Influence of systematic variations of the stimulation profile on responses evoked with a vestibular implant prototype in humans

Abstract

Objective: To explore the impact of different electrical stimulation profiles in human recipients of the Geneva-Maastricht vestibular implant prototypes.

Approach: Four implanted patients were recruited for this study. We investigated the relative efficacy of systematic variations of the electrical stimulus profile (phase duration, pulse rate, baseline level, modulation depth) in evoking vestibulo-ocular (eVOR) and perceptual responses.

Main results: Shorter phase durations and, to a lesser extent, slower pulse rates allowed maximizing the electrical dynamic range available for eliciting a wider range of intensities of vestibular percepts. When either the phase duration or the pulse rate was held constant, current modulation depth was the factor that had the most significant impact on peak velocity of the eVOR.

Significance: Our results identified important parametric variations that influence the measured responses. Furthermore, we observed that not all vestibular pathways seem equally sensitive to the electrical stimulus when the electrodes are placed in the semicircular canals and monopolar stimulation is used. This opens the door to evaluating new stimulation strategies for a vestibular implant, and suggests the possibility of selectively activating one vestibular pathway or the other in order to optimize rehabilitation outcomes.

Figures

Figure 1.

Schematic illustration of the experimental procedure for determination of the efficacy of the stimulation paradigm. (a) Determination of the dynamic range (DR) for a given stimulation profile. The threshold (dark blue dotted line) and UCL (dark red dotted line) were measured by consecutive increases of the current amplitude (green bars) of the stimulating pulse train, delivered to one of the vestibular electrodes. The reported DR was the current range between these two values (grey shaded area). Note that the threshold and UCL were determined using subjective reports from the patient, on a 0–8 visual analog scale (dark yellow plot) after each stimulation trial (red arrows). (b) Vestibulo-ocular responses were evaluated at three baseline levels: 30% (red dotted line), 50% (pink dotted line), and 70% (cyan dotted line) of the DR using sinusoidal modulation profiles with a frequency of 2Hz. The effect of modulation depth was investigated using two amplitude levels of the modulation signal: 1.5dB (solid lines) and 3dB (dash-dotted line, only for a baseline of 50% of the DR). The data shown were simulated for illustration purposes, and although representative, do not correspond to particular results of any given patient.

Figure 2.

Influence of the stimulation profile on the vestibular threshold (blue circles), upper comfortable level (UCL – red circle), and corresponding dynamic range (DR – grey columns). The panels on the left display individual data in each experimental condition for: (a) stimulation profiles with a variable phase duration and constant pulse rate (400pps) and (b) stimulation profiles with variable pulse rate and constant phase duration (200μs). The black box in each plot represent mean results (±standard deviation – SD) across electrodes for a given condition. Statistically significant differences (p

Figure 3.

Slope of intensity growth functions of percepts evoked upon stimulation versus PD (left column) and PR (right column). Individual slopes in each condition are presented as coloured dots. Average and standard deviation (SD) values are presented as black solid and dotted lines, respectively.

Figure 4.

Mean normalized PEV (± standard deviation, SD) as a function of mean DR (upper horizontal axes) and of (a) variable PD and (b) variable PR (lower horizontal axes). Normalized PEV values were calculated from eVOR recordings obtained upon sinusoidal (2Hz) modulation of three electrical baseline levels (30%, 50%, and 70% of the DR) with a modulation depth of 1.5dB (respectively red, blue and dark yellow plots). For the baseline level of 50% of the DR an additional modulation depth of 3dB was tested (green plot).