Psychoacoustic and electrophysiological electric-acoustic interaction effects in cochlear implant users with ipsilateral residual hearing.

Cochlear implant users with ipsilateral residual hearing combine acoustic and electric hearing in one ear, this is called electric-acoustic stimulation (EAS). In EAS users, masking can be shown for electric probes in the presence of acoustic maskers and vice versa. Masking effects in acoustic hearing are generally attributed to nonlinearities of the basilar membrane and hair cell adaptation effects. However, similar masking patterns are observed more centrally in electric hearing. Consequently, there is no consensus so far on the level of interaction between the two modalities. Animal studies have shown that electric-acoustic interaction effects can result in reduced physiological responses in the cochlear nerve and the inferior colliculus. In CI users with residual hearing, it has recently become feasible to record intracochlear potentials with a high spatial resolution via the implanted electrode array. An investigation of the electrophysiological effects during combined electric-acoustic stimulation in humans might be used to assess peripheral mechanisms of masking. Seventeen MED-EL Flex electrode users with ipsilateral residual hearing participated in both a behavioral and a physiological electric-acoustic masking experiment. Psychoacoustic methods were used to measure the changes in behavioral thresholds due to the presence of a masker of the opposing modality. Subjects were stimulated electrically with unmodulated pulse trains using a research interface and acoustically with pure tones delivered via headphones. Auditory response telemetry was used to obtain objective electrophysiological changes of electrically evoked compound action potential and electrocochleography for electric, acoustic and combined electric-acoustic presentation in the same subjects. Behavioral thresholds of probe tones, either electric or acoustic, were significantly elevated in the presence of acoustic or electric maskers, respectively. 15 subjects showed significant electric threshold elevation with acoustic masking that did not depend on the electric-acoustic frequency difference (EAFD), a measure for the proximity of stimulation sites in the cochlea. Electric masking showed significant threshold elevation in eleven subjects, which depended significantly on EAFD. In the electrophysiological masking experiment, reduced responses to electric and acoustic stimulation with additional stimulation of the opposing modality were observed. Results showed a similar asymmetry as the psychoacoustic masking experiment. Response reduction was smaller than threshold elevation, especially for electric masking. Some subjects showed reduced responses to acoustic stimulation with electric masking, especially for small EAFD. The reduction of electrically evoked responses was significant in some subjects. No correlation was observed between psychoacoustic and electrophysiological masking results. From present study, it can be concluded that both electric and acoustic stimulation mask each other when presented simultaneously. Electrophysiological measurements indicate that masking effects are already to some extent present in the periphery.

Electric-acoustic forward masking in cochlear implant users with ipsilateral residual hearing.

In order to investigate the temporal mechanisms of the auditory system, psychophysical forward masking experiments were conducted in cochlear implant users who had preserved acoustic hearing in the ipsilateral ear. This unique electric-acoustic stimulation (EAS) population allowed the measurement of threshold recovery functions for acoustic or electric probes in the presence of electric or acoustic maskers, respectively. In the electric masking experiment, the forward masked threshold elevation of acoustic probes was measured as a function of the time interval after the offset of the electric masker, i.e. the masker-to-probe interval (MPI). In the acoustic masking experiment, the forward masked threshold elevation of electric probe stimuli was investigated under the influence of a preceding acoustic masker. Since electric pulse trains directly stimulate the auditory nerve, this novel experimental setup allowed the acoustic adaptation properties (attributed to the physiology of the hair cells) to be differentiated from the subsequent processing by more central mechanisms along the auditory pathway. For instance, forward electric masking patterns should result more from the auditory-nerve response to electrical stimulation, while forward acoustic masking patterns should primarily be the result of the recovery from adaptation at the hair-cell neuron interface. Electric masking showed prolonged threshold elevation of acoustic probes, which depended significantly on the masker-to-probe interval. Additionally, threshold elevation was significantly dependent on the similarity between acoustic stimulus frequency and electric place frequency, the electric-acoustic frequency difference (EAFD). Acoustic masking showed a reduced, but statistically significant effect of electric threshold elevation, which did not significantly depend on MPI. Lastly, acoustic masking showed longer decay times than electric masking and a reduced dependency on EAFD. In conclusion, the forward masking patterns observed for combined electric-acoustic stimulation provide further insights into the temporal mechanisms of the auditory system. For instance, the asymmetry in the amount of threshold elevation, the dependency on EAFD and the time constants for the recovery functions of acoustic and electric masking all indicate that there must be several processes with different latencies (e.g. neural adaptation, depression of spontaneous activity, efferent systems) that are involved in forward masking recovery functions.