Table Of ContentTHEORETICAL AND APPLIED ASPECTS
OF EYE MOVEMENT RESEARCH
SelectedEdited Proceedings of
The Second European Conference on
Eye Movements, Nottingham, England
19-23S eptember, 1983
Edited by
Alastair G. GALE
Division of Radiology
Queen’s Medical Centre
Nottingham
England
and
Frank JOHNSON
Oxford Medical Systems
Abingdon
Oxford
England
1984
NORTH-HOLLAND
AMSTERDAM . NEW YORK . OXFORD
ELSEVIER SCIENCE PUBLISHERS B.V.. 1984
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Library of Congress Cataloging in Publication Data
European Conference on me Movements (2nd : 1983 : Not-
tingham, Nottinghemshire )
Theoretical and applied aspects of eye movement
research.
(Advances in psychology ; 22)
Bibliography: p.
Includes indexes.
1. me-Movements--Congresses. I. Gale, Alastair G.
11. Johnson, Frank. 111. Title. IV. Series: Advances
in psychology (Amsterdam, Netherlands) ; 22.
QP477.5.Eg 1983 612'.846 84-10240
ISM 0-444-87551-3
PRINTED INTHE NETHERLANDS
V
PREFACE
When Galileo protested ‘but does it move’ he may well have been describing that
much smaller globe, the eye. For this motion, so necessary for our vision, is a fascination
to which the work here attests. The study of eye movements encompasses researchers
from different scientific disciplines and progresses in relatively disparate directions
ranging from fundamental research’to the problems encountered in various visual
tasks. Thus while on the one hand state of the art technology allows such niceties as
display modification during a saccade, in other situations far less complex arrange-
ments can also contribute to our understanding of visual behaviour. With such diver-
sity in mind we set out to organise a Conference which would bring together repre-
sentatives from these different areas in the belief that we could all learn something
new from one another.
In September 1983 the Second European Conference on Eye Movements was held
at the Queen’s Medical Centre, Nottingham, England. The Conference was the second
meeting of the European Group for Eye Movement Research and was the successor
to the meeting in Bern in 1981. It was run under the auspices of the Biological
Engineering Society in association with the Applied Vision Association. Originally
envisaged as a small European Conference the response was such that it rapidly grew
into an international meeting which still met our original aim of promoting the
wider exchange of information concerning eye movement research in all its diverse
fElds. Some 160 delegates attended and were presented with 66 papers over the
three day period. This volume presents the edited proceedings of that meeting and
its title reflects the wide range of topics addressed by the authors. The chapters
are gathered together in sections adhering to the conference structure. Four of these
were collections of individually submitted papers and five were organised symposia,
namely; properties of the saccadic eye movement system, medical image perception,
visual search, reading and peripheral vision and neurophysiology of eye movements.
Each section has a short introduction by the appropriate chairperson.
The organisation of the Conference and subsequent production of this volume
could not have succeeded without the dedicated help of a few individuals to whom we
would like to express our gratitude: Mrs. Elaine Wilkin and Mrs. Evelyn Pawley
have tirelessly performed the miriad tasks 90 necessary to the smooth running of an
international meeting and in the preparation of these proceedings. Mr. Keith Copeland,
MIS. Margot Gale and Mukesh Pate1 assisted greatly with the Conference arrange-
vi Preface
ments. Professor Brian Worthington allowed us to overrun his department with the
paperwork. Professor Rudolf Groner, the originator of the European Group for Eye
Movement Research, offered much helpful advice. Dr. John Findlay, Dr. Geoffrey
Underwood, Dr. Laurence Harris and Mr. Murray Siclair acted as symposia organi-
sers and Dr. Ted Megaw, Mr. Keith Copeland and Dr. Deborah Levy kindly agreed to
be chairpersons for the other sessions. Dr. Kees Michielsen of Elsevier Science Pub-
lishers B.V. (North-Holland) has been most helpful with the preparation of this work.
Finally special appreciation is accorded to Professor E. Llewellyn Thomas who was
our guest at the Conference and whose address concluded the meeting.
