Brain-computer interfaces allow a user with a neurological condition to regain some of their lost abilities by enabling them to use robotic limbs for movement or virtual keyboards and customized cursors for operating a computer. These devices can be controlled by the user’s brain activity. On the other hand, a bidirectional BCI not only allows the user to operate these devices but also provides feedback by translating the sensor information into an electric simulation pattern and transmitting it to the relevant region of the brain. In the case of a robotic arm, for instance, not only will bidirectional BCI aid in gripping the object, but it will also allow you to feel the objects held by the arm by providing the required tactile input to the brain.
Bidirectional brain-computer interface technology necessitates capturing brain activity as well as providing feedback by activating neurons. We’ve already looked at the various strategies for recording cerebral activity here, now let’s have a look at some of the stimulation approaches developed by scientists.
Cortical Surface Stimulation
Implantable ECoG grids with electrodes can be put on the cortex’s surface to record electrical activity in the brain. These same electrodes can be used to activate the cortex and provide tactile feedback or any type of stimulation to the neurons.
This is a more intrusive approach wherein microelectrodes are implanted inside the cortex to stimulate the target neurons directly. This method requires less current because the electrodes are situated near the relevant neurons.
Light can be used to observe and trigger neurons in a biological process called optogenetics. To achieve this, green fluorescent protein (GFP), which responds to the presence of intracellular calcium, is introduced into the genetic code of neuronal cells. The transformed cells are activated whenever there is an activity in any region of the brain. Image sensors can be used instead of electrodes to record neuronal activity.
Neurons can be stimulated by flashing light of a specific wavelength at them. Light-sensitive ion channels are expressed in cells by genetic engineering. Genetically identifiable cells can be targeted and either excited or inhibited by shining light of different wavelengths.
The application of a magnetic field is yet another method utilized to stimulate neurons. An electric field created by a varying magnetic field can stimulate neurons. Transcranial magnetic stimulation (TMS) is the most prevalent form of non-invasive magnetic stimulation and involves placing a magnetic coil against the skull to deliver magnetic pulses.
The invasive alternative in this approach uses a microcoil implanted in the cortex to produce a highly focused magnetic field, activating the nearby neurons.
Peripheral Nerve Stimulation
If the patient’s spinal cord is still fully functional, this method can be utilized to provide tactile feedback using a small electrical device implanted next to the peripheral nerve. Good models of afferent activation are now being developed, and peripheral interfaces have the advantage of stimulating the sensory system at its most distal point, where the signals are more predictable. At more central locations, sensory information is changed and becomes more complicated, which makes it more challenging to create completely natural percepts.
Tactile feedback stimulation of the thalamus is another clinically validated stimulation approach. It is clinically used for reducing tremors associated with Parkinson’s disease.
Bridging Lost Connections
Sometimes, the target organs may be unable to either receive brain signals from the brain or send information to the brain if the communication pathway has been blocked, which can result in a number of brain illnesses. The use of a bidirectional BCI can help close this distance by establishing an artificial recurrent link that could re-establish communication between the two locations. The brain’s extraordinary plasticity allows it to naturally incorporate bidirectional BCI into behavior, restoring communication between previously separated parties. For instance, in the very first demonstration of bidirectional BCI by Eberhard Fetz, a monkey with a blocked peripheral nerve leading to a paralyzed arm was able to move a computer cursor through the artificial connection created between the cortex and the hand using bidirectional BCI. The artificial connection gathered cortical activity and then stimulated the arm to convey the message. This has shown therapeutic promise in regaining function in amputated limbs. The users can also receive tactile feedback by stimulating the somatosensory cortex.
Strengthening Weak Synaptic Connections
BBCIs have a second use in producing Hebbian plasticity by stimulating a synaptically linked region with a spike. Hebbian plasticity states that synapses grow more robust when there is frequent interaction between two neurons; in other words, neurons that fire together wire together. This idea can be used to strengthen weak connections in the brain caused by neurological disorders like stroke by stimulating neurons to fire at specific events.
Bidirectional BCI has come a long way, but there are still obstacles preventing it from being used in therapeutic settings. Artifacts from stimulation contaminating recordings of brain activity, health risks, and short-term viability of implantable electrodes are a few of such challenges. Significant progress has been made in the electrode field in recent years, and we may see some great devices using this technology in the coming decade.
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