Perceptual interactions between electrodes using focused and monopolar cochlear stimulation

Abstract

In today's cochlear implant (CI) systems, the monopolar (MP) electrode configuration is the most commonly used stimulation mode, requiring only a single current source. However, with an implant that will allow simultaneous activation of multiple independent current sources, it is possible to implement an all-polar (AP) stimulation mode designed to create a focused electrical field. The goal of this experiment was to study the potential benefits of this all-polar mode for reducing uncontrolled electrode interactions compared with the monopolar mode. The five participants who took part in the study were implanted with a research device that was connected via a percutaneous connector to a benchtop stimulator providing 22 independent current sources. The perceptual effects of the AP mode were tested in three experiments. In Experiment 1, the current level difference between loudness-matched sequential and simultaneous stimuli composed of 2 spatially separated pulse trains was measured as function of the electrode separation. Results indicated a strong current-summation interaction for simultaneous stimuli in the MP mode for separations up to at least 4.8 mm. No significant interaction was found in the AP mode beyond a separation of 2.4 mm. In Experiment 2, a forward-masking paradigm was used with fixed equally loud probes in AP and MP modes, and AP maskers presented on different electrode positions. Results indicated a similar spatial masking pattern between modes. In Experiment 3, subjects were asked to discriminate between across-electrode temporal delays. It was hypothesized that discrimination would decrease with electrode separation faster in AP compared to MP modes. However, results showed no difference between the two modes. Overall, the results indicated that the AP mode produced less current spread than MP mode but did not lead to a significant advantage in terms of spread of neuronal excitation at equally loud levels.

FIG. 1

Schematic of the all-polar mode.

FIG. 2

Threshold levels in monopolar and all-polar mode for the 5 participants.

FIG. 3

Results of Experiment 1. Average current level difference between simultaneous and sequential stimuli when adjusted to equal loudness in AP mode (red crosses) and MP mode (black circles) for each participant and their average (bottom right panel). The error bars of the last panel show the standard error of the mean.

FIG. 4

Results of Experiment 2. Top panels: level of an AP masker in dB (re 1 uA) with a probe presented in AP mode (red crosses) and in MP mode (black circles). Also shown are the masker thresholds and maximum comfortable levels (black and gray thick lines). Bottom panels: the same data but with the masker level expressed as % of the dynamic range.

FIG. 5

Results of Experiment 3. Accuracy scores as function of electrode separation (in mm).

FIG. 6

Model showing two hypothetical electrical fields activated simultaneously (black lines) or sequentially (red dashed lines). The left panel illustrates a scenario with two narrow electrical fields centered on electrodes 10 and 13. The right panel illustrates a scenario with two wide electrical fields.