Voluntary Eye Movement Patterns while Viewing

Müller-Lyer Illusion

A New Screening Method for Scotoma Patients

Mika Haapala

, Antti Rantanen

, Aura Falck

, Anja Tuulonen

, Eero Väyrynen

, Tapio Seppänen

and Seppo J. Laukka

Learning Research Laboratory, University of Oulu, Oulu, Finland

Department of Ophthalmology, Oulu University Hospital, Oulu, Finland

Tays Eye Centre, Tampere University Hospital, Tampere, Finland

Department of Computer Science and Engineering, University of Oulu, Oulu, Finland

Keywords: Scotoma, Screening, Eye Movement Pattern, Müller-Lyer Figure.

Abstract: Diabetic maculopathy and especially advanced glaucoma are the most common eye diseases involving

scotomas, blind spots in the visual field. The risk of having scotomas increases significantly with age and

over 60 million people worldwide suffer from different forms of glaucoma of whom at least are half are not

aware of the eye disease. One of the most common scotoma diagnosis tests is perimetry, a visual field test,

which produces a complete map of the visual field, but may not be suitable for large screening of

population. We are aiming to develop a new portable screening device for cost effective screening of eye

diseases. We studied voluntary eye movement patterns evoked by Myller-Lyer illusion figure. Our study

material included six scotoma patients (two with Maculopahia Diabetica, and four with Glaucoma simplex)

and six control subjects. We recorded eye movement patterns with a portable Tobii T120 eye-tracker system

on which a Müller-Lyer figure was projected. As a result, the variation of the y-component of the eye

movement trajectory indicates that the scotoma subjects had more vertical variation in their eye movement

pattern than the control subjects (P< .01). The preliminary experiment suggests that further prospective

studies using our method of analyzing eye movement patterns is warranted with larger sampale sizes and

different types and stages of defects.

1 INTRODUCTION

Visual field defects are common consequences of

different eye diseases. There are different types of

defects, e.g. altitudinal field defects, bitemporal and

homonymous field defects as well as peripheral and

central scotomas. Earlier studies have shown that

patients with central scotoma develop different

fixation (i.e. during fixation the eye is maintained on

a single location) strategies to compensate their

impaired visual field (Sunnes et al., 2005);

(Schuchard, 2005); (Duret et al., 1999). These

fixation strategies involve the use of preferred retinal

locus (PRL). By using PRL, a patient can fixate the

seeing retina on to the object of interest so that the

scotoma is moved away from the object of interest.

We hypothesized that special eye movements related

to these strategies could be detected from the eye

movement data while viewing a visual illusion

figure.

In visual illusion the visual sensation differs

from reality (Gregory, 1998). One of the most

studied visual illusions is a century old Müller-Lyer

illusion. Even though it has a long history of study,

its mechanism is still obscure (Bodanko et al., 2009).

Müller-Lyer illusion is also known as an arrow

illusion. A line with arrow heads pointing out (i.e.

closed part of the figure) looks shorter than a line

with arrow heads pointing in (i.e. open part of the

figure). We used the so called Brentano version in

which the arrow heads are drawn on to a single line

(Fig. 1). We use this well-known illusion

innovatively to evoke voluntary eye movements

while viewing the illusion and, further, to compare

the eye movement data between people with and

without central scotoma.

267

Haapala M., Rantanen A., Falck A., Tuulonen A., Väyrynen E., Seppänen T. and J. Laukka S..

Voluntary Eye Movement Patterns while Viewing Müller-Lyer Illusion - A New Screening Method for Scotoma Patients.

DOI: 10.5220/0004188202670270

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS-2013), pages 267-270

ISBN: 978-989-8565-36-5

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

Diabetic maculopathy and especially advanced

glaucoma are typical eye diseases causing central

scotoma. The damage inflicted by glaucoma is

related to optic nerve head, nerve fiber layer and

visual field. Too high intraocular pressure destroys

retinal fiber pathways leading to defects on the

visual field (scotomas) starting typically from the

peripheral visual field. It has been estimated that

glaucoma is one of the leading causes of blindness

together with diabetic eye disease. The risk of

having glaucoma increases significantly with age.

