Multi-electrode stimulation evokes consistent spatial patterns of phosphenes and improves phosphene mapping in blind subjects.

BACKGROUND: Visual cortical prostheses (VCPs) have the potential to restore visual function to patients with acquired blindness. Successful implementation of VCPs requires the ability to reliably map the location of the phosphene produced by stimulation of each implanted electrode. OBJECTIVE: To evaluate the efficacy of different approaches to phosphene mapping and propose simple improvements to mapping strategy. METHODS: We stimulated electrodes implanted in the visual cortex of five blind and fifteen sighted patients. We tested two fixation strategies, unimanual fixation, where subjects placed a single index finger on a tactile fixation point and bimanual fixation, where subjects overlaid their right index finger over their left on the tactile point. In addition, we compared absolute mapping in which a single electrode was stimulated on each trial, and relative mapping with sequences containing stimulation of three to five phosphenes on each trial. Trial-to-trial variability present in relative mapping sequences was quantified. RESULTS: Phosphene mapping was less precise in blind subjects than in sighted subjects (2DRMS, 16 ± 2.9° vs. 1.9 ± 0.93°; t (18) = 18, p = <0.001). Within blind subjects, bimanual fixation resulted in more consistent phosphene localization than unimanual fixation (BS1: 4.0 ± 2.6° vs. 19 ± 4.7°, t (79) = 24, p < 0.001; BS2 4.1 ± 2.0° vs. 12 ± 2.7°, t (65) = 19, p < 0.001). Multi-point relative mapping had similar baseline precision to absolute mapping (BS1: 4.7 ± 2.6° vs. 3.9 ± 2.0°; BS2: 4.1 ± 2.0° vs. 3.2 ± 1.1°) but improved significantly when trial-to-trial translational variability was removed. Although multi-point mapping methods did reveal more of the functional organization expected in early visual cortex, subjects tended to artificially regularize the spacing between phosphenes. We attempt to address this issue by fitting a standard logarithmic map to relative multi-point sequences. CONCLUSIONS: Relative mapping methods, combined with bimanual fixation, resulted in the most precise estimates of phosphene organization. These techniques, combined with use of a standard logarithmic model of visual cortex, may provide a practical way to improve the implementation of a VCP.

Eye movements and the perceived location of phosphenes generated by intracranial primary visual cortex stimulation in the blind.

BACKGROUND: Restoring sight for the blind using electrical stimulation of the visual pathways is feasible but demands an understanding of the spatial mapping of the visual world at the site of targeted stimulation, whether in the retina, thalamus, or cortex. While a visual cortex stimulator can bypass the eye and create visual percepts, there is an inherent dissociation between this stimulation and eye movements. It is unknown whether and how robustly the brain maintains the oculomotor circuitry in patients with bare- or no-light perception. OBJECTIVE: To critically and quantitatively evaluate the effect of eye movements have on phosphene locations elicited by cortical stimulation that bypasses the eyes in order to restore sight in blind subjects. METHODS: The NeuroPace Responsive Neurostimulator (RNS) and the Orion visual cortical prosthesis devices were used to electrically stimulate the visual cortex of blind subjects with bare or no light perception. Eye positions were recorded synchronized with stimulation and the location of the percepts were measured using a handheld marker. RESULTS: The locations of cortical stimulation-evoked percepts are shifted based on the eye position at the time of stimulation. Measured responses can be remapped based on measured eye positions to determine the retinotopic locations associated with the implanted electrodes, with remapped responses having variance limited by pointing error. CONCLUSIONS: Eye movements dominate the perceived location of cortical stimulation-evoked phosphenes, even after years of blindness. By accounting for eye positions, we can mimic retinal mapping as in natural sight.

The human primary somatosensory cortex encodes imagined movement in the absence of sensory information.

Classical systems neuroscience positions primary sensory areas as early feed-forward processing stations for refining incoming sensory information. This view may oversimplify their role given extensive bi-directional connectivity with multimodal cortical and subcortical regions. Here we show that single units in human primary somatosensory cortex encode imagined reaches in a cognitive motor task, but not other sensory-motor variables such as movement plans or imagined arm position. A population reference-frame analysis demonstrates coding relative to the cued starting hand location suggesting that imagined reaching movements are encoded relative to imagined limb position. These results imply a potential role for primary somatosensory cortex in cognitive imagery, engagement during motor production in the absence of sensation or expected sensation, and suggest that somatosensory cortex can provide control signals for future neural prosthetic systems.

Technical considerations for generating somatosensation via cortical stimulation in a closed-loop sensory/motor brain-computer interface system in humans.

Somatosensory feedback is the next step in brain computer interface (BCI). Here, we compare three cortical stimulating array modalities for generating somatosensory percepts in BCI. We compared human subjects with either a 64-channel "mini"-electrocorticography grid (mECoG; 1.2-mm diameter exposed contacts with 3-mm spacing, N = 1) over the hand area of primary somatosensory cortex (S1), or a standard grid (sECoG; 1.5-mm diameter exposed contacts with 1-cm spacing, N = 1), to generate artificial somatosensation through direct electrical cortical stimulation. Finally, we reference data in the literature from a patient implanted with microelectrode arrays (MEA) placed in the S1 hand area. We compare stimulation results to assess coverage and specificity of the artificial percepts in the hand. Using the mECoG array, hand mapping revealed coverage of 41.7% of the hand area versus 100% for the sECoG array, and 18.8% for the MEA. On average, stimulation of a single electrode corresponded to sensation reported in 4.42 boxes (range 1-11 boxes) for the mECoG array, 19.11 boxes (range 4-48 boxes) for the sECoG grid, and 2.3 boxes (range 1-5 boxes) for the MEA. Sensation in any box, on average, corresponded to stimulation from 2.65 electrodes (range 1-5 electrodes) for the mECoG grid, 3.58 electrodes for the sECoG grid (range 2-4 electrodes), and 11.22 electrodes (range 2-17 electrodes) for the MEA. Based on these findings, we conclude that mECoG grids provide an excellent balance between spatial cortical coverage of the hand area of S1 and high-density resolution.