seminar report

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.

Department of EC 1 H.K.B.K.C.E


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.


ebcid:com.britannica.oec2.identifier.IndexEntryContentIdentifier?idxStructId=65832&library=EB


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.


http://en.wikipedia.org/wiki/Absolute_temperature


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.


http://en.wikipedia.org/wiki/Absolute_temperature

http://en.wikipedia.org/wiki/Volt

http://en.wikipedia.org/wiki/Reversal_potential


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.


http://en.wikipedia.org/wiki/Cell_(biology)

http://en.wikipedia.org/wiki/Membrane_potential


• 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.



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.


http://en.wikipedia.org/wiki/Membrane_potential


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



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.



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.



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.



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)).



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).



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.



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◦.



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.


http://www.ncbi.nlm.nih.gov/books/n/neurosci/A2251/def-item/A2838/





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




http://en.wikipedia.org/wiki/Optical_resolution

http://en.wikipedia.org/wiki/Fovea

http://en.wikipedia.org/wiki/Retina


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.

















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.



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.

.



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



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.



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.



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)


http://en.wikipedia.org/wiki/Concentration

http://en.wikipedia.org/wiki/Activity_(chemistry)

http://en.wikipedia.org/wiki/Nernst_equation

http://en.wikipedia.org/wiki/Deposition_(chemistry)

http://en.wikipedia.org/wiki/Solvation

http://en.wikipedia.org/wiki/Current_(electricity)

http://en.wikipedia.org/wiki/Silver_chloride

http://en.wikipedia.org/wiki/Silver

http://en.wikipedia.org/wiki/Redox_electrode

http://en.wikipedia.org/wiki/PH_meter

http://en.wikipedia.org/wiki/Electrochemistry

http://en.wikipedia.org/wiki/Reference_electrode


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


http://en.wikipedia.org/wiki/Electrolyte

http://en.wikipedia.org/wiki/Standard_hydrogen_electrode


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.



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



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.


http://en.wikipedia.org/wiki/Electrical_network

http://en.wikipedia.org/wiki/BIBO_stability

http://en.wikipedia.org/wiki/Accuracy

http://en.wikipedia.org/wiki/Input_impedance

http://en.wikipedia.org/wiki/Common-mode_rejection_ratio

http://en.wikipedia.org/wiki/Open-loop_gain

http://en.wikipedia.org/wiki/Noise_(physics)

http://en.wikipedia.org/wiki/Drift_(telecommunication)

http://en.wikipedia.org/wiki/Direct_current

http://en.wikipedia.org/wiki/Electronic_test_equipment

http://en.wikipedia.org/wiki/Differential_amplifier


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.


http://en.wikipedia.org/wiki/Common-mode_rejection_ratio


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


http://en.wikipedia.org/wiki/Resistor


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.



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



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.



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



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.



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.


Filter User safety

Saccadic movements detector

Smooth movements detector

Position control

Speed control

Output control

Feedback parameters adjustment

+


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



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


EOG

Visual feedback

Wheel chair

Eye model

Eye position

Command generator


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.

.



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.



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.



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



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



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



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.



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



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



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



[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.


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http://www.sinauer.com/

http://en.wikipedia.org/wiki/Electrooculography