INTERPRETATION OF EOG DATA IN ORDER

TO OBSERVE EYE MOVEMENTS

Tina Meißner and Harald Loose

Department of Computer Science and Media, Brandenburg University of Applied Sciences,

Magdeburger Str. 50, 14770 Brandenburg, Germany

Keywords:

Eye tracking, EOG, Eye movement.

Abstract:

This paper deals with the possibilities of observing eye movements from EOG recordings. First the setup

for the recording of eye movements, then the EOG method, which is used to obtain eye movements in the

experimental context, are explained. The ﬁrst recordings resulted in the detection of eye movements within

the EOG data and the results were put into relation to the gaze points gained from the eye tracking system.

In addition to these subjective observations a ﬁrst attempt at quantiﬁcation of the dependency between EOG

signal and gaze points is presented. Though faced with a few problems, it was possible to put numerical values

for distances in relation to signal amplitudes of the EOG.

1 INTRODUCTION

The capture and interpretation of eye movements ha-

ve been done since the 19th century. Countless stu-

dies have been carried out with different interpretati-

on goals regarding the eye tracking data. There are

eye tracking studies which are focused on usabili-

ty, on human-machine interaction, on behavior (psy-

chological, physiological) and so on. The techniques

have changed from simple observation of the move-

ments and automatic capture of the eye to mobile head

mounted eye trackers which work with digital came-

ras and LEDs. There are several publications on eye

movement and eye tracking methods. Duchowski’s

work (Duchowski, 2007) is a fundamental approach

to the topic of eye tracking methodology. In order to

understand what exactly eye movements are and how

it is possible to retrieve them on a less technical basis,

Joos et al. (Joos et al., 2003) published an overview

of eye movements and their applications.

Nowadays the commercial eye tracking systems

which work with LEDs and camera recordings are the

most common way to gain eye tracking data. They

provide reliable gaze points with little effort. The dis-

advantages are the dependency on these systems and

the limited adaptability to own experimental needs.

Another negative aspect is the physical constraint of a

subject. When the eye tracking system is permanent-

ly installed to a work space, the proband has to be in

front of it during the whole experiment with small to-

lerated head movements. This side effect requires al-

ternatives. The following paper follows the idea to re-

place the eye tracking system with EOG signals. An

experimental setup was created in which eye tracking

data is recorded and an EOG derivation is made. Aim

of this experiment was to detect eye movements in

EOG data and be able to derivate exact gaze points

from the EOG signals.

2 RECORDING AND

PROCESSING OF EYE

MOVEMENTS

The commercial eye tracking system Nyan 2.0XT (In-

teractive Minds

) was used to get reference data. Ny-

an 2.0XT is a commercial software which is distribu-

ted with the eye tracking system of LC Technologies

An alternative is the recording of EOG (Electrooculo-

graphy) signals. In this setup a system from Neuro-

werk (SIGMA Medizin-Technik

) was used to derive

eye movement signals. The characteristics of this me-

thod are introduced in subsection 2.2. In subsection

2.1 an overview of the experimental setup in which

http://www.interactive-minds.com/de/eyetracking-

software/nyan-2

http://www.eyegaze.com/

www.neurowerk.de

393

Meißner T. and Loose H..

INTERPRETATION OF EOG DATA IN ORDER TO OBSERVE EYE MOVEMENTS.

DOI: 10.5220/0003776603930397

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS-2012), pages 393-397

ISBN: 978-989-8425-89-8

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

the recordings and tentative interpretations were ma-

de is explained.

2.1 Experimental Setup

The experimental setup was based on former biosi-

gnal experiments as well as ﬁrst test scenarios for col-

lecting EOG data. A subject had to look at a screen

presentation while the eye movements were simul-

taneously recorded using both devices. The screen

presentation consisted of 24 slides. Each of them is

shown for four seconds (details in section 3).

The subject was seated in front of the monitor wi-

thin a distance of 60-70 cm. The recording was car-

ried out two times to have the possibility to compare

the recordings and their resulting interpretations.

The recorded eye tracking data were visualized

using Nyan 2.0XT. A program called EyeValuation

was developed to import, analyze and visualize eye

tracking data. The proprietary EOG signals were im-

ported into MATLAB

and splitted into slides or

groups of slides for visualization.

