Research briefing by Charles M. Greenspon, PhD published on Nature Biomedical Engineering.
Intracortical microstimulation of the somatosensory cortex evokes tactile sensations, but those of individual electrodes are insufficient for functional tasks. We show that stimulating multiple electrodes with somatotopically matched projected fields improves task performance with bionic hands.
The problem
Intracortical microstimulation (ICMS) of the somatosensory cortex can evoke vivid tactile sensations on the body contralateral to the implanted electrodes, and thus ICMS has become a promising approach for restoring the sense of touch needed to achieve an ultimate goal of providing functional bionic limbs to people with spinal cord injuries1,2. Mapping electrodes that evoke tactile sensations to sensors on the bionic limb with the somatotopically matched location allows information about contact location and force to be used by a bionic limb (an artificial sense of touch). Endowing bionic limbs, especially hands, with the sense of touch is thought to be essential for enabling dexterity and embodiment3. However, owing to the low number of participants implanted with an intracortical electrode array in the somatosensory cortex and the lack of comparative analyses, it has remained unclear how ICMS-enabled artificial touch compares to natural touch and whether it is sufficient to restore a functional sense of touch via neural prostheses.
The discovery
We characterized the performance of individual electrodes of Utah microelectrode arrays (2 × 32 electrodes) implanted in the primary somatosensory cortex of three individuals with spinal cord injuries to provide ICMS-enabled artificial touch. First, we performed a retrospective analysis of the reported locations (projected fields) on the fingers of the ICMS-evoked tactile sensations for each electrode of the Utah array to examine their stability (over 2–8 years). Second, we assessed each participant’s ability to locate brief ICMS events to individual fingers or pairs of fingers. Third, we quantified the ability of participants to discriminate changes in ICMS intensity for each electrode. Fourth, we investigated how the ICMS-evoked sensations of individual electrodes interacted when multiple electrodes with overlapping projected fields were stimulated simultaneously. Last, we repeated experiments using multiple rather than individual electrodes of the Utah arrays to determine whether localization and discrimination task performance could be improved.
Our analysis of the projected field locations found that they tended to be stable for the entire duration of the implant. In addition, by examining the residual receptive fields of the electrodes and matching them to their projected fields, we confirmed that the three individuals had each preserved their original somatotopy and that no reorganization could be observed from examination of the ICMS-evoked sensations over time. We found that individual electrodes were broadly unreliable in locating and discriminating artificial tactile events compared with natural touch, in part because the electrodes were insensitive to stimulus intensity, and thus ICMS-evoked tactile sensations were often weak. However, when simultaneously stimulating electrodes, we found that the locations of the tactile sensations evoked by individual electrodes summed together (Fig. 1). Repeating the localization and discrimination experiments using multiple simultaneous electrodes, we found that participants performed better on both tasks than when tested with individual electrodes.