eye movement recording method
The principle of the EOG as an eye movement recording
method is demonstrated schematically for the horizontal component in fig.1. The
electrodes have to be attached carefully near the eye. The recorded horizontal
potential difference UH is a function of the position α
of the eye with respect to the electrodes and the CRP, which is assumed to be
constant during the recording time. Because the CRP is a slowly changing
function of luminance steps the influence of changes in the CRP can be
disregarded using nearly constant illumination and repeated calibrations. Metabolic
influences e.g. alcohol should be avoided during the recording time. By means
of a suitable offset voltage the DC-amplifier can be adjusted so that the
output voltage is zero for the the straight eye position (reference
compensation).
fig.1
Although
different eye movement recording systems with their advantages and
disadvantages exist (electromagnetic search coil method or the eye trackers for
example), the EOG is applicable in many cases.
Advantages of
the DC-EOG
The
EOG is easy to use, also in children and patients confined to bed
The
signals are measured with respect to the head
Registration
of eye movements with closed eyelids and during sleep are possible
Artifacts
from eyelid blinking can be detected easily
Eye
movement data are analog and the sampling rate of a following
analog-digital-converter (ADC) can be choosen freely
With
naso-temporal electrode application the linearity amounts to ± 20 degrees, with
bi-temporal up to ± 30 degrees
The
EOG is the most inexpensive eye movement recording system
Disadvantages
of the DC-EOG
Superposition
of signals from mimic or chewing muscles (EMG)
Dependency
on changes in CRP
DC-recordings
always have the problem of drift superimposed on the signal of interest
Overcoming the
disadvantages
The
influence of EMG artifacts can be reduced by instructing the subject to keep
mouth open and avoid innervation of the mimic musculature
Dependency
on changes in CRP can be eliminated by reference compensation and repeated
calibration (see below)
The
drifts can be reduced by careful application of the Ag/AgCl-electrodes and
filling them bubblefree with electrode-cream. Technical improvement in EOG
amplifiers can reduce the influence of drifts as well. In fig.2 a dual-channel
DC-EOG amplifier with automatic reference compensation triggered by hand or
computer program is shown. The amplifier is optically isolated to protect the
subject from electrical shocks, in accordance with EU standards.
fig.2
The
procedure of reference compensation and calibration is displayed in fig.3. It
is initiated by an acoustic signal and the illumination of a central LED. During
the reference fixation of this LED the output of the amplifier is set at zero. During
compensation the overall gain is coded in the signal. Calibration can be
carried out by switching two LEDs at a known position, ±20 deg for example.
fig.3
The
linearity of the method is demonstrated in fig.4. Electrodes of each eye are
attached nasally and temporally respectively. As can be seen from fig.1 the eye
is closer to the nasal than to the temporal electrode with respect to the
normal i.e. the straight, position, although eye movements in the nasal
direction should show signal saturation earlier than in the temporal. The
linear range is also dependent on the anatomical situation. As can be seen from fig.4.
fig.4
the
function is linear up to ± 20 degrees. Using a bi-temporal electrode attachment
the function can be linear up to ± 30 degrees ( fig.5) and the signal is larger
with respect to fig.4.
fig.5
Some
examples of eye movement registrations are attached. Figure 6 is a registration
of horizontal and vertical eye movements. In the upper trace the artifacts
during blinking are demonstrated. Short involuntary blinks can be detected and
voluntary longer lid closures distinguished. During the second voluntary lid
closure horizontal eye movements have been recorded.
fig.6
The
second example (fig.7) shows a registration of reading. Fixations and saccades
forwards and backwards are shown. This could be used to record reading in
legasthenic children and monitor therapy.
fig.7
In the last example (fig. 8) horizontal eye movements, mostly saccades, are recorded. Using an algorithm separating saccadic from nonsaccadic eye movement components automatically by use of the acceleration signal delivers a membership function which is +1 for saccades to the right, –1 to the left, and zero for all nonsaccadic eye movements. The membership function is plotted in trace 2 and the time after saccades in trace 3. The maxima represent the intersaccadic intervals.
fig.8
Behrens F., Weiss L.-R. (1992)
An Algorithm Separating Saccadic from Nonsaccadic Eye Movements Automatically
by Use of the Acceleration Signal. Vision Res. Vol. 32,5, 889-803
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