gms | German Medical Science

Introduction: The cochlear nucleus (CN) is a major target region for auditory brainstem implants due to its good surgical accessibility. In the CN, all auditory nerve (AN) fibers end, and different neuron types give rise to parallel auditory pathways that diverge in the brainstem. Despite the well described anatomical and physiological properties of these pathways, their functional relevance for hearing remains largely unknown. A major neuron type of the CN, the bushy cell, mainly projects to the binaural brainstem nuclei and responds to pure tones with higher temporal precision than AN fibers. This suggests that bushy cells are critical to processing of interaural time differences (ITDs), the dominant cue for directional hearing at low frequencies.

Methods: To elaborate this hypothesis, we recorded responses of single AN and CN fibers to broadband noise during acute experiments on cats. Next, we applied a coincidence analysis within the framework of detection theory to investigate the extent to which CN processing affects ITD-sensitivity. Using the monaural responses, we emulated the binaural task of ITD discrimination, and calculated just noticeable differences (jnds).

Results: The ITD jnds derived from single fibers showed a clear dependency on the cochlear origin of the fiber. ITD jnds ranged from 10 to 300 μs and jnds derived from CN fibers were lower than those derived from AN fibers.

Discussion: These results show that bushy cells process AN fiber information in a way that binaural sensitivity to ITDs improves and suggests that the CN houses a pre-processor for binaural interaction.

Unterstützt durch: This work was supported by Fund for Scientific Research – Flanders Grants G.0083.02 and G.0392.05 and Research Fund K. U. Leuven Grant OT/10/42.