gms | German Medical Science

Bilateral cochlear implant (biCIs) patients show often only poor sensitivity to interaural time differences (ITDs), especially after early deafness. However, we were recently able to show that neonatally deafened (ND) rats fitted biCIs in young adulthood and given precisely synchronized binaural stimulation from the outset can develop excellent ITD sensitivity [1]. This suggests that the insensitivity to ITDs seen in human patients may be more due to technical limitations of the devices than issues related to critical periods in development. Todays clinical CI processors employ asynchronous pulsatile stimulation in each ear [2], so that biCI users only experience ITDs in pulse train envelopes (envITD). However, it may not be safe to assume that the CI stimulated auditory pathway is capable of extracting and processing envelope timings from the electric pulsatile stimuli with sufficient precision to permit ITDs to be computed with high precision. In the normal auditory system, envelope extraction of high frequency stimuli is usually performed by the inner hair cells, but in the CI supplied ears of patients, hair cells are defective and bypassed by direct electric stimulation. It therefore seemed likely to us that ITD processing in the biCI stimulated auditory pathway will be dominated by pulse-timing ITDs (ptITDs), rather than envITDs. This motivated us to investigate the relative effectiveness of ptITDs and envITDs in informing lateralization judgments in a well-controlled behavioral animal experiment.

ND biCI rats were initially trained to lateralize sinusoidally enveloped, binaural pulse trains in which envITD and ptITDs co-varied, and they quickly learned to lateralize these ITDs with high accuracy. The animals were then tested with stimuli in which envITDs and ptITDs were drawn independently from \'7b-100, 0, +100\'7d μs. Various combinations of two pulse rates \'7b900, 4500\'7d pps and three envelope modulation rates \'7b5, 20, 100\'7d Hz were tested. Probit analysis was used to quantify how strongly animals based their lateralization judgments on envITDs or ptITDs at each condition.

At all conditions tested, lateralization behavior was determined exclusively by ptITDs. EnvITDs never significantly influenced the lateralization behavior.

We conclude that, while the healthy auditory pathway is undoubtedly highly sensitive to acoustic envITDs [3], this does not extrapolate to the biCI fitted, electrically stimulated pathway. Our results demonstrate that the biCI stimulated auditory pathway is highly sensitive to ptITDs, but insensitive to envITDs. If future improvements in CI techniques are to provide better binaural hearing, more attention will need to be given to precise and appropriate pulse timing.