  Recent works on the relationship between the electro-encephalogram (EEG) data
and psychological stimuli show that EEG recordings can be used to recognize an
auditory stimulus presented to a subject. The recognition rate is, however,
strongly affected by technical and physiological artifacts. In this work,
subjects were presented seven auditory simuli in the form of English words
(first, second, third, left, right, yes, and no), and the time-locked electric
field was recorded with a 64 channel Neuroscan EEG system. We used the surface
Laplacian operator to eliminate artifacts due to sources located at regions far
from the electrode. Our intent with the Laplacian was to improve the
recognition rates of auditory stimuli from the electric field. To compute the
Laplacian, we used a spline interpolation from spherical harmonics. The EEG
Laplacian of the electric field were average over trials for the same auditory
stimulus, and with those averages we constructed prototypes and test samples.
In addition to the Laplacian, we applied Butterworth bandpass digital filters
to the averaged prototypes and test samples, and compared the filtered test
samples against the prototypes using a least squares metric in the time domain.
We also analyzed the effects of the spline interpolation order and bandpass
filter parameters in the recognition rates. Our results show that the use of
the Laplacian improves the recognition rates and suggests a spatial isomorphism
between both subjects.
