed a stronger activation for stimuli away from the horizontal plane. The results indicate that at the level of the brainstem and primary auditory cortex binaural interaction is mostly affected by interaural cues (ITD, ILD). Potentials evoked by stimuli with natural combinations of ITD, ILD, and spectral cues were not larger than those elicited by diotic chirps.