2.2 Functionality of the EOG

The electrooculography is the measurement of the po-

tential difference between the positively charged cor-

nea and the negatively charged retina. The eye’s elec-

trical ﬁeld is measurable through surface electrodes

near the eye: Electrodes are placed left to the left eye

and right to the right eye to obtain horizontal eye mo-

vements, above and under the left eye for vertical mo-

tions. Two separate signals with positive and negati-

ve amplitudes as an indicator of eye movement are

received. When the eyes move to the right the vol-

tage changes to a positive value. During a leftwards

movements, the voltage inverts to a negative value

(Malmivuo and Plonsey, 1995, p. 580). During an up-

ward (vertical) motion of the eyes, the signal’s volta-

ge drops into the negative area and vice versa when

the eyes move downward. By having both signals it

is pretty easy to get a general idea of where the eyes

are moving towards, but it is difﬁcult to pinpoint the

exact gaze position. The analysis of the EOG signals

is presented in section 3.

3 RESULTS

As mentioned before the commercial eye tracking sy-

stem was used in former experiments and its reliabi-

lity was tested. In this experiment the eye tracking

The MathWorks, Inc.: MATLAB http://www.math-

works.com/

system acts as a reference for comparing the EOG

data to the correct eye movements and gaze points.

It provides information about the location and dura-

tion of a gaze point and enables an interpretation of

the EOG data. The following subsections presents the

eye tracking and EOG data received during the expe-

riment for selected slides and a ﬁrst quantiﬁcation of

the EOG values in regard to exact gaze points.

3.1 Task Slides

After an initial slide there are task slides with instruc-

tions for the proband. The ﬁxation points of the sub-

ject and saccades are visible in ﬁg. 1.

Figure 1: Fixation points and saccades from task slide.

The gaze points that exceed a certain gaze time du-

ration (ﬁxations) are represented by circles. The larger

the diameter of the circle, the longer the gaze durati-

on. The sequence of reading is apparent through the

movements from one ﬁxation point to the other (sac-

cades). The subject started at the top left and succes-

sively read the four lines. Looking at the EOG data of

the task slide (see ﬁg. 2) the reading rhythm is reco-

gnizable. On both sides (horizontal and vertical mo-

Figure 2: EOG data for task slide (left: horizontal move-

ments, right: vertical movements).

vements) the reading of the four lines of the task slide

can be identiﬁed. The horizontal eye movement fol-

lows a regular ﬂow from left (negative) to right (po-

sitive), then the eye moves left to the beginning of a

new line and moves to the right again. This happens

for each line. The length of the lines corresponds to

the transition from the negative to the positive volta-

ge range. The third left to right development is the

BIOSIGNALS 2012 - International Conference on Bio-inspired Systems and Signal Processing

394

longest which is in conformity with the length of the

third line. This similarity indicates a direct dependen-

cy between distances on screen and values between

negative and positive peaks.

The vertical movements show the same dependen-

cies between distance and voltage values like the ho-

rizontal movements. The difﬁculty of analyzing the

EOG signals is the quantiﬁcation of the voltage va-

lues.

3.2 Angle Slides

In the experiment there were angle slides where a red

dot wanders from the center of a bar to the right in

steps of six, back to the center, then six steps to the left

and back to the center. The distance between each step

covers about 10 degrees, which is about 100 pixels on

the screen (of the experiment). The sequence of the

moving point can be detected best in the ﬁg. 3 which

was generated in the EyeValuation program.

Figure 3: Gaze points for angle slide (slide is integrated).

Every point had to be ﬁxated for four seconds

which the subject did quite smoothly. The complete

EOG signal for the angle slide section is shown in ﬁ-

gure 4. The horizontal movements were made evenly.

Figure 4: EOG data for angle slides (left: horizontal move-

ments, right: vertical movements).

It is possible to detect the six steps to the right (ca. 100

µV each). The distance between the steps on screen

was about 100 pixels, ergo 100 pixels are represented

through 100µV. The noticeable drop towards the ne-

gative voltage spectrum is due to the jump from the

outer right point to the center (movement to the left).

The six steps to the left can be detected as well (-100

µV). Though, not in this ﬁgure apparent, both peaks

from the sides to the center have approximately the

same value which proves the dependency between di-

stance on screen and the measured EOG amplitude. If

100 pixels are represented by 100 µV, 600 pixels must

be aquivalent to 600 µV and this is what was measu-

red (see ﬁg. 4).

Since the task was to follow the red dot on a bar,

there are no major vertical movements. The noticea-

ble outliers are simply blinks of the subject and should

not disturb the analysis.

3.3 Fixation Crosses

In order to receive vertical movements, a ﬁxation

cross sequence on ﬁve slides was created: cross right,

left, up, down and center. The gaze points for the who-

le ﬁxation cross process as well as the combined cross

slides are presented in ﬁg. 5.

