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Electrooculography and it’s applications
Chapter 1
INTRODUCTION
Our window into the large universe has always been a fused two-piece unit
called as the eye. The eye is a complex optical system which collects light from the
surrounding environment, regulates its intensity and focuses it through an adjustable
assembly of lenses to form an image, converts this image into a set of electrical signals,
and transmits these signals to the brain.
The new advancements in the field of biomedical electronics and in the field of
electronics and communication system have changed the perception of eye from an
ordinary sense organ which enables us to see , in to, an organ which generates trigger
pulses to activate and control various electronic devices.
The new methods of efficient human machine interfaces by using the eye
movements and eye blinks are realized by using a very new bio-electric signal processing
technique called as Electrooculography (EOG). Electrooculography is a technique for
measuring the resting and action potential of the retina. The resulting signal is called the
electrooculogram. Usually, pairs of electrodes are placed either above and below the eye
or to the left and right of the eye. If the eye is moved from the center position towards one
electrode, this electrode "sees" the positive side of the retina and the opposite electrode
"sees" the negative side of the retina. Consequently, a potential difference occurs between
the electrodes. Assuming that the resting potential is constant, the recorded potential is a
measure for the eye position.
The hardware components generally required to detect the EOG signals are four to
five electrodes, and the amplifiers and filters are required for amplification and filtering
processes respectively. The signals are processed using controllers or dsp processors
depending up on the complexity of the application.
Some of the important applications of EOG are in electrooculographic guidance of
a wheel chair, retina controlled mouse, eye controlled switching on and off of electronic
and electric devices, interactive gaming systems etc. The use of EOG for guiding of
missiles in the battle field is a new project under research by the defense systems.
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Electrooculography and it’s applications
Chapter 2
BIOELECTRICPOTENTIALS
Bioelectricpotentials refers to the electrical, magnetic or electromagnetic
fields produced by living cells, tissues or organisms. Bioelectric potentials are generated
by a variety of biological processes and generally range in strength from one to a few
hundred millivolts. Biological cells use bioelectricity to store metabolic energy, to do
work or trigger internal changes and to signal one another. Bioelectricity is the electric
current produced by action potentials along with the magnetic fields they generate
through the phenomenon of electromagnetism.
Bioelectric potentials are identical with the potentials produced by devices such as
batteries or generators. In nearly all cases, however, a bioelectric current consists of a
flow of ions (i.e., electrically charged atoms or molecules), whereas the electric current
used for lighting, communication, or power is a movement of electrons.
If two solutions with different concentrations of an ion are separated by a
membrane that blocks the flow of the ions between them, the concentration imbalance
gives rise to an electric-potential difference between the solutions. In most solutions, ions
of a given electric charge are accompanied by ions of opposite charge, so that the solution
itself has no net charge. If two solutions of different concentrations are separated by a
membrane that allows one kind of ion to pass but not the other, the concentrations of the
ion that can pass will tend to equalize by diffusion, producing equal and opposite net
charges in the two solutions.
In living cells the two solutions are those found inside and outside the cell. The
cell membrane separating inside from outside is semi permeable, allowing certain ions to
pass through while blocking others. In particular, nerve- and muscle-cell membranes are
slightly permeable to positive potassium ions, which diffuse outward, leaving a net
negative charge in the cell.
The bioelectric potential across a cell membrane is typically about 50 mill volts;
this potential is known as the resting potential. All cells use their bioelectric potentials to
assist or control metabolic processes, but some cells make specialized use of bioelectric
potentials and currents for distinctive physiological functions, such as the nerve cell.
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Electrooculography and it’s applications
2.1 Mechanism behind Bio Potentials
Concentration of potassium (K+) ions is 30-50 times higher inside as compared to
outside and Sodium ion (Na+) concentration is 10 times higher outside the membrane than
inside. In resting state the member is permeable only for potassium ions, thus resulting in
the Potassium ion flowing outwards leaving an equal number of negative ions inside, but
the Electrostatic attraction pulls potassium and chloride ions close to the membrane and
an inward directed electric field is formed.
The below figure shows the ion transfer into the cell and out of the cell
Fig 2.1: Inter cellular ion movement
The bioelectric potentials of a cell is given by mainly two equations and they are as
follows
2.1.1 Nernst Equation
Vk=(-RT/Zkf)ln(Ci,k/Co,k)………………………………………………………...…....(2.1)
Where ,
Vk is the potential of the potassium ion .
R is the universal gas constant .R=8.3144621(75) JK-1mol-1
T is the absolute temperature.
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Zk is the number of moles of potassium ions transferred in the cell reaction.
Ci,k is the potassium ion concentratrion inside the cell.
Co,k is the potassium ion concentration outside the cell.
2.1.2Goldman-Hodgkin-Katz equation:
This equation is used to determine the equilibrium potential across a cell's
membrane, taking into account all of the ions that are permeant through that membrane.
Vm =-(RT/ZkF)ln((PkCo,k+PnaCo,na+PclCi,cl)/(PkCi,k+PnaCi,na+PclCo,cl))........................(2.2)
Where,
Vm= the membrane potential in volts.
R is the universal gas constant .R=8.3144621(75) JK-1mol-1
T is the absolute temperature.
Zk is the number of moles of potassium ions transferred in the cell reaction.
Ci,ion is the ion concentratrion inside the cell.
Co,kion is the ion concentration outside the cell.
The different types of potentials generated are
2.2 The Membrane Potential
A potential difference usually exists between the inside and outside of any cell
membrane, including the neuron. The membrane potential of a cell usually refers to the
potential of the inside of the cell relative to the outside of the cell i.e. the extracellular
fluid surrounding the cell is taken to be at zero potential. When no external triggers are
acting on a cell, the cell is described as being in its resting state. A human nerve or
skeletal muscle cell has a resting potential of between -55mV and -100mV. This potential
difference arises from a difference in concentration of the ions K+ and Na+ inside and
outside the cell. The selectively permeable cell membrane allows K+ ions to pass through
but blocks Na+ ions. A mechanism known as the ATPase pump pumps only two K+ ions
into the cell for every three Na+ cells pumped out of the cell resulting in the outside of the
cell being more positive than the inside.
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Electrooculography and it’s applications
2.3 The Action Potential
Action potential is a short-lasting event in which the electrical membrane potential
of a cell rapidly rises and falls. Action potential generally occurs in neuron cells and
muscle cells in human beings and animals. As mentioned already, the function of the
nerve cell is to transmit information throughout the body. A neuron is an excitable cell
which may be activated by a stimulus. The neuron’s dendrites are its stimulus receptors.
If the stimulus is sufficient to cause the cell membrane to be depolarized beyond the gate
threshold potential, then an electrical discharge of the cell will be triggered. This
produces an electrical pulse called the action potential or nerve impulse. The action
potential is a sequence of depolarization and depolarization of the cell membrane
generated by a Na+ current into the cell followed by a K+ current out of the cell. The
stages of an action potential are shown in Figure
Figure 2.2: An Action Potential.
The above graph shows the change in membrane potentials as a function of time when an
action potential is elicited by a stimulus.
•Stage 1 – Activation: When the dendrites receive an “activation stimulus” the Na+
channels begin to open and the Na+ concentration inside the cell increases, making the
inside of the cell more positive. Once the membrane potential is raised past a threshold
(typically around -50mV), an action potential occurs.
• Stage 2 – Depolarization: As more Na+ channels open, more Na+ ions enter the cell and
the inside of the cell membrane rapidly loses its negative charge. This stage is also known
as the rising phase of the action potential. It typically lasts 0.2 - 0.5ms.
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Electrooculography and it’s applications
• Stage 3 – Overshoot: The inside of the cell eventually becomes positve relative to the
outside of the cell. The positive portion of the action potential is known as the overshoot.
