Intracortical Somatosensory Stimulation to Elicit Fingertip Sensations in an Individual With Spinal Cord Injury

Abstract

Background and objectives: The restoration of touch to fingers and fingertips is critical to achieving dexterous neuroprosthetic control for individuals with sensorimotor dysfunction. However, localized fingertip sensations have not been evoked via intracortical microstimulation (ICMS).

Methods: Using a novel intraoperative mapping approach, we implanted electrode arrays in the finger areas of left and right somatosensory cortex and delivered ICMS over a 2-year period in a human participant with spinal cord injury.

Results: Stimulation evoked tactile sensations in 8 fingers, including fingertips, spanning both hands. Evoked percepts followed expected somatotopic arrangements. The subject was able to reliably identify up to 7 finger-specific sites spanning both hands in a finger discrimination task. The size of the evoked percepts was on average 33% larger than a finger pad, as assessed via manual markings of a hand image. The size of the evoked percepts increased modestly with increased stimulation intensity, growing 21% as pulse amplitude increased from 20 to 80 µA. Detection thresholds were estimated on a subset of electrodes, with estimates of 9.2 to 35 µA observed, roughly consistent with prior studies.

Discussion: These results suggest that ICMS can enable the delivery of consistent and localized fingertip sensations during object manipulation by neuroprostheses for individuals with somatosensory deficits.

Clinicaltrialsgov identifier: NCT03161067.

Figure 1. Intracortical Microstimulation Projected Field Map

(A) Portions of the hand that were reported as part of a percept for any single electrode on multiple days. Hue denotes finger; saturation denotes finger segment; and hatching denotes a dorsal hand percept. (B) Microelectrode array implantation locations, as localized via intraoperative photos, shown as gray boxes. Colored circles denote intraoperative electrocorticographic high gamma responses, manually filtered for most salient results. (C) Colored array maps refer to their reconstructed positions and orientations on the MRI in panel B. For each electrode on the array, the set of colors present within the square corresponds to a finger segment in panel A that was reported for that electrode. Gray squares are electrodes not wired for stimulation, and white squares did not elicit any single percept multiple times. C.S. = central sulcus.

Figure 2. Drawn Projected Fields

(A) Example projected fields (elicited at 80 µA) drawn by the participant. Drawn percepts on each hand were elicited through a different single electrode in each drawing. Top right example (showing perception on the right thumb) elicited sensation on both palmar and dorsal sides of the finger. In the leftmost hand on the bottom row, small dots within the projected field indicate “dull pencil point” sensations at those specific points. (B) Projected fields for 4 stimulation intensities through 2 distinct electrodes. Electrode numbers are included below each hand drawing in panels A and B for reference against eFigure1 (Supplement http://links.lww.com/WNL/B694). (C) Distributions of projected field sizes, log-normalized to the size of the index fingertip. (D) Number of electrodes (of the full 96 tested) for which stimulation yielded projected fields on ≥1 fingers at each of the 4 stimulated intensities. (E) Projected field size of all electrodes with nonnull percepts, normalized to the within-electrode mean. Error bars denote the SEM. Outlying projected field sizes, determined from the log-normal distribution, were omitted from this plot. Hand images used for recording and reporting subject responses in panels A and B adapted from a publication by Lameira et al.

Figure 3. ICMS Detection Thresholds

(A) Example psychometric function of intracortical microstimulation (ICMS) amplitude detection delivered to a single electrode with a projected field on the right ring finger. (B) ICMS detection thresholds were estimated for 32 stimulation sites and ranged from 9.2 to 35 µA, with a median value of 18.6 µA. (C) Detection thresholds were estimated at sites using either 1 or 2 (i.e., multi) electrodes. Thresholds at stimulation sites with 2 electrodes appear to be more clustered with less variance. Thresholds estimated with the method of constant stimuli are marked with an x. It should be noted that each threshold data point is color coded according to the finger region that encompasses the projected field of that site.