Some 60 million people worldwide suffer from the

disease, half of them unawares. Demographical

studies and screenings of population have shown

that especially open angle glaucoma patients have

not been detected by the opportunistic case finding

and there is lack of simple appropriate screening

methods (Wong, 2004).

One of the most common glaucoma diagnostic

test is perimetry, a visual field test, which produces

map of the visual field (Humphrey´s and Octopus

static perimetries and Goldmann´s kinetic

perimetry). These methods have been approved for

clinical studies, but they may not be suitable for

large screening of the population.

Our aim was to test the hypothesis and develop a

new method of collecting and analyzing eye

movement data in order to advance the development

of a new portable screening device for patients with

central scotoma patients.

2 MATERIALS AND METHODS

2.1 Data Collection

Total of 12 voluntary subjects participated in the

experiment: six scotoma patients and six control

subjects. Two of the six scotoma patients were

diagnosed with Maculopahia Diabetica and four

patients were diagnosed with Glaucoma simplex.

The scotoma patients were recruited at routine

checkups at the University Hospital of Oulu. The

scotoma group included 4 male and 2 female

patients aged between 30 and 74 years, with the

mean 62.8 years.

The control group consisted of 3 male and 3

female students and faculty members of the

University of Oulu who were aged between 23-30

years with the mean 27.2 years.

The eye movements of the subjects were

recorded using a Tobii T120 Eye Tracker. Tobii

T120 records eye movements using an enhanced

version of the commonly used Pupil Centre Corneal

Reflection (PCCR) technique. The resolution of the

Tobii T120 tft-display, on which the Müller-Lyer

figure (M-L) was presented, was 1280x1024 pixels.

The line length of the presented M-L figure was 22.5

cm and the line thickness was 0.79 mm, see Fig. 1.

The M-L illusion figure consisted of a starting error

of 7.5 cm: the closed part of the figure was two

times longer than the open part of the figure. The

starting error was added in order to clearly show that

the line which was to be adjusted, with closed

arrowheads, was considerably longer than the line

with open arrowheads. The visual angle of the

presented M-L figure was 21.2 degrees (i.e. the

angle opening from the viewer towards the ends of

the line). The angle of the M-L figure arrow head

was 45 degrees and its length was 2cm. The sample

rate of the Tobii T120 Eye Tracker was 17 ms (60

Hz). The eye tracker was controlled by a laptop

computer. The M-L figure was presented on the eye

tracker screen by EMMI® software (LudoCraft Ltd,

Oulu, Finland).

Figure 1: Brentano version of the Müller-Lyer figure as

presented on screen. The closed side was adjusted.

The subject was sitting approximately 60 cm

from the Tobii T120 tft-monitor. A total of five right

pointing M-L figures were presented consecutively

on the screen using EMMI® software. The subject

was instructed to adjust M-L figure´s lines to be

equally long using the left and right arrow buttons of

a keyboard. After the adjustment of each M-L

figure, the subject had to press the enter-key.

2.2 Data Processing

Each Tobii T120 recording consists of a sequence of

triplets (t,x,y) following the eye movement

trajectory on the monitor, in which t is the

timestamp and x/y are the coordinates of a trajectory

point. These data points were further assigned to the

same fixation if the velocity remained below a

threshold of 30 degrees per second. In addition, the

maximum allowed time between fixations was set to

75 milliseconds and the maximum allowed angle

between fixations was set to 0.5 degrees; and the

minimum allowed fixation duration was set to 60

milliseconds.

EMMI® software recorded the participants’

BIOSIGNALS2013-InternationalConferenceonBio-inspiredSystemsandSignalProcessing

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visuo-motor activity and the magnitude of the

illusion at a sample rate of 17 ms (60 Hz). The data

from the Tobii eye tracker and EMMI® software

were integrated and synchronized with an algorithm

written in Matlab programming language.

Several indices of eye movement dynamics were

computed for both eyes while watching the M-L

figures: the illusion magnitude, the number of

fixations, the duration of fixation, the standard

deviation of the vertical direction on the display, and

the standard deviation of the horizontal direction on

the display. Differences between the groups were

analyzed statistically using the Mann-Whitney U –

test.

