The EOG value varies from 50 to 3500 lV with a frequency
range of about DC-100 Hz. Its behaviour is practically linear for
gaze angles of ±30 and changes approximately 20 lV for each degree
of eye movement. It should be pointed out here that the variables
measured in the human body (any biopotential) are rarely
deterministic. Its magnitude varies with time, even when all possible
variables are controlled. This means that the variability of the
electrooculogram reading depends on many factors that are diffi-
cult to determine: perturbations caused by other biopotentials
such as EEG (electroencephalogram), EMG (electromiogram), in
turn brought about by the acquisition system, plus those due to
the positioning of the electrodes, skin-electrode contacts, head
and facial movements, lighting conditions, blinking, etc. In
(Glenstrup & Engell, 1995) various studies were made of the accuracy
and precision of the EOG in tracking the eye gaze. To eliminate
or minimise these defects, a considerable effort had to be made in
the signal acquisition stage to make sure it is captured with the
minimum possible perturbations and then during the study and
processing thereof to obtain the best possible results. In our case,
to avoid this problem an AC high-gain differential amplifier
(1000–5000) is used, together with a high pass filter with cut-off
frequency at 0.05 Hz and relatively long time constant and a low
pass filter with cut-off frequency at 35 Hz. The signals are sampled
100 times per second. Fig. 1b shows the EOG signal corresponding
to look at right and left obtained with a bioamplifier with gain
1000.