Brain-brain interface (BBI) refers to interface technology that allows for communication between biological brains. These interfaces typically employ a brain-machine interface (BMI; also called brain-computer interface or BCI) to establish a connection from one brain to a machine, which then transmits the received neural information to another brain. Brain-to-brain interfaces have been utilized among non-human animals, and recent developments have also allowed for successful communication between humans and non-human animals, as well as across human brains. Once a subject is accustomed to receiving foreign, incoming brainwaves, then BBI's can be utilized without strict limitations, such as across long geographical distances, which was demonstrated on subjects, in this case rats, that were on different continents, yet still able to send and receive information to one another. BBI's have been used for communication in the form of behavior, and speech, but using brain signals is novel and was first displayed by researchers at the University of Washington in 2013. 
Brain-computer interface (BCI) also called neural control interface (NCI), mind–machine interface (MMI)., direct neural interface (DNI), or brain–machine interface (BMI)  is novel technology that bridges the capabilities of modern computers and the internet with an organic brain by allowing communication to an external device. A connection via an implantable brain-machine interface allows exponential growth of the organic brain’s processing power. Intricate research and controversial studies such as testing the memory and cognitive skills of humans while spelling complex words, controlling a robotic quadcopter in three-dimensional space using neurological brain signals,  and allowing monkeys to play video games  are just a few of the possibilities that are of interest.
- 1 Research in BBIs
- 2 Commercialization of BBIs/BCIs
- 3 Ethical Considerations
- 4 References
Research in BBIs
Recent work in brain-to-brain interfaces include examples of brain-to-brain communication between animals of the same species (such as between rats), cross-species communication (such as between a rat and a human), and communication between two or more humans. Major milestones in this field of research are detailed below.
Brain-to-brain interfaces between rats were first introduced in 2013 by Miguel Nicolelis at Duke University.  Through this investigation, two rats were taught to press either a right or left lever, and then a brain-machine interface was used to record cortical and sensorimotor signals containing information on which lever one rat pressed, known as the "encoder" rat. The information was then passed through and decoded by a computer, and subsequently transmitted to the brain of the second rat, known as the "decoder" rat. The overall goal of this procedure was to teach the "decoder" rat to press the correct lever after receiving the brain signals of its counterpart. As noted by the study, the receiving rat learned to press the correct lever seven out of ten times.
Despite Nicolelis's success, the study received mixed reviews. Neuroscientist Ron D. Frostig at the University of California, Irvine, described it as “an amazing paper” that demonstrated information could be transferred from one brain to another in real time, while Andrew B. Schwartz, a neuroscientist at the University of Pittsburgh, stated the study was “very simplistic”.
In 2013, associate professor of radiology, Seung-Schik Yoo, led a team of Harvard researchers in the development of the first interspecies BBI between human and rat subjects. Using a non-invasive method of BBI, the study used a flashing light to generate a signal from the human brain. The generated signal was then transmitted to the brain of an anesthetized rat, causing the animal to move its tail. This was done by attaching a "focused ultrasound machine" (FUS) machine to the rat designed to stimulate the specific lobes of the brain associated with tail movement. In parallel, the BBI technology was used to read the neural signals of a human using an electroencephalogram (EEG). By connecting these two systems, the flashing light that the human participant looked at triggered a burst of ultrasound waves connected to the rat’s brain, causing it to move its tail. Further investigation is currently being conducted in order to send more sophisticated messages through BBI, as well as exploring the idea of 'neural coupling.'
In 2013, researchers at the University of Washington established brain-to-brain communication in six participants. Described as the “first direct brain-to-brain interface in humans,” participants were asked to play a cooperative computer game after being paired up with one another. Within each group, one player acted as the “sender” and the other as the “receiver.” The players were separated into different buildings on campus, located one mile apart. During the experiment, the “sender” was able to see the game on a computer screen but had no control over the in-game mechanics. On the other hand, the "receiver" was unable to see the game, but had a touchpad that could be used to interact with the game. Using electroencephalography (EEG), electrical brain activity containing the motor information to operate the cannon was recorded from the “sender” and sent to a brain-computer interface server. Transcranial magnetic stimulation (TMS) was then utilized; the information was transmitted to a TMS server and created stimulation of specified brain regions in the “receiver,” causing motor movement of the hand in the receiving participant to click or move the mouse in the game.