• Stage 4 – Repoarization: The Na+ channels close and the K+ channels open. The cell
membrane begins to repolarise towards the resting potential.
• Stage 5 – Hyperpolarisation: The membrane potential may temporarily become even
more negative than the resting potential. This is to prevent the neuron from responding to
another stimulus during this time, or at least to raise the threshold for any new stimulus.
• Stage 6: The membrane returns to its resting potential.
2.3.1Propagation of the Action Potential
An action potential in a cell membrane is triggered by an initial stimulus to the
neuron. That action potential provides the stimulus for a neighboring segment of cell
membrane and so on until the neuron’s axon is reached. The action potential then
propagates down the axon, or nerve fibre, by successive stimulation of sections of the
axon membrane. Because an action potential is an all-or-nothing reaction, once the gate
threshold is reached, the amplitude of the action potential will be constant along the path
of propagation. The speed, or conduction velocity, at which the action potential travels
down the nerve fibre, depends on a number of factors, including the initial resting
potential of the cell, the nerve fibre diameter and also whether or not the nerve fibre is
myelinated. Myelinated nerve fibres have a faster conduction velocity as the action
potential jumps between the nodes of Ranvier.
2.3.2 Synaptic Transmission
The action potential propagates along the axon until it reaches the axonal ending.
From there, the action potential is transmitted to another cell, which may be another nerve
cell, a glandular cell or a muscle cell. The junction of the axonal ending with another cell
is called a synapse. The action potential is usually transmitted to the next cell through a
chemical process at the synapse.
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Electrooculography and it’s applications
2.4 Resting Potential
The relatively static membrane potential of non active or stationary cells is called
the resting membrane potential In resting potential state the member is permeable only
for potassium ions. The resting potential is generated as follows,
Potassium flows outwards leaving an equal number of negative ions inside. Thus
electrostatic attraction pulls potassium and chloride ions close to the membrane and forms
an inward directed electric field. This electric field gives rise to the resting potential
Nerve and muscle cells are encased in a semi-permeable membrane that permits
selected substances to pass through while others are kept out. Body fluids surrounding
cells are conductive solutions containing charged atoms known as ions. In their resting
state, membranes of excitable cells readily permit the entry of K+ and Cl- ions, but
effectively block the entry of Na+ ions (the permeability for K+ is 50-100 times that for
Na+). Various ions seek to establish a balance between the inside and the outside of a cell
according to charge and concentration. The inability of Na+ to penetrate a cell membrane
results in the polarization that is called as Resting Potential.
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Electrooculography and it’s applications
Chapter 3
ELECTROOCULOGRAPHY
3.1 Electrooculography (EOG) Principle
Electrooculography (EOG) is a new technology of placing electrodes on user’s
forehead around the eyes to record eye movements. This technology is based on the principle of
recording the polarization potential or corneal-retinal potential (CRP), which is the resting potential
between the cornea and the retina. This potential is commonly known as electrooculogram. (EOG)
is a very small electrical potential that can be detected using electrodes. The EOG ranges
from 0.05 to 3.5 mV in humans and is linearly proportional to eye displacement.
Compared with the electroencelography (EEG), EOG signals have the
characteristics as follows: the amplitude is relatively the same (15-200uV), the
relationship between EOG and eye movements is linear, and the waveform is easy to
detect. Considering the characteristics of EOG mentioned above, EOG based HCI is
becoming the hotspot of bio-based HCI research in recent years.
Basically EOG is a bio-electrical skin potential measured around the eyes but first we have to understand eye itself:
3.2 Anatomy of the Eye
Fig 3.1: Anatomy of the eye
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Electrooculography and it’s applications
The main features visible at the front of the eye are shown in Fig 3.1 .The lens,
directly behind the pupil, focuses light coming in through the opening in the centre of the
eye, the pupil, onto the light sensitive tissue at the back of the eye, the retina. The iris is
the coloured part of the eye and it controls the amount of light that can enter the eye by
changing the size of the pupil, contracting the pupil in bright light and expanding the
pupil in darker conditions. The pupil has very different reflectance properties than the
surrounding iris and usually appears black in normal lighting conditions. Light rays
entering through the pupil first pass through the cornea, the clear tissue covering the front
of the eye. The cornea and vitreous fluid in the eye bend and refract this light. The
conjuctiva is a membrane that lines the eyelids and covers the sclera, the white part of the
eye. The boundary between the iris and the sclera is known as the limbus, and is often
used in eye tracking.
The light rays falling on the retina cause chemical changes in the photosensitive
cells of the retina. These cells convert the light rays to electrical impulses which are
transmitted to the brain via the optic nerve. There are two types of photosensitive cells in
the retina, cones and rods. The rods are extremely sensitive to light allowing the eye to
respond to light in dimly lit environments. They do not distinguish between colours,
however, and have low visual acuity, or attention to detail. The cones are much less
responsive to light but have a much higher visual acuity. Different cones respond to
different wavelengths of light, enabling colour vision. The fovea is an area of the retina of
particular importance. It is a dip in the retina directly opposite the lens and is densely
packed with cone cells, allowing humans to see fine detail, such as small print. The
human eye is capable of moving in a number of different manners to observe, read or
examine the world in front of them.
3.3 The Electrooculogram
The electrooculogram (EOG) is the electrical signal produced by the potential
difference between the retina and the cornea of the eye. This difference is due to the large
presence of electrically active nerves in the retina compared to the front of the eye. Many
experiments show that the corneal part is a positive pole and the retina part is a negative
pole in the eyeball. Eye movement will respectively generates voltage up to 16uV and
14uV per 1° in horizontal and vertical way. The typical EOG waveforms generated by
eye movements are shown in Fig 3.2.
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Electrooculography and it’s applications
In Fig 3.2 the diagram top figure shows the three types of eye movements and the
bottom figure shows the original EOG waveform.
Positive or negative pulses will be generated when the eyes rolling upward or
downward. The amplitude of pulse will be increased with the increment of rolling angle,
and the width of the positive (negative) pulse is proportional to the duration of the eyeball
rolling process.
When the eyes are stationary or when the eyes are looking straight ahead, there is
no considerable change in potential and the amplitude of signal obtained is approximately
zero.
Fig 3.2 EOG generation using the eye movements and EOG waveform
When the eyes are made to move upwards, then there results an action potential, which
when measured will give a value of -0.06v to +0.06v. Similarly a downward movement of
the eyes will give a similar voltage with opposite polarities to that obtained due to the left
movement.
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3.4 EOG Spectrum and Amplitude
Fig 3.3: Spectrum of various biomedical signals
The above fig 3.3 shows the spectrum of EOG signal along with other biomedical
signals. As it can be seen from the figure, EOG signals have an amplitude range from
10µvolts to approximately 1 millivolts. The frequency ranges from 0.1 Hz to 10 Hz, thus
the bandwidth is only 9.9 Hz.
The important factor regarding the EOG signal is that it does not fall in the
amplitude or frequency range of the EMG signal ,thus during the process of measurement
of the EOG signals ,the head or other parts of the body can be moved ,as these muscular
activities will not interfere with the EOG signals and can be filtered easily.
The ECG signal can be easily filtered out from the EOG signals by using a low
pass filter, as the ECG signals have a higher bandwidth. One more interesting factor
regarding the ECG signals are that, it does not interfere with the EOG signals, because
when EOG is measured using precision electrodes, and as ECG is generated by the heart
it does not get detected by the electrodes placed near the eye.
The EEG signal shown in the above fig is obtained by placing many electrodes on
the head region, but in the EOG measurements the electrodes are placed only near the eye
region and thus there is no interference from EEG signals also.
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Electrooculography and it’s applications
3.5 EOG Signals
The below figure shows the two types of EOG signals which are detected using
electrodes:
Fig. 3.4: EOG signals during eye movement and blinking. (a) HEOG signals. (b) VEOG
signals.
