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| 4.
BCI - Brain–computer interfaces |
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4.2. Present-day BCIs
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4.2.1.
Visual evoked potentials |
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In
the 1970s, Jacques Vidal developed a system that satisfied the
current definition of a dependent BCI (Vidal, 1973, 1977).
This system used the VEP recorded from the scalp over
visual cortex to determine the direction of eye gaze,
and thus to determine the direction in which the user wished
to move a cursor.
Brain response interface (BRI) uses the VEPs
produced by brief visual stimuli and recorded from the scalp
over visual cortex. The user faces a video screen displaying
64 symbols (e.g. letters) in an 8 x 8 grid and looks at the
symbol he or she wants to select. Subgroups of these 64 symbols
undergo an equiluminant red/green alternation or a fine red/green
check pattern alternation 40–70 times/s. Each symbol is included
in several subgroups, and the entire set of subgroups is presented
several times. Each subgroup’s VEP amplitude about 100 ms after
the stimulus is computed and compared to a VEP template already
established for the user. From these comparisons, the system
determines with high accuracy the symbol that the user is looking
at. A keyboard interface gives access to output devices. Normal
volunteers can use it to operate a word processing program at
10–12 words/min.
Middendorf (2000) reported another method for using
VEPs to determine gaze direction.
Several virtual buttons appear on a screen and
flash at different rates. The user looks at a button and the
system determines the frequency of the photic driving response
over visual cortex. When this frequency matches that of a button,
the system concludes that the user wants to select it. These
VEP-based communication systems depend on the user’s ability
to control gaze direction.
Source:
Brain–computer
interfaces for communication and control, Clinical Neurophysiology
113 (2002) 767–791, Jonathan R. Wolpaw, Niels Birbaumer, Dennis
J. McFarland, Gert Pfurtscheller, Theresa M. Vaughan |
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