Brain-Computer Interface Technology and Applications
Brain-computer interfaces (BCI) allow subjects to control devices using their own brain signals. Brain signals are recorded by one of several methods available: electroencephalography (EEG), single unit recordings (SU) and electrocorticography (ECoG). The signals are sent through a series of filters and amplifiers to a digital signal processor where they are converted into an output communicating the user’s intent. BCI is very useful for patients with severe motor disabilities because it gives them a way to communicate and interact with their environment through non-muscular means. BCI is also an extremely useful neuroscientific tool to investigate new hypotheses on cortical population representations. Work with BCI in the laboratory setting will lend researchers insight into the function of neural networks during specific tasks as well as during free behavior. BCI technology was patented at Washington University in St. Louis by Dr. Eric Leuthardt, Dr. Gerwin Schalk, Dr. Daniel Moran, Dr. Jonathan Wolpaw, and Dr. Jeffrey Ojemann in 2006. Methods of BCI have evolved over time from using EEG to primarily SU recordings and most recently to the introduction of ECoG as a method of recording brain signals. The most significant limitation of current BCI methods is the necessity of cords plugged into the electrode array on the subject's brain to carry signals from the brain to a digital signal processor.
Clinical applications of BCI include providing a means of communication and control for patients with severe motor disabilities, returning muscle control to patients with spinal cord injuries, and sensing the on and off states of Parkinson's disease for potential stimulation therapy. In order for BCI to be used clinically, however, it must be low risk and completely implantable so that electrodes on the brain are not exposed for long periods of time. This requires the device to transmit signals wirelessly to a signal processor or a computer.
Clinical applications of BCI include providing a means of communication and control for patients with severe motor disabilities, returning muscle control to patients with spinal cord injuries, and sensing the on and off states of Parkinson's disease for potential stimulation therapy. In order for BCI to be used clinically, however, it must be low risk and completely implantable so that electrodes on the brain are not exposed for long periods of time. This requires the device to transmit signals wirelessly to a signal processor or a computer.
Methods of Recording Brain Signals
- ELECTROENCEPHALOGRAPHY Electroencephalography takes recordings of electrical activity in the brain from electrodes placed on the scalp. It is non-invasive, very safe for the patient, and a convenient and inexpensive method of acquiring brain signals. Its use in BCI, however, is limited because recordings from EEG have low spatial resolution and are extremely susceptible to artifacts such as electromyographic (EMG) signals. Extensive user training is also required for the effective use of EEG.
- SINGLE UNIT RECORDING Single-unit (SU) recordings aim to measure and record the activity of a single neuron. In order for these recordings to be taken an electrode must be placed into the brain, penetrating the parenchyma and entering the brain tissue. The electrode can be placed in close proximity to the neuron of interest, which allows SU recordings to have incredibly high spatial resolution. These high fidelity signals can be very useful for BCI because they contain very few artifacts and provide a clear signal for processing. There are consequences, however, that arise from placing a foreign object in a subject’s brain. The electrodes in the brain tissue cause local neural and vascular damage and pose a risk of central nervous system (CNS) infection which could lead to encephalitis, or acute inflammation of the brain. This is incredibly dangerous for the subject undergoing the procedure and thus rules SU recording out as a clinically applicable method of BCI. A second disadvantage of SU is the difficulty in achieving and maintaining stable recordings. The irritation caused by the presence of a microelectrode in the brain causes a cascade of cell responses leading to the creation of a high impedance gliotic sheath surrounding the electrode. This sheath isolates the electrode from the surrounding neural tissue and the currents from the firing neuron of interest find other pathways around the electrode rather than flowing through the high impedance sheath. SU recordings are therefore unreliable as well as dangerous, and are not a reliable source of recording brain signals for BCI.
- ELECTROCORTICOGRAPHY Electrocorticography (ECoG) is an alternative method of recording brain signals that provides higher quality signals than EEG and is less invasive than SU recording. The electrodes used in ECoG sit on the surface of the brain, but do not penetrate the parenchyma thus they do not provide a path for CNS infection. ECoG BCI offers many advantages over both EEG and SU BCI. ECoG has much higher spatial resolution than EEG, (tenths of millimeters versus centimeters) as well as a superior frequency range (0-200 Hz versus 0-49 Hz). ECoG can achiever higher spatial resolution than EEG because the ECoG electrode array is place on the surface of the cortex whereas the EEG electrodes are placed on the surface of the head so signals from the brain must travel a farther distance through the skull before reaching the EEG electrodes. ECoG BCI also records higher amplitude signals than EEG (50-100 μV versus 10-20 μV), is much less susceptible to EMG artifacts, and has a much higher signal-to-noise ratio (SNR). Advantages of ECoG BCI over single-unit recordings lie in the fact that ECoG is recorded by epidural electrode arrays and thus does not require electrodes that penetrate into the cortex. This allows for greater long-term stability in recordings and is much safer for the patient. ECoG has evolved as the most effective method of recording brain activity for BCI; it combines the high fidelity signals of single-unit recording and the safety of EEG.