The EOG signals detected by using the electrodes are two types depending on the eye
movements, they are:
Horizontal electrooculogram signals (HEOG)
Vertical electrooculogram signals (VEOG)
3.5.1 HEOG signals
This type of signals is obtained for the horizontal eye movements. With reference
to the figure 3.4(a) shows the HEOG signals .When the eye is motion less the detected
voltage is constant, but when the eyes moves from center to left direction a small positive
spike of voltage is detected and this amplitude remains constant ,for a time duration as
long as the eyes are to the left(indicated by 1in fig 3.4(a)).The voltage comes to a stable
value when the eyes come back to the center from the left(indicated by 2 in fig 3.4(a)).
When the eyes move from the center to the right a negative spike of voltage is
detected and this amplitude remains constant, for time duration as long as the eyes are to
the right (indicated by 3in fig 3.4(a)). The voltage comes to a stable value when the eyes
come back to the center from the right (indicated by 4 in fig 3.4(a)).
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Electrooculography and it’s applications
3.5.2 VEOG signals
This type of signals is obtained for the vertical eye movements. With reference to
the figure 3.4(b) shows the VEOG signals .When the eye is motion less the detected
voltage is constant, but when the eyes moves from center to top direction a small positive
spike of voltage is detected and this amplitude remains constant ,for a time duration as
long as the eyes are pointed to the top (indicated by 6 in fig 3.4(b)).The voltage comes to
a stable value when the eyes come back to the center from the top (indicated by 7 in fig
3.4(a)).
When the eyes move from the center to the bottom a negative spike of voltage is
detected and this amplitude remains constant ,for a time duration as long as the eyes are
pointed downwards(indicated by 8 in fig 3.4(a)). The voltage comes to a stable value
when the eyes come back from down to center position (indicated by 9 in fig 3.4(a)).
The VEOG signals have a slightly lesser amplitude, when compared with the
HEOG signals .This makes it easy to detect and differentiate these two signals easily.
3.5.3 Blink signals
Blink signals are the Eog signals which are a result of blinking of the eyes. There
are two types of blink signals they are:
Voluntary blink signals
Involuntary blink signals
3.5.3.1: Voluntary blink signals
These are the EOG signals which are detected by the voluntary eye blinks. When
the eyes are blinked voluntarily a large positive spike of voltage can be detected, this
detected spike is very instantaneous and remains only for a time period of 1 millisecond.
This voltage is shown in 11 of fig 3.4(b).
3.5.3.1: Involuntary blink signals
These are the EOG signals which are detected by the involuntary eye blinks. This
detected voltage spike is very small compared to voluntary blink, and thus can be filtered
out .this spike is indicated in 10 of fig 3.4(b).
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Electrooculography and it’s applications
3.6 Advantages of the EOG over Other Methods
The principle advantages of EOG over other bioelectric signals are as follows
3.6.1 Range
The EOG typically has a larger range than visual methods which are constrained
for large vertical rotations where the cornea and iris tend to disappear behind the eyelid.
Angular deviations of up to 80 can be recorded along both the horizontal and vertical
planes of rotation using electrooculography.
3.6.2 Linearity
The reflective properties of ocular structures used to calculate eye position in
visual methods are linear only for a restricted range, compared to the EOG where the
voltage difference is essentially linearly related to the angle of gaze for ±30◦ and to the
sine of the angle for ±30◦ to ±60◦
3.6.3 Head Movements are Permissible
The EOG has the advantage that the signal recorded is the actual eyeball position
with respect to the head. Thus for systems designed to measure relative eyeball position
to control switches (e.g. looking up, down, left and right could translate to four separate
switch presses) head movements will not hinder accurate recording.
3.6.4 Non-invasive
Unlike techniques such as the magnetic search coil technique, EOG recordings do
not require anything to be fixed to the eye which might cause discomfort or interfere with
normal vision. EOG recording only requires three electrodes (for one channel recording),
or five electrodes (for two channel recording), which are affixed externally to the skin.
3.6.5 Obstacles in Front of the Eye
In visual methods, measurements may be interfered with by scratches on the
cornea or by contact lenses. Bifocal glasses and hard contact lenses seem to cause
particular problems for these systems. EOG measurements are not affected by these
obstacles.
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Electrooculography and it’s applications
3.6.6 Cost
EOG based recordings are typically cheaper than visual methods, as they can be
made with some relatively inexpensive electrodes, some form of data acquisition card and
appropriate software,
3.6.7 Lighting Conditions
Variable lighting conditions may make some of the visual systems unsuitable or at
least require re-calibration when the user moves between different environments. One
such scenario which could pose problems is where the eye tracking system is attached to
a user.
3.6.8 Eye Closure is Permissible
The EOG is commonly used to record eye movement patterns when the eye is
closed, for example during sleep. Visual methods require the eye to remain open to know
where the eye is positioned relative to the head, whereas an attenuated version of the
EOG signal is still present when the eye is closed.
3.6.9 Real-Time
The EOG can be used in real-time as the EOG signal responds instantaneously to
a change in eye position and the eye position can be quickly inferred from the change.
The EOG is linear up to 30◦.
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Electrooculography and it’s applications
Chapter 4
EYE MOVEMENTS
A basic knowledge of different types of eye movements and its applications are
very necessary for the detection of the EOG signals. The amplitude and duration of the
EOG signals obtained will depend up on the different types of eye movements.
Mainly there are four types of eye movements and they are:
Saccades
Smooth pursuit movements
Vergence movements
Vestibulo-ocular movements
4.1 Saccades
Saccades are rapid, ballistic movements of the eyes that abruptly change the point
of fixation. They range in amplitude from the small movements made while reading, for
example, to the much larger movements made while gazing around a room. Saccades can
be elicited voluntarily, but occur reflexively whenever the eyes are open, even when
fixated on a target. The rapid eye movements that occur during an important phase of
sleep are also saccades. The time course of a saccadic eye movement is shown in fig 4.1
Fig 4.1: saccadic eye movement delay
The metrics of a saccadic eye movement: The red line indicates the position of a fixation
target and the blue line the position of the fovea. When the target moves suddenly to the
right, there is a delay of about 200ms before the eye begins to move.
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Electrooculography and it’s applications
One reason for the saccadic movement of the human eye is that the central part of
the retina—known as the fovea—plays a critical role in resolving objects. By moving the
eye so that small parts of a scene can be sensed with greater resolution, body resources
can be used more efficiently.
Saccades are the fastest movements produced by the human body. Saccades to an
unexpected stimulus normally take about 200 milliseconds (ms) to initiate, and then last
from about 20–200 ms, depending on their amplitude (20–30 ms is typical in language
reading).
The electrooculography technique can be used to record the saccadic movements.
The saccadic movements are fast and generate typical EOG signals, because of its fast
nature it requires precision electrodes to measure the EOG signals produced due to
saccadic movements. The EOG signals of saccade are very useful for sleep studies.
4.2 Smooth pursuit movements
Smooth pursuit movements are much slower tracking movements of the eyes
designed to keep a moving stimulus on the fovea. Such movements are under voluntary
control in the sense that the observer can choose whether or not to track a moving
stimulus fig 3.2 .Surprisingly, however, only highly trained observers can make a smooth
pursuit movement in the absence of a moving target. Most people who try to move their
eyes in a smooth fashion without a moving target simply make a saccade.
Fig 4.2: smooth pursuit eye movements
The metrics of smooth pursuit eye movements. These traces show eye movements
(blue lines) tracking a stimulus moving at three different velocities (red lines). After a
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Electrooculography and it’s applications
quick saccade to capture the target, the eye movement attains a velocity that matches the
velocity of the target
The electroocullography technique can be used to record the smooth pursuit eye
movements also. The smooth pursuit eye movements are slower compared to the
saccades, but the amplitude of EOG signals generated for both the movements are
almost the same.
