Fig 1.
ECoG grid and activation thresholds.
A: Photograph of the high-density ECoG grid used in this study. The camera is looking at the back of the grid, the non-brain facing surface. The grid was composed of a silicone sheet (4 cm × 4 cm in size, 1 mm thick) with 64 platinum electrodes. The electrodes were 2 mm in diameter and 4.5 mm apart (center-to-center), except for the two ground electrodes that were 3 mm in diameter and 5 mm from the neighboring electrodes (center-to-center). All electrodes were connected to platinum lead wires that formed four 60-cm long leads. B: Post-operative skull radiograph showing the ECoG grid. C: Grid location on brain surface. The blue electrodes are above the motor cortex (M1). The yellow electrodes are above the somatosensory cortex (S1). The green and grey electrodes are dura-facing ground and reference electrodes, respectively. The white electrodes did not generate a response to any cortical stimulation attempted within the safety limits of our study (less than 57 μC/cm2 per phase). D: Activation thresholds. G1 and G2 (black electrodes) were the return electrodes for Electrodes 1–32 and Electrodes 33–64, respectively. The threshold values are presented in terms of charge exchange per second (μC•Hz) required to elicit a response. The gray line separates the electrodes above the motor and somatosensory cortices. The stimulation pulse amplitude, width, and frequency ranged from 1 to 7 mA, 50 to 400 μs, and 50 to 500 Hz, respectively. The color bar represents the range of the charge exchange.
Table 1.
Terminology and definition.
Fig 2.
A stimulus train is a group of bi-phasic pulses drawn in red. This figure is not drawn to actual time scale.
Table 2.
Stimulation paradigms and their corresponding parameters.
Detection amplitude was specific to each electrode that is tested, and it was established in Phase 2 (Stimulation detection). Detection amplitude was also established for 200 and 400 μs pulse widths for a subset of electrodes following the same style as Phase 2 (Stimulation detection). Δ50 Hz and Δ10 Hz mean 50Hz and 10Hz increments, respectively.
Fig 3.
Motor and somatosensory cortical electrode activation thresholds and electrode impedances.
A: Median and interquartile range of activation thresholds for electrodes above the somatosensory cortex (light gray) and those above the motor cortex (dark gray). Activation thresholds were significantly different between the somatosensory and motor cortical electrodes (Wilcoxon rank sum test, p<0.05). B: Median and interquartile range of impedances for electrodes above the somatosensory cortex (light gray) and those above the motor cortex (dark gray). Electrode impedances were tested at the end of the implantation surgery using the clinical XLTEK® system (non-programmable default testing signal frequency of 1000 Hz). There was no significant difference in electrode impedance between the somatosensory and motor cortical electrodes (Wilcoxon rank sum test, p = 0.66).
Fig 4.
Sensation fields perceived by the participant.
A: Electrode groups that correspond to the eight patterns of evoked sensations indicated by the participant. The white electrodes had no response to cortical stimulation. B: The eight patterns of activity, corresponding to various sensation fields, perceived by the participant during stimulation of the sensory electrodes. The sketches were made by the participant using his right hand on a template.
Table 3.
Electrodes and the corresponding activation thresholds (stimulation parameters needed to elicit a motor or sensory response).
Fig 5.
Perceived intensity as a function of stimulation pulse amplitude and frequency.
The figures indicate the percentage of perceived intensity (y-axis) for a second stimulus train with respect to a preceding reference stimulus train. All trials consisted of two stimulus trains separated by a 2 s time gap. The x-axis indicates the pulse amplitude or frequency. The reference pulse amplitude and frequency are marked by black arrows. The black dots and error bars represent mean and 95% confidence interval, and the linear fit was drawn as a black line. A: The figure corresponds to 10 repetitions of stimulations at Electrode 38 (Linear fit: R2 = 0.99, p< 0.05). A similar monotonic pattern was observed with three other electrodes (3, 11, and 20). B: The figure corresponds to 3 repetitions of stimulations at Electrode 38 (Linear fit: R2 = 0.48, p< 0.05). A similar pattern was observed with three other electrodes (3, 20, and 46). C: Perceived intensity reported by the participant for a smaller frequency range (100 to 200 Hz) compared to B. The figure corresponds to 10 repetitions of stimulations at Electrode 38 (Linear fit: R2 = 0.71, p< 0.05). A similar pattern was observed with three other electrodes (3, 20, and 46).
Fig 6.
Perceived sensation for two types of stimulus trains.
The two types of stimulus trains were 0.5 s in duration and had pulse widths of 200 μs and 400 μs. The participant reported two distinct sensations (tingling and electrical buzz) for four of the eight electrodes (20: 100%. 38: 90%. 39: 90%. 46: 90%) when the stimulus train pulse width was changed from 400 μs (10 repetitions) to 200 μs (10 repetitions). The sensation types of tingling, electrical buzz (E. Buzz), vibration, and vertigo are indicated by yellow, cyan, green, and blue colored tiles, respectively. The white colored tiles indicate no perceived sensation.
Fig 7.
Spatial discrimination for cortical stimulation.
ECoG grid showing the pairs of electrodes that were tested for spatial discrimination and their corresponding perceived body parts indicated by colored outlines. The pulse width, pulse frequency, and stimulus train duration were 400 μs, 500 Hz, and 0.5 s, respectively. The pulse amplitude for Electrodes 3, 11, 19, 12, 20, 38, 39, and 46 were 2.5 mA, 1.5 mA, 1.5 mA, 2 mA, 1.5 mA, 2.5 mA, 2 mA, and 2 mA, respectively.