3 RESULTS

First we compared the magnitude of illusion, the

mean number of fixations and fixation durations

between the Scotoma Group and the Control Group.

Table 1 shows that the illusion magnitude was

higher in the Scotoma group (M=38.03%,

SD=35.30) than in the Control group (M=35.57%,

SD= 14.26) but the differences were not statistically

significant. The mean number of fixations in the

Scotoma Group was 307.83 (SD= 118.16), and in

the Control Group 221.17 (SD= 73.70). There was

no statistical difference between the groups. The

mean duration of fixations was significantly longer

in the Scotoma Group (M=774.50 ms, SD=221.03)

than in the Control Group (M=527.33 ms,

SD=138.11) with P<.01.

Table 1: Illusion magnitudes, mean number of fixations,

and mean duration of fixations (** P< .01).

Groups Magnitude %

Mean number

of fixations

Mean duration

of fixation, ms

Scotoma Group 38.03 % 307.83 774.50**

Control Group 35.57 % 221.17 527.33

Figure 2: Scatter dot chart of the eye movements (i.e. eye

positions in pixels, y- versus x-coordinates) for the

Scotoma Group (right eye on the left; left eye on the

right).

Figure 3: Scatter dot chart of the eye movements (i.e. eye

positions in pixels, y- versus x-coordinates) for the

Control Group (right eye on the left; left eye on the right).

Then we compared the standard deviations of x-

and y-coordinates separately (i.e. eye movements)

between the groups. The most significant difference

in the eye movement data was the variance in the y-

component of the eye movement trajectory over the

M-L figure. For visualization purposes scatter dot

charts of both eyes were drawn separately for both

groups (see Fig. 2 and Fig. 3).

The standard deviation of y-coordinate was

significantly greater in the Scotoma Group than in

the Control Group in both eyes (P< .01) (see Table

2).

Table 2: Standard deviations of y-coordinates of the eye

movement trajectories for the Scotoma Group and the

Control Group, subjects presented separately.

Subjects Scotoma Group Control Group

Right eye SD (pixels) SD (pixels)

1 60.2 24.22

2 73.46 21.95

3 58.6 24.5

4 69.33 17.96

5 35.63 19.03

6 69.3 18.78

Left eye SD (pixels) SD (pixels)

1 69.12 25.39

2 38.1 20.92

3 41.34 23.24

4 64.91 20.55

5 36.01 18.37

6 69.96 17.82

The standard deviations of the x-coordinates of

the right and left eye did not differ between the

groups (see Table 3).

4 CONCLUSIONS

We recorded voluntary eye movements of the

Scotoma group and the Control group, consisting of

six subjects each, while viewing the Müller-Lyer

illusion figure. We analysed the data in regards to

illusion magnitude, number and duration of fixations

and the x- and y-coordinates of the eye’s trajectories

across the screen and compared these parameters

VoluntaryEyeMovementPatternswhileViewingMüller-LyerIllusion-ANewScreeningMethodforScotomaPatients

269

between the Scotoma and Control groups.

The main novel finding was that the eye

movement dynamics differ significantly between the

Scotoma and the Control group. This observation

was confirmed with two interrelated parameters: the

vertical variation of eye trajectories over the

M-L figure (P<.01) and the fixation duration

(P<.01).

The different trajectories are an indication of

compensatory eye movements: Scotoma patients

compensate their visual field deficiencies with

abnormal eye movement trajectories. The longer

fixation duration is also an indication of

compensation for scotomas in the Scotoma Group

(Duret et al., 1999).

The presented novel method warrants further

research with larger sample size as well as different

types and stages of eye diseases.

Table 3: Standard deviations of the x-coordinates of the

eye movement trajectories for the Scotoma Group and the

Control Group, subjects presented separately.

Subjects Scotoma Group Control Group

Right eye SD (pixels) SD (pixels)

184.71 177.17

179.59 204.13

198.02 220.03

263.45 228.36

181.59 170.24

261.28 189.45

Left eye SD (pixels) SD (pixels)

186.51 172.34

165.09 201.16

183.95 223.07

284.06 235.73

179.03 166.59

246.01 186.18

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