Brain–Computer Interfaces - Index of

360 B.Z. Allison
contact, check the impedance between the skin and the electrode, and repeat this
process until impedances are reduced. Fortunately, there has been some progress
toward improved sensors that require less or no gel [65] and may not require expert
assistance. Several companies develop systems to play games, communicate, diagnose
or monitor patients, and/or perform various functions using EEG. Some of
these systems also measure other physiological signals. Such work should facilitate
wider development of head-mounted physiological sensors, if not improved BCI
electrodes.
Users must perform intentional mental tasks to control any BCI. Different BCI
approaches reflect different task alphabets. In some BCIs, imagination of left vs.
right hand movement might convey information. Other BCIs cannot understand such
neural signals, but instead allow communication via attention to visual targets. The
term “alphabet” reflects that users must usually combine several BCI signals to send
a message or accomplish another goal.
Hybrid BCIs could detect activity stemming from both imagined movement and
selective attention. Hybrid BCIs were suggested over 10 years ago [10], but were
not developed until very recently, as discussed below.
In the distant future, as BCIs learn to recognize larger alphabets, these mental
tasks or “cognemes” might be compared to graphemes or phonemes in written or
spoken language [4]. Of course, most written and spoken languages have vocabularies
with many thousands of different elements. Modern BCIs recognize very few
different signals. It is unlikely that BCIs will have alphabets comparable to natural
languages for a very long time.
However, there is some progress toward task alphabets that are larger and more
consistent with natural languages. Some recent work describes a BCI that can identify
some vowels and consonants that a user is imagining [31, 69]. This feat requires
an invasive BCI. Efforts to reliably identify individual words or sentences through
the EEG have failed [78]. While noninvasive BCIs will develop larger alphabets,
a noninvasive system will never allow a vocabulary as large as a natural language
without a dramatic revolution in neuroimaging technology. Similarly, a “literal” BCI
that can directly translate all of your thoughts into messages or commands is very
far from reality.
Recent work has discussed BCIs based on new or better defined tasks, such
as quasi-movements, first person movement imagery, or imagined music [23, 52,
56]. Brain activity relating to perceived error, image recognition, or other experiences
might be used in other systems based on the EEG, perhaps combined with
other signals [13, 18, 25, 29, 54]. Ideally, the mental tasks used in BCIs should be
easy to generate, difficult to produce accidentally, and easily mapped on to desired
commands. BCIs might also use existing tasks for novel goals.
The last BCI Catalyst, Brain Effects, reflects changes that may occur in the
brain or body because of BCI use. There could be negative side effects, which
may be similar to the negative side effects of watching too much TV or playing
too many computer games. However, some newer BCI systems are based on a revolutionary
principle: using a BCI to produce positive effects. Although the user
does send command and control signals, the system’s main goal is not to enable