4.3 Vergence movements
Vergence movements align the fovea of each eye with targets located at different
distances from the observer. Unlike other types of eye movements in which the two eyes
move in the same direction (conjugate eye movements), vergence movements are
disconjugate (or disjunctive); they involve either a convergence or divergence of the lines
of sight of each eye to see an object that is nearer or farther away. Convergence is one of
the three reflexive visual responses elicited by interest in a near object. The other
components of the so-called near reflex triad are accommodation of the lens, which brings
the object into focus, and pupillary constriction, which increases the depth of field and
sharpens the image on the retina.
The electrooculography technique can be used to record the vergence movements
also.
4.4 Vestibulo-ocular movements
Vestibulo-ocular movements stabilize the eyes relative to the external world, thus
compensating for head movements. These reflex responses prevent visual images from
“slipping” on the surface of the retina as head position varies. The action of vestibulo-
ocular movements can be appreciated by fixating an object and moving the head from
side to side; the eyes automatically compensate for the head movement by moving the
same distance but in the opposite direction, thus keeping the image of the object at more
or less the same place on the retina. The vestibular system detects brief, transient changes
in head position and produces rapid corrective eye movements.
The EOG technique can be used to measure Vestibulo-ocular movements also.
Although this type of eye movements is a result of the head movements, these head
movements also do not cause a disturbance in measurement of EOG signals.
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Electrooculography and it’s applications
4.5 Eye blinks
Blinking of eyes automatically supplies two forms of moisture to our eyes, to keep
them from drying out, and to keep foreign matter from entering and irritating our eyes.
Blinking also protects the eye from dryness by irrigating the eyelid, through suction,
automatically draws the fluid we cry with from the well we refer to as the tear duct over
the eyeball, to irrigate, and to moisturize the eye. The process is similar to the manner in
which the farmer uses water to irrigate his crops during a dry spell.
There are three types of eye blinks and they are as follows,
4.5.1 Voluntary blink
The opening and closing of the eyes voluntarily is called as voluntary blinking of
the eyes. During the process of voluntary blinking the detected EOG signal has higher
amplitude of the order of millivolt range.
4.5.2 Involuntary blink
Involuntary blinks occur 15 to 20 times per minute .this type of blinking occur to
keep the eyes healthy by keeping the cornea moist. The EOG signal detected due to
involuntary blinks have very small amplitude of the range of microvolts, thus the
voluntary and involuntary EOG signals can be easily separated.
4.5.3 Blink Reflex
Blink reflex is the fast closing of the eyes when the eyes blink to act as a defense
mechanism in response to a potentially harmful stimulus. Generally EOG signals are not
measured for this type of blinks, as they occurrence of such blinks are very rare.
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Electrooculography and it’s applications
Chapter 5
METHODOLOGY OF EOG DETECTION
The electrooculogram signals can be utilized only if it is correctly detected and
processed. The eog signals are to be detected by using electrodes, which are of non
polarisable type. Once the EOG signals are detected, only then it can be processed
(amplification and filtering) so that it can be used for some application.
In this chapter we will mainly concentrate on the above said detection process.
5.1 EOG detection
The primary function in EOG signal estimation and processing is the detection of
the EOG signals. The detection takes place as shown below.
The below figure shows the method of detection of EOG signals using electrodes
Fig 5.1: Electrode placements for EOG detection
As it can be seen from the above figure, four to five electrodes are required for the
detection of the EOG signals. In the process of detection, the electrodes act as a
transducer converting the ion current obtained at the skin to electron current. The
derivation of the EOG is achieved placing two electrodes on the outer side of the eyes to
detect horizontal movement and another pair above and below the eye to detect vertical
movement. A reference electrode is placed on the forehead as shown in the fig 5.1.
.
Department of EC 20 H.K.B.K.C.E
Electrooculography and it’s applications
5.1.1 Placement of electrodes
5.1.1.1 Horizontal electrode placement
Fig 5.2: HEOG electrode placement
As it can be seen from the fig 5.2 the horizontal electrooculogram signals (HEOG)
are best detected by placing the electrodes on the left and right external canthi (the bone
on the side of the eye).Whenever the eyes move from center to left or from center to right
horizontal EOG signals are produced, these signals are very small and have to be
amplified. The electrodes are placed exactly at the canthi because of the availability of
higher amplitude EOG signals at this region when compared to other regions surrounding
the eyes.
5.1.1.2 Vertical electrode placement
Fig 5.3: VEOG electrode placement
As it can be seen from the fig 5.3 the vertical electrooculogram signals (VEOG)
are best detected by placing the electrodes approximately one centimeter vertically above
and below the eye . Whenever the eyes move from center to top or from center to down
Department of EC 21 H.K.B.K.C.E
Electrooculography and it’s applications
vertical EOG signals are produced, these signals are very small and have to be amplified.
The electrodes have to be placed within one cm above or below, if the electrode
separation increases between top and bottom of the eye the amplitude of detected EOG
signals will decrease.
5.1.1.3 Reference electrode placement
Fig 5.4: reference electrode placement
The fig 5.4 shows the reference electrode placement. The reference electrode is
placed to act as a ground with respect to vertical and horizontal electrodes. The reference
electrode can be placed at the forehead or at the neck.
5.1.2 Precautionary measures during placement of electrodes
Place electrodes as close as possible to the eye without causing discomfort.
1. Clean the skin on the cheek near the eyes. The skin should be cleansed of
oils with alcohol or a commercial skin-preparing material
2. Attach Large Adhesive Tape (Micropore) to the electrodes.
3. Apply Electrolyte Gel through the electrode opening.
4. Place the electrodes.
5. Press the electrodes onto skin.
6. Check the impedances. Impedance of the applied electrode should measure
<10 k Ohms over a frequency range that includes 30 to 200 Hz.
7. Secure with tape.
Department of EC 22 H.K.B.K.C.E
Electrooculography and it’s applications
8. If non-disposable electrodes are used, they should be suitably cleaned after
each use to prevent transmission of infectious agents.
5.2 EOG ELECTRODES
Because of the very low amplitude of the EOG, the electrodes represent the
weakest link in the entire recording system. The following properties are desirable in an
EOG electrode:
(a) Stable electrode potential: Spontaneous fluctuations of only 2 or 3mV in the
potential difference between an electrode and the surrounding electrolyte will produce
artifacts very much larger than the EOG.
(b) Equal electrode potentials: A small standing potential difference between a pair of
electrodes will not present major difficulties, apart from producing a temporary deflection
of the trace and possibly blocking of the amplifiers when the electrodes are first
connected to the recorder. However, if the current flow between the electrode varies
owing to changing contact resistances, artifact may result, As it is in practice never
possible to ensure that conventional electrodes are of equal potential, it follows that a
third desirable characteristic is constant electrode contact resistances
(c) Equal electrode resistances: EOG recording is bedeviled by electrical interference -
particularly from ac mains; there are generally unwanted changes in potential difference
between the subject and the ECG machine that are seen as common mode signals and can
he rejected by the use of differential amplifiers. Unequal electrode resistances, however,
unbalance the system and produce an out-of-phase component that will appear in the
tracing.
(d) Low electrode resistance: With modern amplifier design, it is now easy to ensure
that the electrode resistances are very much less than the input impedance so that as much
as possible of the ECG signal is applied at the input of the amplifier. The effects of
unequal electrode resistances are less marked when the actual values are low. In general
when the other criteria above are satisfied, the electrode resistance is to be less than 5k
and measurement of resistance provides a good check on the quality of electrode
preparation and application.
Department of EC 23 H.K.B.K.C.E
Electrooculography and it’s applications
The desirable characteristics above can generally be satisfied by the use of non-
polarisable electrodes, so far as identical physical and chemical structure, securely
attached to skin that has first been cleaned and abraded to remove the outer layer which is
of high resistance.