Figure 5: Gaze points from ﬁxation crosses slide (red cros-

ses: ﬁxation crosses from slides).

The gaze points are almost identical to the ﬁxation

crosses; the paths from one cross to another are reco-

gnizable. The EOG data for the whole ﬁxation crosses

section is shown in ﬁg.6.

Figure 6: EOG data from ﬁxation crosses (left: horizontal

movements, right: vertical movements).

INTERPRETATION OF EOG DATA IN ORDER TO OBSERVE EYE MOVEMENTS

395

At 94 seconds there is a very strong amplitude

in the negative voltage area of the horizontal signal.

This is due to the crossing of the slide from the right

cross to the left cross. The values validate again that

the distance the eye has to cover is proportional to

the change of the EOG signal.The same is observable

when the subject had to look at the upper cross. Du-

ring the left-to-top-movement the signal of the verti-

cal movement changes, too. The upward eye move-

ment is clearly detectable (-300 µV). The same is ob-

served for the down movement from the top cross to

the lower cross (450 µV). Afterwards the signal con-

verts into a negative run because the eye moves from

the lower cross to the middle cross and the sequence

is over. During those movements the horizontal signal

evens out at around zero because no further left-right-

movements are made.

It was easy to detect eye movements through an

EOG system. The proband is not restricted in the usu-

al movements and a technical calibration, like it has

to be done for the commercial eye tracking system,

is not needed. The disadvantage of the EOG system,

compared to the gaze positions from the eye tracking

data, is the lack of quantiﬁcation of the voltage values

in order to receive pixel positions. An attempt of such

quantiﬁcation is mentioned in subsection 3.4.

3.4 Quantiﬁcation of EOG Data and

Eye Movements

The ﬁltered (Notch ﬁlter) EOG signal, provided by

the Neurowerk software, was analyzed to derive exact

gaze points from voltage values. The quantiﬁcation

was done for a task slide which consists of four lines

to be read (see ﬁgure 1). The signal has been ”split-

ted” into four parts to be able to measure and compare

voltage values and gaze points (see ﬁg. 7). The ampli-

tudes of the line breaks (end of one line to the begin-

ning of the next line) for vertical and horizontal mo-

vements were chosen for a sample quantiﬁcation. The

length of the signal change was measured and quan-

tiﬁed. In order to compare those values to a distance

covered on screen, the gaze points for the reading sec-

tion have been examined within EyeValuation.

During vertical movements one pixel can be re-

presented by 0.95 µV, during horizontal movements

one pixel covers about 1.76 µV. It was possible to

calculate approximated values for the vertical move-

ments. First the starting point had to be determined

from the position of last upward movement until the

peak of normalization (striving towards 0). The cal-

culated starting point is around 385 px. The starting

point according to the gaze points in EyeValuation is

375 px, so it is a pretty good result.

Figure 7: Filtered and labeled EOG data (from Neurowerk).

For the starting point of line two the distance from the

last peak up until the starting of the downward mo-

vement (maximum amplitude) is needed. The calcu-

lation of the starting point of line 2 result in a value

of around 478 px. Compared to the gaze point from

EyeValuation, which is around 480, the result is very

satisfying. The same happens with the other two li-

nes, the results are always in the proximity of the eye

tracking position.

The calculation of horizontal positions is a bit mo-

re complicated. The micro jumps caused be ﬁxations

and saccades while reading have to be included into

the calculations, therefore increasing the amount of

work. These calculations are not part of this paper.

4 CONCLUSIONS

The information gain of the EOG when compared to

proven measuring systems (eye tracking system) is

high. It is recommendable to use the EOG system in

combination with commercial eye tracking systems to

obtain veriﬁed results. EOG data is useful in the ﬁeld

of eye movement analysis. The problems described in

section 3 are the initial point for the continuation of

this experiment. The ﬁrst results were satisfying for

vertical eye movements.

The development of an eye tracking system with

an integrated EOG system to visualize and interpret

both data sets could be the next goal, offering ﬁelds

of application in health business.

REFERENCES

Duchowski, A. (2007). Eye Tracking Methodology: Theory

and Practice. Springer Verlag.

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Joos, M., Ritting, M., and Velichkovsky, B. (2003). Die

Bewegungen des menschlichen Auges: Fakten, Me-

thoden, innovative Anwendungen. In Psycholingui-

stik: Ein internationales Handbuch, pages 142–168.

De Gruyter.

Malmivuo, J. and Plonsey, R. (1995). Bioelectromagnetism:

Principles and Applications of Bioelectric and Bioma-

gnetic Fields. Oxford University Press.

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