Envelope Interactions in Multi-Channel Amplitude Modulation Frequency Discrimination by Cochlear Implant Users

Abstract

Rationale: Previous cochlear implant (CI) studies have shown that single-channel amplitude modulation frequency discrimination (AMFD) can be improved when coherent modulation is delivered to additional channels. It is unclear whether the multi-channel advantage is due to increased loudness, multiple envelope representations, or to component channels with better temporal processing. Measuring envelope interference may shed light on how modulated channels can be combined.

Methods: In this study, multi-channel AMFD was measured in CI subjects using a 3-alternative forced-choice, non-adaptive procedure ("which interval is different?"). For the reference stimulus, the reference AM (100 Hz) was delivered to all 3 channels. For the probe stimulus, the target AM (101, 102, 104, 108, 116, 132, 164, 228, or 256 Hz) was delivered to 1 of 3 channels, and the reference AM (100 Hz) delivered to the other 2 channels. The spacing between electrodes was varied to be wide or narrow to test different degrees of channel interaction.

Results: Results showed that CI subjects were highly sensitive to interactions between the reference and target envelopes. However, performance was non-monotonic as a function of target AM frequency. For the wide spacing, there was significantly less envelope interaction when the target AM was delivered to the basal channel. For the narrow spacing, there was no effect of target AM channel. The present data were also compared to a related previous study in which the target AM was delivered to a single channel or to all 3 channels. AMFD was much better with multiple than with single channels whether the target AM was delivered to 1 of 3 or to all 3 channels. For very small differences between the reference and target AM frequencies (2-4 Hz), there was often greater sensitivity when the target AM was delivered to 1 of 3 channels versus all 3 channels, especially for narrowly spaced electrodes.

Conclusions: Besides the increased loudness, the present results also suggest that multiple envelope representations may contribute to the multi-channel advantage observed in previous AMFD studies. The different patterns of results for the wide and narrow spacing suggest a peripheral contribution to multi-channel temporal processing. Because the effect of target AM frequency was non-monotonic in this study, adaptive procedures may not be suitable to measure AMFD thresholds with interfering envelopes. Envelope interactions among multiple channels may be quite complex, depending on the envelope information presented to each channel and the relative independence of the stimulated channels.

Fig 1. Illustration of the summation-adjusted current levels and DRs used for subject S3.

The colored solid and dashed lines show the original single-channel T and MAL levels before adjusting for multi-channel loudness summation, respectively. The ovals represent the summation-adjusted DRs, and also represent the AM depth used to measure AMFD (i.e., between the summation-adjusted T and MAL levels); the number within each oval indicates the electrode. The ovals on the left side of the figure show single-channel stimuli; the ovals in the middle and right side of the figure show multi-channel stimuli. The filled ovals indicate the channels that received the target AM frequency and the white ovals indicate the channels that received the reference AM frequency.

Fig 2. Examples of experimental stimuli.

The reference stimuli are shown in the left column and the probe stimuli are shown in the middle and right columns. The top row shows probe stimuli with the 102 Hz target AM frequency and the bottom row shows probe stimuli with the 132 Hz target AM frequency. The left column shows the reference AM frequency delivered to all 3 channels, the middle column shows the target AM frequency delivered to 1 of 3 channels (with the reference AM delivered to the other 2 channels), and the right column shows the target AM frequency delivered to all 3 channels. The x-axis shows time (in ms). The y-axis shows the nominal summation-adjusted current levels. The figure accurately shows the timing of the pulse trains and order of interleaving over a 20 ms range. The close-up of the stimulation pattern shows the current levels for the target AM channel (red) and the reference AM channels (blue and green) for the 102 Hz target AM frequency.

Fig 3. Illustration of electrode spacing conditions.

The wide spacing is shown at top and the narrow spacing is shown at bottom. The gray regions indicate the target AM frequency channels and the white regions indicate the reference AM frequency channels. The target AM was delivered to a single channel (left), 1 of 3 channels (middle), or to all 3 channels (right).

Fig 4. Percent correct AMFD at each probe frequency with wide electrode spacing.

Each row shows individual subject data. Each column shows data when the target AM was delivered to only the basal (left), middle (middle), or apical channel (right), or to all 3 channels. The white triangles show data from the present study, in which the target AM was delivered to 1 of 3 channels. The black symbols show data from a related previous study [21], in which the target AM was delivered to a single channel (circles) or to all 3 channels (squares). The caret symbol (^) indicates the target AM channel(s). The dashed line shows chance level performance (33% correct).

Fig 5. Percent correct AMFD at each probe frequency with narrow electrode spacing.

Each row shows individual subject data. Each column shows data when the target AM was delivered to only the basal (left), middle (middle), or apical channel (right), or to all 3 channels. The white triangles show data from the present study, in which the target AM was delivered to 1 of 3 channels. The black symbols show data from a related previous study [21], in which the target AM was delivered to a single channel (circles) or to all 3 channels (squares). The caret symbol (^) indicates the target AM channel(s). The dashed line shows chance level performance (33% correct).

Fig 6. Mean percent correct AMFD across all probe frequencies.

Individual and average data is shown. The top and bottom panels show mean AMFD for the wide and narrow spacing, respectively. The black bars show performance when the target AM was delivered to all 3 channels and the hatched colored bars show performance when the target AM was delivered to a single channel; data are from a previous related study [21]. The filled bars show performance when the target AM was delivered to a 1 of 3 channels. The caret symbol (^) indicates the target AM channel(s). The dashed line shows chance-level performance (33.3% correct).

Fig 7. Mean difference in percent correct (across subjects) when the target AM was delivered to 1 of 3 channels or to all 3 channels, relative to a single channel.

The left and right columns show data for the wide and narrow spacing conditions, respectively. The top, middle, and bottom rows show data relative to the single basal, middle, or apical channel, respectively. The caret symbol (^) indicates the target AM channel(s). The black squares show the mean difference when 2 target AM channels were added to the single target AM channels; the white triangles show performance when 2 reference AM channels were added to the single target AM channels. The error bars show the standard error. Data for the single-channel reference and when the target AM was delivered to all 3 channels (black squares) were collected in a previous related study [21].