Alastair Gale
Frank Johnson
Nottingham
March, 1984
Theoretical and Applied Aspects of Eye Movement Research
AG. Gale and F. Johnson (Editors)
0 Elmrier Science Publishers B.V. (North-Holland), 1984 3
EYE MOVEMENT RECORDING
Frank Johnson
Oxford Medical Systems,
Abingdon, England.
This section contains contributions on three aspects of instrumentation.
Two papers (Frietman and Frecker) describe eye movement recording techniq-
ues. Two papers address the problems of analysis (Widdel and Kliegl), and
the paper by McConkie is concerned with using eye movement recording as a
basis for making stimulus manipulations.
At a conference where NAC revealed their latest "Eye Mark" system which in-
cluded 3 solid-state head-mounted cameras, it may be wondered whether
worthwhile instrumentation development is maintained by individual research
workers. These papers indicate that valuable work is being done to improve
the hardware.
The Eye-sistant described by Frietman develops the processing of the signals
from reflection to provide an accuracy of 6 minutes of arc. No drift is
reported and the bandwidth extends to 1.5 kHz.
The system is used as a comnunication aid and has easy extension to function
as a pupillometer. Head movement was coped with by measuring the position
of further light sources positioned on the frames of the device. This
could resolve head movements to 1.5 minutes of arc over 150 mm movement
range.
Frecker and colleagues describe a system which is the result of ten years
development along many avenues at the University of Toronto. The system
utilizes the first Purkinje image formed when infra-red light is reflected
from the eye. The position of the image is recorded in two dimensions by
1 inear detector arrays, processed using techniques analogous to those used
in gamma cameras. This system records eye movements with a noise level of
30 arc seconds over the dynamic range of 30 arc seconds to 36 degrees.
Applications of this device are to psychopharmacology and contingent disp-
lay control is feasible.
The volume of data generated by the detectors is recognised by the papers
by Widdel and Kliegl. Widdel first addresses the problem of definition of
a fixation. This has been rarely considered in previous work and yet is
fundamental to any work on analysis of eye movements. He recommends appro-
priate window sizes for visual search experiments.
The modular programmes of Kliegl form a valuable contribution towards a
generalised approach to eye movement analysis. Routines which many labor-
atories have written are included in a cohesive suit of programes and
Kliegl was willing to make them available to other research workers.
The use of contingent stimulus control was mentioned by Frecker and is
fully considered by McConkie and colleagues. His primary concern was in
the choice of a display which could react within a sufficiently short time.
The eymovement recording system clearly has to have a comparable real-time
response. Performance using a system with a throughput of 4 msec. was
4 F. Johnson
described.
This section on instrumentation confirms that there is more work to be done
and demonstrates the contributions being made by individuals. Despite
growing commercial interest there remains scope for development by research
workers.
Theoretical and Applied Aspects of Eye Movement Research
A.G. Gale and F. Johnson (Editors)
0 Ekvier Science Publishers B.V. (North-Holland), 1984 5
THE DETECTION OF EYEBALL MOVEMENTS WITH THE
EYE-SISTANT
Edward E.E. Frietman
Dept.of Applied Physics, Delft University of Technology, Lorentzweg 1,2628 CJ
Maarten M. Joon, Gijs K. Steenvoorden
Institute of Applied Physics, Stieltjesweg 1, 2600 AD, Delft, Holland
The EYE-SISTANT' , a portable instrument developed for the
detection of eyeball movements detects horizontal and ver-
tical eyeball movements separately on a non-contact basis.
The final result, expressed in analogue form, is propor-
tional to the amount of reflected Lnfra zed (IR) energy
coming from the iris, pupil and sclera. The IR sources and
light-sensitive elements are mounted on oculist spectacles,
making the device also suitable for persons wearing spec-
tacles. The EYE-SISTANT has already been applied in funda-
mental research and in the domain of communication aids
for the disabled.
INTRODUCTION
The EYE-SISTANT consists of two separate sensor assemblies, suitable for the
detection of horizontal and vertical eyeball movements, mounted on a pair of
spectacle frames (see Fig. 1). The electronics perform the amplification,
analysis and processing of the signals derived from the sensor assemblies
(see Fig. 2). Both eyes are simultaneously lit by the energy from two differ-
ent Infra Red Light Emit-
ting Diode (IRLED) parts.
