Psychophysical versus physiological spatial forward masking and the relation to speech perception in cochlear implants

Identifying cochlear implant channels with poor electrode-neuron interface: partial tripolar, single-channel thresholds and psychophysical tuning curves

OBJECTIVE

The goal of this study was to evaluate the ability of a threshold measure, made with a restricted electrode configuration, to identify channels exhibiting relatively poor spatial selectivity. With a restricted electrode configuration, channel-to-channel variability in threshold may reflect variations in the interface between the electrodes and auditory neurons (i.e., nerve survival, electrode placement, and tissue impedance). These variations in the electrode-neuron interface should also be reflected in psychophysical tuning curve (PTC) measurements. Specifically, it is hypothesized that high single-channel thresholds obtained with the spatially focused partial tripolar (pTP) electrode configuration are predictive of wide or tip-shifted PTCs.

DESIGN

Data were collected from five cochlear implant listeners implanted with the HiRes90k cochlear implant (Advanced Bionics Corp., Sylmar, CA). Single-channel thresholds and most comfortable listening levels were obtained for stimuli that varied in presumed electrical field size by using the pTP configuration for which a fraction of current (sigma) from a center-active electrode returns through two neighboring electrodes and the remainder through a distant indifferent electrode. Forward-masked PTCs were obtained for channels with the highest, lowest, and median tripolar (sigma = 1 or 0.9) thresholds. The probe channel and level were fixed and presented with either the monopolar (sigma = 0) or a more focused pTP (sigma > or = 0.55) configuration. The masker channel and level were varied, whereas the configuration was fixed to sigma = 0.5. A standard, three-interval, two-alternative forced choice procedure was used for thresholds and masked levels.

RESULTS

Single-channel threshold and variability in threshold across channels systematically increased as the compensating current, sigma, increased and the presumed electrical field became more focused. Across subjects, channels with the highest single-channel thresholds, when measured with a narrow, pTP stimulus, had significantly broader PTCs than the lowest threshold channels. In two subjects, the tips of the tuning curves were shifted away from the probe channel. Tuning curves were also wider for the monopolar probes than with pTP probes for both the highest and lowest threshold channels.

CONCLUSIONS

These results suggest that single-channel thresholds measured with a restricted stimulus can be used to identify cochlear implant channels with poor spatial selectivity. Channels having wide or tip-shifted tuning characteristics would likely not deliver the appropriate spectral information to the intended auditory neurons, leading to suboptimal perception. As a clinical tool, quick identification of impaired channels could lead to patient-specific mapping strategies and result in improved speech and music perception.

Principles of design and biological approaches for improving the selectivity of cochlear implant electrodes

The perceptual performance of cochlear implant recipients seems to have reached a plateau in recent years. This may be attributable to inadequate neural selectivity of available intracochlear electrodes, caused by current spread and electrode interactions. Attempts to improve electrode selectivity have included manipulating the number and configuration of electrodes that are stimulated at any one time, displacing perilymph from the cochlea to restrict current flow along the cochlea, and reducing the distance between electrodes and neurons. One experimental approach by which the distance between neurons and electrodes may be reduced is to use neurotrophic factors to promote the regeneration of the peripheral dendrites of auditory neurons and guide them towards intracochlear electrodes. The likely requirements of a system for regenerating auditory neurons towards the cochlear electrode include either a stable release of neurotrophin, or transient neurotrophin followed by electrical stimulation; a close proximity of electrode to osseous spiral lamina or a polymer to bridge the gap between the two; guidance signals to attract neurons towards the electrode; patterning of the electrode surface to direct dendrites to electrode contacts and a 'stop' signal to arrest regeneration once the electrode has been reached.

Psychophysical and physiological measures of electrical-field interaction in cochlear implants

The primary purpose of this study was to determine whether the electrically evoked compound action potential (ECAP) can be used to predict psychophysical electrical-field interaction patterns obtained with simultaneous stimulation of intracochlear electrodes. The second goal was to determine whether ECAP patterns are affected by recording location because differences might influence the relation between ECAP and psychophysical measures. The third goal was to investigate whether symmetrical threshold shifts are produced with phase inversion of the interaction stimulus. Nine adults with Advanced Bionics cochlear implants participated. ECAP and psychophysical thresholds were obtained for basal, middle, and apical probe electrodes in the presence of a subthreshold interaction stimulus delivered simultaneously to each of seven to eight interaction electrodes per probe. The results showed highly significant correlations between ECAP and psychophysical threshold shifts for all nine subjects, which suggests that the ECAP can adequately predict psychophysical electrical-field interaction patterns for subthreshold stimuli. ECAP thresholds were significantly higher for recordings from the basal (versus apical) side of the probe, which suggests that recording location may affect relations between ECAP and psychophysical measures. Interaction stimulus phase inversion generally produced symmetrical threshold shifts for psychophysical measures but not for half of ECAP measures.