Binaural encoding of electric and acoustic stimuli

Small differences in the arrival time of sound at the two ears (ITDs) provide important cues for directional hearing. However, deaf subjects with bilateral cochlear implants (CIs) benefit little from ITDs. It is unclear whether this deficit is due to between-ear mismatches in activation sites, differences in binaural brain circuits activated by electric versus acoustic stimulation, or deafness-induced degradations in neural ITD processing.

To identify potential limitations of electric ITD coding, normal hearing gerbils were bilaterally implanted with round window electrodes, and neuronal coding of electric and acoustic ITDs in auditory midbrain (inferior colliculus) was directly compared.

Neural discrimination thresholds for electric and acoustic ITDs were virtually identical (˜100µs). Across populations of neurons within a given hemisphere, ITD tuning was similar for electric and acoustic stimuli. In contrast, even short-term uni- or bilateral hearing loss resulted in a clear degradation and interhemispheric asymmetry in population ITD tuning.

The similarities between electric and acoustic ITD coding in normal hearing animals suggest that similar binaural networks are involved in the processing of electric and acoustic stimuli. The poor ITD sensitivity observed in bilateral CI users is likely due to deafness-induce changes in binaural signal processing.

Publication History

Publication Date:
23 April 2019 (online)