Fully-implantable, bi-directional brain-computer interfaces (BCIs) necessitate simultaneous cortical recording and stimulation. This is challenging since electrostimulation of cortical tissue typically causes strong artifacts that may saturate ultra-low power (ULP) analog front-ends of fullyimplantable BCIs. To address this problem, we propose an efficient hardware-based method for artifact suppression that employs an auxiliary stimulator with polarity opposite to that of the primary stimulator. The feasibility of this method was explored first in simulations, and then experimentally with brain phantom tissue and electrocorticogram (ECoG) electrode grids. We find that the canceling stimulator can reduce stimulation artifacts below the saturation limit of a typical ULP front-end, while delivering only ∼10% of the primary stimulator's voltage.
I. INTRODUCTIONElectrocorticography (ECoG) is a promising braincomputer interface (BCI) platform for restoring motor function to individuals with severe paralysis [1], [2]. Normally, these BCIs exploit visual feedback to achieve closed-loop operation, which is suboptimal for movement restoration where persistent somatosensory feedback is crucial. Recent human studies [3], [4] used subdurally implanted ECoG grids to elicit somatosensation via cortical electrostimulation, suggesting that future ECoG-based BCIs should be able to restore both motor and sensory functions. These "bidirectional" BCIs will operate in a more biomimetic fashion, and their performance and ease-of-use would likely improve upon the present-day BCIs [5]. Additionally, ECoG-based bi-directional BCIs could be realized as fully-implantable systems, which would greatly improve their utility [6], [7].A major technical challenge to developing a fullyimplantable, ECoG-based, bi-directional BCI is the presence of strong artifacts created by cortical stimulation. Specifically, a fully-implantable BCI must operate in an ultra-low-power (ULP) regime, which limits the nominal supply voltage of its analog front-end [7]. If stimulation artifacts reach or exceed this voltage-a likely scenario in bi-directional BCI operation-recording amplifiers will be saturated, thereby leading to unrecoverable loss of data.
Objective. Electrocorticogram (ECoG)-based brain-computer interfaces (BCIs) are a promising platform for the restoration of motor and sensory functions to those with neurological deficits. Such bi-directional BCI operation necessitates simultaneous ECoG recording and stimulation, which is challenging given the presence of strong stimulation artifacts. This problem is exacerbated if the BCI’s analog front-end operates in an ultra-low power regime, which is a basic requirement for fully implantable medical devices. In this study, we developed a novel method for the suppression of stimulation artifacts before they reach the analog front-end. Approach. Using elementary biophysical considerations, we devised an artifact suppression method that employs a weak auxiliary stimulation delivered between the primary stimulator and the recording grid. The exact location and amplitude of this auxiliary stimulating dipole were then found through a constrained optimization procedure. The performance of our method was tested in both simulations and phantom brain tissue experiments. Main results. The solution found through the optimization procedure matched the optimal canceling dipole in both simulations and experiments. Artifact suppression as large as 28.7 dB and 22.9 dB were achieved in simulations and brain phantom experiments, respectively. Significance. We developed a simple constrained optimization-based method for finding the parameters of an auxiliary stimulating dipole that yields optimal artifact suppression. Our method suppresses stimulation artifacts before they reach the analog front-end and may prevent the front-end amplifiers from saturation. Additionally, it can be used along with other artifact mitigation techniques to further reduce stimulation artifacts.
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