Chronic Deafness Degrades Temporal Acuity in the Electrically Stimulated Auditory Pathway

Area of Science:

• Neuroscience
• Biomedical Engineering
• Auditory Prosthetics

Background:

• Conventional cochlear implants (CI) provide limited frequency selectivity due to broad electrical field spread.
• Intraneural (IN) electrodes offer potential for greater frequency selectivity by stimulating auditory nerve fibers more precisely.
• Concerns exist regarding the efficacy of IN electrodes in deafened ears with reduced auditory nerve fiber counts.

Purpose of the Study:

• To evaluate the effectiveness of IN and CI stimulation in chronically deafened cats.
• To assess the impact of auditory nerve fiber loss on stimulation thresholds and dynamic ranges.
• To investigate changes in temporal acuity with chronic deafness using IN and CI stimulation.

Main Methods:

• Electrical stimulation using IN and CI electrodes in acutely and chronically deafened cats.
• Monitoring auditory pathway activation via recordings from the central nucleus of the inferior colliculus (ICC).
• Measuring thresholds, dynamic ranges, and synchronization rates of ICC neurons to electrical stimulation.

Main Results:

• Chronically deafened cats (153-277 days) showed a ~30% loss of auditory nerve fibers compared to acutely deafened cats.
• No significant impairment in excitation thresholds or dynamic ranges was observed for IN or CI stimulation in deafened cats.
• Temporal acuity significantly decreased in chronically deafened cats, with reduced maximum synchronization rates to electrical stimulation (e.g., IN stimulation synchronized up to 79 pps vs. 203 pps in acute deafness).

Conclusions:

• Intraneural electrodes maintain efficacy in stimulating the auditory nerve even with significant fiber loss.
• Chronic deafness severely degrades temporal acuity, potentially contributing to poor temporal fine structure perception in cochlear implant users.
• Observed temporal acuity deficits in cats suggest that cochlear implants may partially restore temporal processing, but significant challenges remain.