Decoding movement-related cortical potentials from electrocorticography

Abstract

Object: Control signals for brain-machine interfaces may be obtained from a variety of sources, each with their own relative merits. Electrocorticography (ECoG) provides better spatial and spectral resolution than scalp electroencephalography and does not include the risks attendant upon penetration of the brain parenchyma associated with single and multiunit recordings. For these reasons, subdural electrode recordings have been proposed as useful primary or adjunctive control signals for brain-machine interfaces. The goal of the present study was to determine if 2D control signals could be decoded from ECoG.

Methods: Six patients undergoing invasive monitoring for medically intractable epilepsy using subdural grid electrodes were asked to perform a motor task involving moving a joystick in 1 of 4 cardinal directions (up, down, left, or right) and a fifth condition ("trigger"). Evoked activity was synchronized to joystick movement and analyzed in the theta, alpha, beta, gamma, and high-gamma frequency bands.

Results: Movement-related cortical potentials could be accurately differentiated from rest with very high accuracy (83-96%). Further distinguishing the movement direction (up, down, left, or right) could also be resolved with high accuracy (58-86%) using information only from the high-gamma range, whereas distinguishing the trigger condition from the remaining directions provided better accuracy.

Conclusions: Two-dimensional control signals can be derived from ECoG. Local field potentials as measured by ECoG from subdural grids will be useful as control signals for a brain-machine interface.

Fig. 1

Representative grid schematic of the electrode contact locations for participant 153. On electrical stimulation, the red contact (105) was noted to induce left-arm tingling, whereas stimulating the green contact (113) caused left-hand tingling.

Fig. 2

Time-frequency analysis (upper) and averaged evoked potential (lower) for contact 105, with all contralateral movement conditions collapsed together. Zero corresponds to visual stimulus onset. Note the visual evoked potential synchronized to the visual stimulus between 0 and 0.5 seconds and associated low frequency increase in power. A later increase in high-frequency power and drop in low-frequency power associated with actual movement occurred between 0.5 seconds and 1.5 seconds. Finally, there was a persistent low-frequency increase in power associated with the steady-state movement response after 1.5 seconds.

Fig. 3

Time-frequency analysis (upper) and averaged evoked potential (lower) for contact 105, with all contralateral movement conditions collapsed together. Zero corresponds to movement onset. Note the disappearance of the visual evoked potential but the persistence of the high-frequency/low-frequency inversion that occurs during movement (at the zero time point).

Fig. 4

Time-frequency analysis (upper) and averaged evoked potential (lower) for contact 105, with all ipsilateral movement conditions collapsed together. Zero corresponds to visual stimulus onset. Note the persistence of the visual evoked potential, even in the hemisphere ipsilateral to movement. Note also the persistence of the high-frequency/low-frequency inversion associated with movement.

Fig. 5

Time-frequency analysis performed for channel 105 (marked in red on the grid schematic), separating the 4 movement directions and the trigger condition. Although there is a power inversion associated with movement, there is no clear separation between the movement conditions. Trials synchronized to joystick movement onset at 0 seconds.

Fig. 6

Time-frequency analysis performed for channel 113 (marked in green on the grid schematic), separating the 4 movement directions and the trigger condition. With this channel, the trigger condition separates clearly from the other conditions, with a marked increase in high-frequency power relative to the other 4 directional movement conditions. Trials synchronized to joystick movement onset at 0 seconds.

Fig. 7

Graph of normalized power for channel 113 between 70 and 130 Hz over time showing clear separation of the trigger condition (blue) relative to the other 4 movement conditions (down [green], right [red], up [cyan], left [magenta]). Trials synchronized to joystick movement onset at 0 seconds.