A study published in Nature in 2019 by a joint collaboration between researchers at the University of Washington & Carnegie Mellon University introduced BrainNet. Described as the “first multi-person non-invasive direct brain-to-brain interface for collaborative problem solving” and “a next-generation BBI that addresses many of the limitations of past BBIs,” BrianNet is designed to allow for brain-to-brain communication on a collaborative task between more than two participants. The study specifically involved three human subjects playing a game similar to Tetris, in which information from the brains of two “senders” was recorded through the use of EEG and transmitted to the brain of the “receiver” over the internet through a brain-computer interface based on TMS.
The authors of the paper identified three ways BrainNet improved upon prior human brain-to-brain interfaces:
- “BrainNet expands the scale of BBIs to multiple human subjects working collaboratively to solve a task.”
- “BrainNet is the first BBI to combine brain recording (EEG) and brain stimulation (TMS) in a single human subject, eliminating the need to use any physical movements to convey information”
- “Using only the information delivered by BrainNet, Receivers are able to learn the reliability of information conveyed to their brains by other subjects and choose the more reliable sender. This makes the information exchange mediated by BrainNet similar to real-life social communication, bringing us a step closer to a ‘social network of brains’.”
Commercialization of BBIs/BCIs
Starting in the mid-2010s, BBI/BCI/NaaS (Neuroscience as a Service) companies and startups began to pop up around the world. With recognition from large governmental agencies such as DARPA with their Brain Initiative, the commercial world began to pick up on what the future might have in store for commercializing research into the mind and its various interfaces. In 2016, two presently influential American BCI/NaaS startups were born – Neuralink and Kernel, respectively. There exist a slew of others, but these are specifically notable for their aims and interface development. While greater research is necessary, possible uses for this technology have already started to arise in mainstream culture.
NeuraLink, the latest development out of the Nueralink corporation, is an invasive neurointerface that has the potential of becoming the basis for new communication systems and advanced assistive technologies for humans. It also plays an important role in the study of the brain because it is able to carry out neurological data extrapolation. Because of its complex nature, engineers had to create special robots that perform the unique surgical approach needed to implant the device into the skull, around the same approximate area as behind the left and right ear.  Data extrapolated from the brain is then used to feed sophisticated machine learning algorithms to help a subject, such as a monkey, communicate with an external device, which amplifies and processes neural signals, subsequently allowing the monkey to play video games with it's mind by coordinating hand movements via the motor cortex.
The idea of utilizing brain stimulation as a means of alleviating chronic disorders began in the 1960s with Bergstrom et al. testing stimulation of the thalamic and subthalamic area in patients with cerebral palsy. As the technology developed and research continued, the use of implanted pulse generators – essentially two metal spikes connected to electrodes – was accepted as a treatment for various neurological diseases and disorders such as Parkinson's and Tourettes. These devices acted as a kind of crude electronic pacemaker, and current commercial BBI/BCI companies are looking to make these treatments much more accurate and much more available to individuals in need. Medical aims of BCI company Neuralink are currently attempting to create a means to reverse damage to the spinal cord, giving people with paralysis and potentially a whole host of other neurological disorders a highly accurate, simple means of taking control of their agent once more. Though technically classified as a “neuroscience as a service” company, Kernel also has noninvasive BCI products the “Flow” and “Flux”, but is aiming to tackle broader issues of health and the mind such as sleep and impulse control, effects of psilocybin, and other areas of mental health.
BBI technology has already shown promising results in the future of medical devices. Neural implants, such as cochlear implants, bypass the auditory apparatus and allow a person to hear sounds from outside by converting them into electrical signals that directly reach the brain. Furthermore, some prosthetics, like artificial arms and legs, have already been made to move in response to a patient's thoughts.
Other possible innovations through BBI technology include restoring function to people disabled by neuromuscular disorders such as cerebral palsy, strokes, or spinal cord injuries.  However, experts agree that in order for greater progress to be made in the use of this technology, "day-to-day and moment-to-moment reliability of BCI performance must be improved". At the moment, the sophistication of BCIs do not approach the reliability of natural functions.
Technological Interfacing Capacity
Commercialization also reaches to non-medical applications, in fact, one of Neuralink’s foremost goals is to create a smartphone application that allows for one to interact with their device using their brain. Once interfacing with something like a smartphone keyboard has been achieved and integrated, expansion into a whole host of other applications can be expected for brain-computer interfaces. Though BCI’s are still unable to interact with more than a select few, simplistic technological interfaces, the capacity for low latency interaction with technological tools will enable the creation of complex new forms of technology as the lines between mind and machine blur. Though ethical concerns arise concerning access to these technologies, the proposition that we one day may literally be able to dream designs for new things in a suite such as photoshop seems like a quantum leap in our society’s capacity for creative action. The boundaries of our mind may soon expand, or rather, decalcify, as BCI’s begin to enable seamless interaction with the internet of things around us.
