Brain Computer Interface Terms

•“A brain computer interface (BCI) is a communication system in which messages or commands that an individual sends to the external world do not pass through the brain’s normal output pathways of peripheral nerves and muscles” 

•– Wolpaw et al, 2002

Points to ponder about Wolpaw’s definition: 

1. What exactly constitutes a message or command? 
2. Could some systems (such as alertness monitors or alpha biofeedback systems) be used to communicate contrary to the designer’s intent? Would they then be BCIs? 
3. Could some systems (such as EEG based lie detectors) be used to communicate contrary to the user’s intent? Would they be BCIs?

• Locked in syndrome: Any disorder that causes someone to lose all voluntary muscle control while remaining cognitively intact. 
• Causes include Lou Gehrig’s disease (ALS), stroke, or injury.

1. Mu: Based on changes in mu activity, also called ERD/ERS, which are associated with real or imagined movements. 
2. Mental task: Based on changes in other free running EEG rhythms, which are associated with imagining certain tasks like singing, math, or writing. 
3. P300: Based on the P300 component of the ERP, which depends on paying attention to certain events and ignoring others. 
4. SSVEP: Based on the SSVEP, which depends on attention to one rapidly flashing LED or area of a monitor while ignoring others. 
5. SCP: Based on changes in slow cortical potentials, which depend on emotional or sexual imagery. 
6. Cortical neurons: Based on information recorded directly from cortical neurons (in an invasive BCI). These depend on imagined or real movements. 

• Dependent: In a dependent BCI, some activity from peripheral nerves and muscles is needed to produce the necessary changes in brain activity. Hence, locked in patients cannot use them. 
• Independent: No activity from peripheral nerves and muscles is needed. According to Wolpaw et al (2002), SSVEP BCIs are dependent, and all others are independent. 

•Invasive (aka implanted): An invasive BCI requires neurosurgery to implant the electrodes used to detect brain activity on or in the brain. The patient should see the doctor periodically to minimize the risk of infection. 

•Noninvasive: A noninvasive BCI can be used without neurosurgery. Most noninvasive BCIs use EEG electrodes placed on the surface of the scalp. BCIs based on fMRI, fNIR, or other approaches are also possible. 

• Synchronous or cue-based: The user must pay attention to external cues, such as flashes, scrolling letters, other prompts on a monitor, or sounds such as tones or words. 
• Asynchronous or non-cue-based: No attention to specific stimuli is necessary to operate the BCI. 

•Midas touch problem: When a user accidentally triggers a BCI. For example, if a mu BCI translates imagined movements into commands, the user may be thinking about movement (without intending to send a message or command) and activate the BCI. This problem is most serious in synchronous BCIs. 

If you’re locked in, how do you turn a BCI on or off? 

1. Spontaneous: A BCI that uses spontaneous EEG components (such as mu). 
2. Evoked: A BCI that uses event related potentials (such as P300). All evoked BCIs are synchronous. 
3. Adaptive: A BCI that adapts its pattern recognition mechanism based on the user’s EEG. 
4. Nonadaptive: A BCI that uses fixed pattern recognition parameters. These are increasingly rare. 
5. Man-machine learning dilemma (MMLD): The problem of a computer trying to adapt to the user’s EEG, while the user’s EEG is simultaneously adapting to the computer. (from Pfurtscheller) 
6. Cursor cortex: Brain tissue that is used for no purpose other than controlling a BCI. (from Kennedy) 
7. Cogneme: The smallest amount of mental activity required to convey meaning in a BCI. It is analogous to a morpheme or grapheme in spoken or written language. 

Note that cognemes will vary depending on the pattern recognition approach used.

•Literal: In a literal BCI, the user’s intent is translated literally to an output. For example, the user thinks of the word “tree” and that word appears on the screen. Or a user thinks of moving her left arm up, and this moves a prosthetic arm up. 

•Interpretive: In an interpretive BCI, the user performs mental activities unrelated to the task. For example, a user may imagine singing, or detect specific flashes, to move a prosthetic arm up. 

These terms are not absolutes, but endpoints of a continuum. 

•Simple: A BCI that uses only one type of signal, such as P300 or mu activity. 

•Hybrid: A BCI that uses more than one type of brain signal, such as both P300 and mu. 

•Interleaved: In this type of hybrid BCI, the different brain signals each send different information. For example, someone may use mu activity to move a cursor and P300 activity to select an item. The added brain activity increases N.

•Simultaneous: In this type of hybrid BCI, the different brain signals are combined to confirm each other. For example, if someone imagines movement, both mu and cortical neural activity could be used to detect this. The added brain activity increases P.