By taking in to consideration the desirable factors of EOG electrodes ,the most
suitable electrode used for EOG measurement is the Ag-AgCl electrode. The reason is
because it is a type of electrodes in which current passes freely across the electrode-
electrolyte interface, requiring no energy to make the transition. These electrodes see no
over potentials. These electrodes are perfect for recordings and measurements.
5.3 Ag-AgCl electrode
A silver chloride electrode is a type of reference electrode, commonly used in
electrochemical measurements. For example, it is usually the internal reference electrode
in pH meters.
The electrode functions as a redox electrode and the reaction is between
the silver metal (Ag) and its salt — silver chloride (AgCl, also called silver (I) chloride).
The corresponding equations can be presented as follows:
Ag+ + e- Ag(s)……………..……………………….…………………………..(5.1)
Agcl(s) Ag+ + Cl-………………………………...…………………………...(5.2)
or an overall reaction can be written:
Agcl(s)+e- Ag(s) + Cl-…………………………….…………………………….(5.3)
This reaction is characterized by fast electrode kinetics, meaning that a
sufficiently high current can be passed through the electrode with the 100% efficiency of
the redox reaction (dissolution of the metal or cathodic deposition of the silver-ions). The
reaction has been proved to obey these equations in solutions with pHs of between 0 and
13.5.
The Nernst equation below shows the dependence of the potential of the silver-
silver (I) chloride electrode on the activity or effective concentration of chloride-ions:
E=E(0)- (RT/F)ln acl-…………………………….………………………… (5.4)
Department of EC 24 H.K.B.K.C.E
Electrooculography and it’s applications
The standard electrode potential E0 against standard hydrogen electrode (SHE) is
0.230V ± 10mV. The potential is however very sensitive to traces of bromide ions which
make it more negative. (The more exact standard potential given by an IUPAC review
paper is 0.22249 V, with a standard deviation of 0.13 mV at 25 °C).
Fig 5.5: a silver chloride shown in cross section
Commercial reference electrodes consist of a plastic tube electrode body. The
electrode is a silver wire that is coated with a thin layer of silver chloride, either
physically by dipping the wire in molten silver chloride, or chemically by electroplating
the wire in concentrated hydrochloric acid.
A porous plug on one end allows contact between the field environment with the
silver chloride electrolyte. An insulated lead wire connects the silver rod with measuring
instruments.
The electrode has many features making is suitable for use in the field:
Simple construction
As mentioned above the construction of an Ag AgCl electrode is simple and requires
very less no of component.
Inexpensive to manufacture
The manufacturing process is inexpensive, as most of the components are easily
available at the market.
Stable potential
The potential generated by the electrode is stable for a variety of temperature
ranges.
Non toxic components
Department of EC 25 H.K.B.K.C.E
Electrooculography and it’s applications
The components used for manufacture are non toxic, thus making it for excellent
usage in medical applications.
5.4 Metal disk electrodes
Fig 5.6: metal disk electrodes for EOG measurement
Metal disk and Cup electrodes are generally made of high purity tin, silver, gold
or even surgical steel, or some combination of these (i.e. gold plated silver or silver
chloride). They usually have a diameter that is within 4-10 mm as smaller than 4mm, or
larger than 10mm,. The application site near the eye region is determined and prepared by
sterilizing with alcohol, using an abrasive to remove dead skin. Once the electrode is
secure, the cup is filled with a conductive gel which aids conductivity. These electrodes
can also be placed on other parts of the body to monitor skin potentials and filter these
out, increasing the reliability of the readings.
Fig 5.7: an Ag-AgCl disk electrode
The Ag-AgCl disk electrode as shown in the fig 5.7 are used for the EOG signal
detections. These electrodes are attached to the skin by using adhesive tapes, these
electrodes detect the EOG signals with almost 99% accuracy.
Department of EC 26 H.K.B.K.C.E
Electrooculography and it’s applications
Chapter 6
EOG SIGNAL FILTERING AND ACQUISITION SYSTEM
The EOG signal detected by using the electrodes are very weak as result of the
occurrence of DC drifts and numerous artifacts along with power-line interference, thus it
has made the EOG signal quite unattractive for biomedical applications. Therefore it
becomes necessary to eliminate these DC drifts and other artifacts in order to maintain
signal linearity.
Thus it is necessary to have an EOG signal acquisition system that counters all the
above mentioned problems making it suitable for both theoretical analysis as well as
industrial applications.
6.1 EOG biopotential amplifier
Fig 6.1: Block diagram of first stage of EOG biopotential amplifier
As shown in the fig 6.1 the vertical and horizontal EOG signals detected by the
electrodes are passed to the first stage of the amplifier consisting of instrumentation
Department of EC 27 H.K.B.K.C.E
Electrooculography and it’s applications
amplifier for primary amplification of the EOG signals. The amplified EOG signals are
given to a pair of High pass filters for the elimination of low frequency noise signals and
to pass high frequency EOG signals. Then the signals are passed to a pair of low pass
filters which are used to eliminate the high frequency noise signals. The function of low
pass and high pass filters could be performed by using a band pass filter with cut off
frequencies fc1=0.1 Hz and fc2=40 Hz respectively.
6.1.1 Instrumentation amplifier
The primary amplifier required for the amplification of the EOG signals are the
instrumentation amplifiers. The first stage of any EOG biopotential amplifier is the
Instrumentation amplifier which provides the initial amplification while reducing the
effect of signals such as power-line interference and skin muscle artifacts owing to its
high Common Mode Rejection Ratio (CMRR). Two instrumentation amplifiers are
employed for this purpose, one for each of the two channels.
An instrumentation amplifier is a type of differential amplifier that has been
outfitted with input buffers, which eliminate the need for input impedance matching and
thus make the amplifier particularly suitable for use in measurement and test equipment.
Additional characteristics include very low DC offset, low drift, low noise, very high
open-loop gain, very high common-mode rejection ratio, and very high input impedances.
Instrumentation amplifiers are used where great accuracy and stability of the circuit both
short- and long-term are required.
Although the instrumentation amplifier is usually shown schematically identical to
a standard op-amp, the electronic instrumentation amp is almost always internally
composed of 3 op-amps. These are arranged so that there is one op-amp to buffer each
input (+, −), and one to produce the desired output with adequate impedance matching.
Department of EC 28 H.K.B.K.C.E
Electrooculography and it’s applications
Fig 6.2: op-amp based instrumentation amplifier
The most commonly used instrumentation amplifier circuit is shown in the fig 6.2. The
gain of the circuit is
(Vout/V2-V1)=(1+(2R1/Rgain)(R3/R2)……………………………………....(6.1)
The rightmost amplifier, along with the resistors labeled R2 and R3 is just the standard
differential amplifier circuit, with gain = R3/R2 and differential input resistance = 2·R2.
The two amplifiers on the left are the buffers. With Rgain removed (open circuited), they
are simple unity gain buffers; the circuit will work in that state, with gain simply equal to
R3/R2 and high input impedance because of the buffers. The buffer gain could be
increased by putting resistors between the buffer inverting inputs and ground to shunt
away some of the negative feedback; however, the single resistor Rgain between the two
inverting inputs is a much more elegant method: it increases the differential-mode gain of
the buffer pair while leaving the common-mode gain equal to 1
. This increases the common-mode rejection ratio (CMRR) of the circuit and also
enables the buffers to handle much larger common-mode signals without clipping than
would be the case if they were separate and had the same gain. Another benefit of the
method is that it boosts the gain using a single resistor rather than a pair, thus avoiding a
resistor-matching problem (although the two R1’s need to be matched), and very
conveniently allowing the gain of the circuit to be changed by changing the value of a
single resistor. A set of switch-selectable resistors or even a potentiometer can be used for
Rgain, providing easy changes to the gain of the circuit, without the complexity of having
to switch matched pairs of resistors.