BCI-enabled automobiles stand to play an important role in the development of safer vehicles. While car accidents are one of the biggest causes of death worldwide, BCI-enabled vehicles could help prevent these accidents by recognizing thoughts in a driver's mind and taking a decision faster than the driver could. Nissan, an automobile manufacturer, is already conducting research on a system that would allow the vehicle to slow down or turn the steering wheel 0.2 to 0.5 seconds faster than the driver could.
In addition, studies have shown that EEG signals could potentially be used in car control applications, allowing for individuals with limited mobility, such as those with locked-in disorders, to operate vehicles while also supporting healthy individuals in safe driving. 
BCI technology could also be used in the office place in multiple ways. For example, BCI could detect when attention levels are too low and could trigger an alert to focus, or even adapt the lighting in the room to affect a person. A startup named "Muse" is already developing tools that could do this. Their sensing startup can give real-time information on brain activity and give insight on engagement levels, which could help individuals complete their tasks.
Additionally, BCI technology has the ability to help "situational disabled’ users, or those who are able to use their hands, feet, gaze, etc., to add an extra control to their abilities. One example of this would be a pilot able to better modulate their hand-eye coordination and vision.  Through the development of these technologies a greater variety of fields would become available to the average person as the degree of skill required to complete specific tasks is reduced.
Several gaming companies have also started to utilize this technology to make their games more immersive. Game developers Valve and Neurable have already made devices. Valve is in development for a passive interface, rather than implanted, that controls a game using a wearer's mind. Neurable, on the other hand, currently already has a device that can control an escape game using sensors in a cap, but has since moved on to military applications.
While brain-to-brain interface technology is still in an infantile stage of research, several concerns have been identified in the discussion of ethical of brain-to-brain interface use and integration.
Autonomy and Privacy
Elisabeth Hildt from the Center for the Study of Ethics in the Professions at the Illinois Institute of Technology in Chicago has discussed various issues with multi-person BBIs related to autonomy, privacy, agency, accountability, and identity. Concerning autonomy in brain-to-brain interfaces, there is a risk of information taken from individual brains being transmitted and shared within the network without the consent of the individuals involved. Privacy issues may arise if brain information is recorded and used from a person unaware or against the information being recorded and distributed. In addition, those receiving brain information in a network may also have things they do not wish to receive. It is also unclear whether receivers in BBI networks could be harmed through their use of these technologies, as their brains are affected by the signals being transmitted and there could exist some risk of overstimulation.
Other scientists have discussed the ethical concern of biological enhancement. Advanced brain-to-brain interface technology could be used to modify human cognition through the potential enhancement of the rate of knowledge or skill acquisition. Such enhancement could be useful for learning, yet these technologies would likely be expensive and only available to certain groups of people, highlighting problems of social inequities in education and other areas.
Cross-Species Neural Interfacing
It is also important to consider the ethical implications of cross-species neural interfacing. The UK Academy of Medical Sciences has speculated on this topic, in the report, "Animals Containing Human Material," where they identify a category of research as, "Substantial modification of an animal’s brain that may make the brain function potentially more ‘human-like', particularly in large animals." The possibility of conducting this research is well within the scope of brain-to-brain interface technology. Scientists at Harvard Medical School have already conducted the first interspecies brain interfacing study, by using EEG to take recorded signals from a human scalp, and use them to stimulate movement in the tail of an anesthetized rat. They have also suggested the possibility of using this technology to transmit neural stimulation from non-human animals to humans, with the possibility of enhancing human sensory systems. Modifying the human brain to have animal-like functions and instincts presents a need for further consideration of ethical concerns before moving forward.
There are also concerns with brain-to-brain interfaces, combined with brain-machine interfaces, being utilized in a military setting. The Defense Advanced Research Projects Agency (DARPA) is one of several military-affiliated groups with an interest in BBIs; researchers at Rice University exploring applications of BBIs and BMIs received $8 million in funding from DARPA in 2021, an addition to $18 million received in 2018. Use of these technologies in the military has raised concerns pertaining to agency and identity. 
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