The IR sources used are
modulated with a 5 kHz
square wave, in order to
make the detection of the
eyeball movements indepen-
dent of noise and distur-
bances from surrounding
light sources. Horizontal
eyeball movements are
sensed by one pair of sili-
con Photo Transistors
(PTR's), which measure di-
rectly the difference in
reflectivity of the iris-
to-sclera boundary, called
the limbus. Fig. la indi-
cates the detection areas
of the horizontal part.
Fig. 1. Photograph of the detection areas, as Both the PTR's and the IR
seen by the PTR's, and the sensor assemblies sources are mounted in
mounted in a Universal Measuring frame. front of the eyes so as to
6 E. E. E. Frietman et al.
minimise obstruction of the field of view, while maintaining the capability
to accurately monitor the position of the eyeball. In this way a field of
view of about 18' to the left and right is assured. Vertical eyeball move-
ments are sensed by two pairs of silicon PTR's, which measure the difference
in reflectivity of the pupil-and-iris boundaries. Fig. Ib indicates the de-
tection areas of the vertical part. In this case the field of view will be
'8 (upwards) and -10' (downwards). The reason for the asymmetry is that the
upper eyelid usually covers the upper part of the eye itself, which results
in a smaller deflection area upwards. The final result in the form of an
analogue voltage is obtained by the technique of synchronous detection.
Safety precautions are taken to prevent any part of the eyes from being dam-
aged by the IR energy. The IRLED source is adjusted in such a way that the
energy produced is less than the maximum given by the American National
Standard Institute' (ANSI 1978 and 1980). Precautions are also taken to make
the instrument fail-safe. The standards of the IEC 601/1 guarantee the
electrical safety.
EYEBALL MOVEMENT DETECTION.
Basic Principles:
In both horizontal and vertical cases the recording of eyeball movements
(Dutch patent NO.O.A.78.01616) is based on the detection of differences in
the reflection of diffused circular surface areas located on the eyes, as
can be seen in Fig. la and Ib. In the case of the detection of the horizon-
tal eyeball movement the input to the synchronous detector (SD) consists of
merely the combined result of two PTR's (see Fig-. 2). For the vertical eye-
ball movement detection, however, the
. .
SD is provided with a combination of
signals coming from four PTR's (see
Fig. 2). This means that the EYE-SIS-
TANT must consist of at least two sep-
aracely functioning analysing parts,
which will provide signals propor-
tional to the horizontal and vertical
eyeball movements. The SD contains a
suppressed carrier multiplier. The
....... output signal is then fed into a low
pass filter (LPF), whose function is
Fig. 2. Block diagram of the to remove higher-order harmonics and
EYE-SISTANT their derivitives.
Both IRLED sections, driven in a chopped mode, illuminate the eyes homoge-
neously with monochromatic light with a wavelength of 930 nm, so that no
pupil diameter variations will occur. The amount of energy produced is
7 mW/cm2. The temperature rise of the cornea due to the irradiation has been
measured by means of thermography. In an unfavourable situation it will rise
0.4' C at the most. The values lie within the physiological limits of the
normal variations in cornea temperature. A current failure part will switch
off the current through the IRLED in case of a malfunction.
Horizontal eyeball movement detection:
A shift of the eyeball in horizontal direction causes an alteration of the
reflected energy due to the change of the reflection coefficients of the
iris-to-sclera boundary (see Fig. I). This change, characterized by Sn, is
detected by the two PTR's and presents a different output signal eh.
Eye-Sistant 7
S =riAni + rsAns n=l,2
n
where r. and r represent the reflection coefficients of iris and sclera and
A
A. and theis corresponding detection areas.
eh= a1 (Sl-Sz) (2)
with a1 as a constant to match the dimensions. The indices I and 2 refer to
the circular areas 1 and 2 in Fig. la. The position of the two PTR's with
respect to the eye is such that no virtual image of the IRLED is allowed to
be detected. A most demanding condition required for a linear relation be-
tween the eyeball movements and output signal is to avoid direct radiation
from the IRLED's into the PTR's. Wrongly positioned components will cause a
severe distortion.