Department of EC 29 H.K.B.K.C.E
Electrooculography and it’s applications
The ideal common-mode gain of an instrumentation amplifier is zero. In the
circuit shown, common-mode gain is caused by mismatches in the values of the equally-
numbered resistors and by the mis-match in common mode gains of the two input op-
amps.
The instrumentation amplifiers for bio medical recordings such as EOG are
readily available in the form of integrated circuits(IC’s). Generally IC AD 640 is used for
the bio medical signal amplifications of ECG,EOG etc.
6.1.2 Filtering of EOG
Filters are to be made present either before or after the amplification process .The
EOG signals are filtered out to remove the unwanted noise components .Instead of using
individual high pass and low pass filters, it is always advantageous to use a pair of band
pass filters to filter the EOG signals.
Fig 6.4: pre- filtering and post filtering of EOG signals
. T1 in the above figure indicates Small inductors or ferrite beads in the lead wires
which block the high frequency electromagnetic interferences. The RF filtering is done by
using small capacitors such as C1.The high pass filtering is done at the first stage near the
input terminals and the low pass filtering is done at the second stage.
6.1.3 DC Drift Elimination scheme
Department of EC 30 H.K.B.K.C.E
Electrooculography and it’s applications
Fig 6.5: Dc drift eliminating scheme
The block diagram of the DC drift elimination scheme used in the biopotential
amplifier design is shown in fig. 6.5 and is used to eliminate the DC drifts completely
instead of suppressing them as in the conventional design. A second order low pass filter
is used in the feedback path and a subtractor. The DC drift value that is acquired at the
output of the low pass filter is continuously given as input to the subtractor stage without
much delay and is subtracted from the original signal, thus providing an effective solution
to eliminate the DC drifts from the EOG signal.
The drift elimination scheme described above removes the DC component of the
EOG signal also. Therefore, even if the eye-balls are held at a particular position for some
duration continuously, the signal output would not remain constant. Though this loss of
the DC portion of the EOG signal may not hamper the working of many systems that
employ EOG signal processing, it may be a potential source of error in systems that are
eye-ball position dependent. This error can be corrected by using a set of ‘N’ D-latches to
obtain the ‘N’ level quantized digital equivalent of the DC offset value. A/D and D/A
converters are used before and after the set of D-latches. The digital drift value is updated
using a push button that is manually controlled by the user. The modified DC drift
elimination scheme is shown in fig.6. 6.
Department of EC 31 H.K.B.K.C.E
Electrooculography and it’s applications
Fig 6.6: The block diagram of the revised DC drift elimination scheme that preserves DC
content of the EOG Signal.
6.1.4 Power-line Interference Elimination Scheme
The filter that is used to eliminate the 50 Hz power-line interference must possess
linear response in the frequency range of the EOG signal and a small transition BW.
The fig 6.7 shows the notch filters, these filters are used to remove the power line
interference ,these interfaces if not removed will Overlaps with the measurement
bandwidth and distort the measurement result and have an effect on the recorded EOG
signal. R4 makes a provision for the notch tuning. The RC filter combination of R1C1,
R2C2 and R3C3 acts as notch filters.
Fig 6.7: Notch filter
A Type II Chebyshev low pass filter that is constructed using a switched capacitor
Filter can be used for the purpose of a notch filter. This is chosen because of the
requirements of the system which demands linearity in the frequency range of the EOG
signal, a very narrow transition band and maximum possible attenuation in the stop band,
achieving all of which would be difficult with just discrete components. The Type II
Chebyshev low pass filter was preferred over other filters because it has linear response in
its pass band, has equiripple behavior in its stop band and the filter requires the least
possible order for the same transition bandwidth when compared with other IIR filters of
the same specifications.
6.2 EOG signal acquisition system
Department of EC 32 H.K.B.K.C.E
Electrooculography and it’s applications
Fig 6.8: EOG signal acquisition system
This system is found to acquire the EOG signal efficiently, while completely
eliminating the DC drifts and interferences. The loss of the DC component of the EOG
signal that occurs in the drift elimination stage has been completely avoided by adding
appropriate A/D and D/A converters and latches. The response of the overall EOG signal
acquisition system is found to be remarkably linear and the overall system is much
cheaper than existing bioamplifiers for the same purpose.
The EOG acquisition system can be used in applications in medical
instrumentation such as reliable hospital alarm systems. The significant feature of this
system is its versatility, for it can used to work on biomedical applications of EOG signal
processing as well as aid in theoretical analysis experiments.
This significant circuit is found to be ideal for both theoretical analysis of the
EOG signal as well as for practical signal processing applications based on EOG.
With this chapter we finish the basic concepts of detection and acquisition of the
electrooculogram signals .Once these signals are acquired correctly they can be used for a
variety of purposes.
Department of EC 33 H.K.B.K.C.E
Electrooculography and it’s applications
Chapter 7
APPLICATIONS OF ELECTROOCULLOGRAPHY
In the previous chapters the detection, amplification, filtering, dc drifting etc of
the EOG signals were explained in detail. The previous chapter dealt with the acquisition
of the EOG signals. But only acquiring the signal is of no use .This acquired signal can be
effectively utilized for a variety of applications, which will be dealt in this chapter.
In this chapter we will concentrate on two important applications of the EOG
signals and they are:
Electrooculographic guidance of a wheelchair using eye movements.
A portable wireless eye movement-controlled Human-Computer Interface for the
Disabled.
7.1 Electrooculographic Guidance of a Wheelchair using Eye
Movements.
Here we discuss about a robotic wheelchair system based on Electrooculography.
This system allows the users to tell the robot where to move in gross terms and will then
carry out that navigational task using common sensical constraints, such as avoiding
collision. This wheelchair system is a general purpose navigational assistant in
Department of EC 34 H.K.B.K.C.E
Electrooculography and it’s applications
environments with accessible features such as ramps and doorways of sufficient width to
allow a wheelchair to pass. This robotic wheelchair interacts with its user, making the
robotic system semiautonomous.
To realize this wheel chair it is necessary to detect the EOG signals as a result of
eye movement and the eye gaze, using the EOG detections an eye model based on
electrooculography is created. Using the eye model a guidance system for the wheel chair
is created.
7.1.1 EOG acquisition
The discrete electrooculographic control system (DECS) is based in recording the
polarization potential or corneal-retinal potential (CRP). This potential is
electrooculogram. The EOG ranges from 0.05 to 3.5mV in humans and is linearly
proportional to eye displacement.
This system may be used for increasing communication and/or control. The
analog signal from the oculographic measurements has been turned into signal suitable
for control purposes. The derivation of the EOG is achieved placing two electrodes on the
outerside of the eyes to detect horizontal movement and another pair above and below the
eye to detect vertical movement. A reference electrode is placed on the forehead or on the
neck. Figure 7.1 shows the electrode placement.
Fig 7.1: Electrodes placement.
The EOG signal changes approximately 20 microvolts for each degree of eye
movement. In this system, the signals are sampled 10 times per second. The record of
EOG signal has several problems. Firstly, this signal seldom is deterministic, even for
same person in different experiments .The EOG signal is a result of a number of factors,
including eyeball rotation and movement, eyelid movement, different sources of artifact
such as EEG, electrodes placement, head movements, influence of the luminance, etc.
Department of EC 35 H.K.B.K.C.E
Electrooculography and it’s applications
For this reasons, it is necessary to eliminate the shifting resting potential (mean
value) because this value changes. To avoid this problem is necessary to have an ac
differential amplifier where a high pass filter with cutoff at 0.05 Hz and relatively long
time constant is used. The amplifiers used have programmable gain ranging from 500,
1000, 2000 and 5000.
7.1.2 Eye model based in EOG
Once the EOG signals are acquired, a system capable of obtaining the gaze
direction detecting the eye movements is designed. For this, a model of the ocular motor
system based on electrooculography is required as shown in the fig 7.2 (Bidimensional
dipolar model of EOG).
VEOG and HEOG
Fig 7.2: Bidimensional dipolar model of EOG.
This model allows separating saccadic and smooth eye movements and calculating
the eye position into its orbit with good accuracy (less than 2 º). The filter eliminates the
effects due to other biopotentials such as EEG, just as the blinks over to the EOG signal.
Department of EC 36 H.K.B.K.C.E
Filter User safety
Saccadic movements detector
Smooth movements detector
Position control
Speed control
Output control
Feedback parameters adjustment
+
Electrooculography and it’s applications
The security block detects when the eyes are closed and in this case, the ouput is disabled.
After that, the EOG signal is clasified into saccadic or smooth eye movements by means
of two detectors. If a saccadic movement is detected, a position control is used, whereas if
a smooth movement is detected, a speed control is used to calculate the eye position. The
final position (angle) is calculated as the sum of the saccadic and smooth movements.
Besides, the model has to adapt itself to the possible variations of acquisition conditions
(electrodes placement, electrode-skin contact, etc). To do this, the model parameters are
adjusted in accordance with the angle detected.
A person, in a voluntary way, only can make saccadic movements unless he tries
to follow an object in movement. Therefore, to control some interface it is convenient to
focus the study in the detection of saccadic movements (rapid movements).
This process can be done processing the derivate of the EOG signal. To avoid
problems with the variability of the signal (the isoelectric line varies with time, even
though the user keeps the gaze at the same position), a high pass filter with a very small
Cutoff frequency (0.05 Hz) is used. The process followed can be observed in fig 7.3
where the results of a process in which the user made a sequence of saccadic movements
of ±10º.±40º in horizontal derivation are shown. It is possible to see that the derivate of
the electrooculographic signal allows us to determinate when a sudden movement is made
in the eye gaze. This variation can be easily translated to angles (figure7. 3 d).
Fig 7.3: Eog signals
Department of EC 37 H.K.B.K.C.E
Electrooculography and it’s applications
Fig 7.4 EOG controlled wheelchair
The above figure shows the EOG controlled wheelchair.
7.1.3 Wheel chair guidance system
Fig 7.5: Guidance system.
Figure 7.5 shows a diagram of the control system. The EOG signal is recorded
using Ag-AgCl electrodes and this data, by means of an acquisition system are sent to a
Department of EC 38 H.K.B.K.C.E
EOG
Visual feedback
Wheel chair
Eye model
Eye position
Command generator
Electrooculography and it’s applications
PC, in which they are processed to calculate the eye gaze direction. Then, in accordance
with the guidance control strategy, the control commands of the wheelchair are sent. The
commands sent to the wheelchair are the separate linear speed for each wheel. It is
possible to see that there exists a visual feedback in the system by means of a tactile
screen that the user has in front of him.
Fig 7.6: User interface
Figure 7.6 shows the user interface where the commands that the user can
generate are: Forward, Backwards, Left, Right and Stop.
Here the direct access guidance of a wheel chair is implemented. In direct access
guidance, the user can see the different guidance commands in a screen (laptop) and
select them directly. In this way, when the user looks at somewhere, the cursor is
positioned where he is looking, then, the users can select the action to control the
wheelchair movements. The actions are validated by time, this is, when a command is
selected, it is necessary to stay looking at it for a period of time to validate the action. In
“scan” guidance, it is necessary to do an eye movement (a “tick”) to select among the
different commands presented in the screen. The actions are validated by time, this is,
when a command is selected, if other “tick” is not generated during a time interval, the
command is validated and the guidance action is executed.
Whenever a particular option is triggered in the screen, electronic relays
connected to motors convert this action to a rotation of the wheels of the wheel chair. In
this manner a person can move the wheel chair in a certain direction by moving the cursor
in the screen through his eyes.
.
Department of EC 39 H.K.B.K.C.E
Electrooculography and it’s applications
Fig 7.7: User-wheelchair interface.
The figure shows the user interface with the EOG controlled wheel chair. This is a
system that can be used as a means of control allowing the handicapped, especially those
with only eye-motor coordination, to live more independent lives. Eye movements require
minimum effort and allow direct selection techniques, and this increase the response time
and the rate of information flow.
.
7.2 A Portable Wireless Eye Movement-Controlled Human-Computer Interface for the Disabled
Human-Computer Interface (HCI) has become an important area of research and
development for the disabled. A portable wireless eye movement-controlled Human-
Computer Interface which can be used for the disabled who have motor paralysis and who
cannot speak in multiple applications (such as communication aid and smart home
applications) is described here.
This Interface consists of four major parts: (1) surface electrodes, (2) a two-
channel amplifier, (3) a laptop (or a micro-processor), and (4) a ZigBee wireless module.
Persons with severe diseases, such as amyotrophic lateral sclerosis (ALS),
brainstem stroke, brain or spinal cord injury, cerebral palsy, muscular dystrophies,
multiple sclerosis, etc., have difficulty conveying their intentions and communicating
with other people in daily life. With the development of Human-Computer Interface
(HCI), methods have been developed to help these people for communication.
Department of EC 40 H.K.B.K.C.E
Electrooculography and it’s applications
The disabled with severe paralysis and patients who need intensive care may not
be able to speak, and the eye muscles are the only muscles they can control. For these
people, HCI methods based on eye movement or blinking can be selected.
In the present system, a novel portable wireless eye movement-controlled HCI for
the disabled is described. This interface is a real-time communication control system
based on EOG signals. In this system a mathematical morphology method is used to
preprocess original EOG signals, also a wireless module based on the ZigBee protocol is
used to increase the scope of applications (communication aid, smart home applications,
etc.) of this system.
7.2.1 System Overview
The system used here has four major parts: (1) five surface electrodes, (2) a two-
channel amplifier, (3) a laptop (or a micro-processor), and (4) a ZigBee wireless module.
Fig. 7.8 is the schematic diagram of this system and the whole system adopts the
star topology, horizontal and vertical EOG signals are measured by five surface
electrodes placed around eyes. After a two-channel amplifier, the EOG signals are
sampled at the rate of 250 Hz and then sent to a coordinator node which is connected
with a laptop or a micro-processor through ZigBee wireless communication technology.
The software on the laptop or micro-processor recognizes the direction of eye movement
and voluntary eye blinking. Programs (typewriter, patient assistant software, etc.) in
Laptop or remote devices (TV, lamps, telephone, etc.) can be controlled by the
recognized results.
Department of EC 41 H.K.B.K.C.E
Electrooculography and it’s applications
Fig. 7.8: Overview of the EOG-based wireless Human-Computer Interface.
7.2.2 Electrodes and the Principle
The cornea of the eye is electrically positive relative to the retina of the eye and
the potential is slowly varying when eyes move. The standing potential can be measured
by electrodes placed around the eyes. The EOG value varies from 0.05-3.5 mV with a
frequency range of about 0-100 Hz. In this system, there are five electrodes in all which
are classified as horizontal, vertical and reference (ground) electrodes. As showed in Fig.
7.8, the vertical electrodes are placed about 1.0 cm above the right eyebrow and 2.0 cm
below the lower lid of the right eye, the horizontal electrodes are placed 2.0 cm lateral to
the each side of outer canthi. And the last electrode is placed on user’s forehead to serve
as a ground.
If the eyes move left, horizontal EOG (HEOG) signal which is the difference
between signals collected by electrode HEOL and HEOR acquires a positive voltage
value. If the eyes turn right, HEOG signal changes into a negative voltage value.