To decrease the influence of disturbances, such as 50 and 100 Hz from sur-
rounding light sources, the technique of synchronous detection is used to
restore information. The input of the SD consists of a Pulse-Amplitude-
Modulated (PAM) signal, carrying amplitude information proportional to the
position of the eyeball with respect to the two detection areas (see Fig.
la), together with the information containing the disturbances. The output
voltage eh of the SD is at every moment proportional to the angular roll Bh
of the eyeball. The contribution of the disturbances to the output voltage
eh is negligible. The spurious spectral components coming from the harmonics
of the chopped signal and the contribution of the disturbances are removed
by an LPF with a cutoff frequency of 1,s kHz and a slope of 48 dBJoctave.
Filtering can be done without affecting the spectrum of the original signal
because of the fact that both spectra fall in different regions (see also
the section on spectral analysis).
Vertical eyeball movement detection:
The vertical eyeball movements, on the other hand, are measured by four
PTR's by collecting the diffused IR energy from the iris-pupil border (see
also Fig. 2). The output voltage ev is the result of summing two preamplifi-
er sections, as shown in Fig. 2. A uniform expression for Sn and ev is:
Sn= rn p A np + rni A n i + rns An s n=l,2,3,4 (3)
4
where rp represents the reflection coefficient of the pupil and his cor-
responding detection area.
eV =az@,+s,) - (S~+S@)] (41
with a2 as a constant to match the dimensions. The indices 1, 2, 3 and 4 re-
fer to the circular surface areas, as can be seen in Fig. Ib. The relation
between the output voltage ey and the vertical angle displacement &, (see
Fig. 3) is linear within limits depending on pupil diameter and eyeball
position.
Spectral Analysis:
Fig. 3 is a schematic representation of the analysing involved in the pro-
cessing of the PAM signal. eh(t) and ev(t) are bandwidth-limited causal sig-
nals. There is a linear relationship between any output signal e(t), avail-
able at the output of the amplifier (see Fig. 2) and an angular displacement
B(t) of the eyeball, which can be expressed as follows:
8 E. E. E. Frietman et al.
clt)
where r represents a factor propor-
tional to a certain reflection co-
efficient. So the si-g nal e(t) comes
into existence through the multi-
Fig. 3. Schematic representation of plication of a certain eyeball po-
the analysing and processing. sition B(t) and the amount of re-
flected energy, which comes from a
pulsating IRLED {c(t) 1 and/or from surrounding intermittent disturbances
{d(t)}. The sum of the signal c(t) and d(t), with corresponding frequencies
fc and fd, multiplicated by e(t) results in a signal g(t). The equivalent
process in the frequency domain is the convolution of the spectrum E(f) with
the sum {C(f) + D(f)) which yields G(f).
F
g(t) ={e(t).{c(t) + d(t)lI-G(f) = {E(f) o{C(f) + D(f)ll (7)
W W
G(f) = ~TAEC SINC(n.').E(f2- nfc) t 2rrBE C SINC(m.$).E(f-mfd) (8)
n=-m m=-m
n#O ; If-nfc)z fe m#O ; )f-mfdIL fe
A,B and E represent respec-
tively the amplitude of a
-f pulsating signal, an inter-
mittent disturbance and a
bandwidth-limited eyeball
position signal. Multiply-
ing g(t) by a derivitive
(see Fig. 2.: "SYNC signal")
of the pulsating signal c(t)
of appropriate dimensions
leads to a signal f(t) in
which both a sample of the
original signal and the
disturbances are worked up.
Fig. 4. Graphical presentation of the proces- The convolution of the
sing of the signals in the frequency domain. spectrum G(f) and C(f) is
performed in the frequency
domain in order to yield F(f).
. F
f ( t ) =g (t ) c ( t )ctF ( f ) =G (f )* C ( f
03
F(f) = 4rr2A2E C SINC{(k-n) .$}.SINC(n.;).E(f-kfc) +
n=-m
n#O ; n#k
m m
+ 47r'ABE C C SINC(n.{).SINC(m.$).E{f-(nfc + mfd)l (9)
n=-m m=-m
n#O m#O
F(f) shows the complete complex spectrum under the following conditions