Identically, if the eyes move from the central position towards upside, vertical EOG
(VEOG) signal which is the difference between signal collected by electrode VEOU and
VEOL acquires a positive voltage value. If the eyes move downside, VEOG signal
changes into a negative voltage value. An eye blinking can be described by EOG signals
as a peak in VEOG but a flat in HEOG. We can distinguish the voluntary and involuntary
blinking by the value and duration of the peak mentioned above. Fig.7.9 shows EOG
Department of EC 42 H.K.B.K.C.E
Electrooculography and it’s applications
signals (after the amplifier) during eye movement and blinking (voluntary and
involuntary).
Fig 7.9: EOG signals during eye movement and blanking. (a) HEOG signals.(b) VEOG signals
7.2.3. Amplifier
The horizontal and vertical eye movement signals captured by the electrodes were
then transmitted to a two-channel amplifier which consists of (1) preamplifiers, (2) band-
pass filters, (3) shift circuits, (4) right-leg driven circuits and (5) power supply. The
schematic of a single channel is shown in Fig. 7.10. The preamplifier is a micro-power
instrumentation amplifier (INA126, Texas Instruments Inc., Dallas, TX, USA) for
accurate and low noise differential signal acquisition. The gain of the preamplifier is set
to be 21 with a single external resistor. The band-pass filter (0.01-41 Hz) is provided with
two Sallen-Key filters (One second-order high-pass filter and one fourth-order low-pass
filters). The following circuits are secondary amplifier with variable gain and shift circuit
to transform the signal level into the range of 0 V to 3 V for adapting the following
analog-to-digital converter (ADC). Right-leg driven circuit connected with the reference
electrode is used to reduce the common-mode components in the signal. Power for the
board is supplied by one common 6V battery, which is then transformed into ± 3.3 V with
AMS1117 and MAX828 respectively
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Electrooculography and it’s applications
Fig. 7.10: The schematic of a single channel. (a) Power supply. (b) Preamplifiers.
(c) Band-pass filters. (d) Shift circuits. (e) Right-leg driven circuits
7.2.4. Wireless module
The Wireless module takes responsibility for transmitting two-channel EOG
signals from one node attached to the user’s body to the coordinator node connected with
the laptop. Meanwhile, the coordinator can send messages to other remote controllers
(TV, lamp, telephone, etc). The ZigBee wireless communication technology, which is
proved to be reliable, low-power and cost-efficient, is used in this system. Compared with
the popular Bluetooth and Wi-Fi technologies, ZigBee has a wider range of
communication and supports more nodes. Most importantly, the power consumption of
ZigBee is very low. Therefore, ZigBee is perfectly suitable in terms of data rate for the
wireless transmission of physiological vital signs or even continuous monitoring. The
module is established using CC2430 (Texas Instruments Inc., Dallas, TX, USA), which is
a true System-on-Chip solution specifically tailored for IEEE 802.15.4 and ZigBee
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Electrooculography and it’s applications
applications. The CC2430 combines RF transceiver with an industry-standard enhanced
8051 MCU, 32/64/128 KB flash memory, 8 KB RAM and many other powerful features.
At the transmission node, analog EOG signals from amplifiers are sampled at the rate of
250Hz and transmitted. At the reception node, EOG signals are transported to laptop with
RS232-USB interface for signal processing. In the prototype software, the protocol is
based on a ZigBee stack called MSSTATE_LRWPAN which implements a ZigBee
subset wireless stack. The program in CC2430 is based on this protocol completely.
7.2.5 EOG Signal processing
Fig 7.11: The flowchart of EOG signal processing.
Fig. 7.11 shows the flowchart of EOG signal processing. The method is based on
the mathematical morphology (MM), differential and integral algorithms to recognize the
direction of eye movement and voluntary blinking. VEOG signals are used to detect
up/down movement and voluntary eye blinking, while HEOG signals are used to detect
left/right movement.
MM Algorithm: The method of MM is widely used in ECG signal processing and other
fields. It provides a good way to remove drift and magnify feature of the signal. The
result of VEOG signals after MM filter is shown in Fig. 7.12b.
Differential Algorithm: The VEOG signals, after MM filter, are feed into the differential
module implemented by (7.1). The result of this step is shown in Fig. 7.12 c
9 9
Y(n)=∑(x(n)+i+10)-∑(x(n)+i)………………………………………………………....(7.1)
i-0 i-9
Where x(n) is the VEOG signals after MM filter, and y(n) is the result after the
differential module.
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Electrooculography and it’s applications
Integral Algorithm: The difference of original VEOG signals (delay 2N points, N is the
length of the structuring element in MM algorithm) and signals after MM filter can be
used for eye blinking recognition. Because the peak value of voluntary blinking is much
larger than involuntary blinking, we can distinguish those two kinds of blinking by the
integral module using (7.2) and the threshold.
19
Y(n)=∑(x(n)+i)/20……………………………………………………………………..(7.2)
i-0
The result of this step is shown in Fig7.12 d .Where x(n) is the difference of original
VEOG signals (delay 2N points) and signals after MM filter, y(n) is the result after the
integral module.
Decision Module: In Fig.7.11, S1, S2 and S3 are the results by the methods
mentioned above. Threshold1 is the voluntary eye blinking threshold, Threshold2 is the
involuntary eye blinking threshold and they are also used as thresholds for up and down
movements, Threshold3 is the movements (left/right) threshold. We can distinguish eye
blinking (voluntary and involuntary) and eight-direction movement through these
thresholds.
Fig7.12 Example of VEOG signal processing. (a) Original VEOG signals. (b)Signals after
MM filter. (c) The result after the differential module. (d) The result of integral algorithm
7.2.7 Application Software Test
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Electrooculography and it’s applications
Two application programs to provide interface to the system are the typewriter and the
patient assistant software. As shown in figure below.
Fig.7.13. User interface of the two applications. (a) Typewriter application test. (b)
Patient assistant application test
The typewriter user interface is showed in Fig.7.13 a. Users make the cursor move
up, down, left and right to select a letter in the table. The letters selected are showed
above the table. The patient assistant software is showed in Fig.7.13 b. In this application,
users move the cursor by eight-directional eye movements, and the size of icon selected is
enhanced. At the same time, the LED which indicates the direction of eye movement is
lighted by the controlling of the remote ZigBee module.
Chapter 8
CONCLUSION
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Electrooculography and it’s applications
The advancements in the field of medical electronics and in the field of electronics
and communication have presented the world with a new technology of
Electrooculography. This technique has resulted in rapid advancements in the design of
human computer interfaces for severely paralyzed patients, with the aid of this technology
many disabled patients who are unable to speak or move their limbs can access many
electronic devices such as fan, light etc only through the movement of their eyes.
There is no doubt that this technology will still expand and will have many other
applications, let us hope for a future where all devices are eye controlled.
BIBLIOGRAPHY
Department of EC 48 H.K.B.K.C.E
Electrooculography and it’s applications
[1] http://en.wikipedia.org/wiki/Electrooculography.
[2] Augustine GJ, Fitzpatrick D, et al., editors.,Neuroscience.,Purves D, Sunderland (MA): Sinauer Associates; 2nd edition , 2001.(types of eye movements).
[3] Brittanica encyclopedia.
[4]. Shubhodeep Roy Choudhury, S.Venkataramanan, Harshal B. Nemade and J.SSahambi ,Design and Development of a Novel EOG Biopotential Amplifier, Department of Electronics and Communication Engineering, Indian Institute of Technology(IIT), Guwahati, INDIA
[5]. Rafael Barea, Luciano Boquete, Manuel Mazo, Elena López and L.M. Bergasa, Electrooculographic guidance of a wheelchair using eye movements codification, Electronics Department. University of Alcala Campus Universitario s/n. 28871Alcalá de Henares. Madrid. Spain
[6]. Xiaoxiang Zheng, Xin Li, Jun Liu, Weidong Chen, and Yaoyao Hao , A portable wireless eye movement-controlled Human-Computer Interface for the Disabled, IEEE, 2009.
Department of EC 49 H.K.B.K.C.E