Seminar Report 6r5552

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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Chapter 2

BIOELECTRIOTENTIALS Bioelectriotentials

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 but not the other, the concentrations of the ion that can 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 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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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. Zk is the number of moles of potassium ions transferred in the cell reaction. Department of EC

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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 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 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.

2.3 The Action Potential Department of EC

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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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• 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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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 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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Chapter 3

ELECTROOCULOGRAPHY 3.1 Electrooculography (EOG) Principle Electrooculography (EOG) is a new technology of placing electrodes on ’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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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 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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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 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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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)). Department of EC

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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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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 lenses. Bifocal glasses and hard lenses seem to cause particular problems for these systems. EOG measurements are not affected by these obstacles.

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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 moves between different environments. One such scenario which could pose problems is where the eye tracking system is attached to a .

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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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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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 quick

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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 vestibuloocular 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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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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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.

.

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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 Department of EC

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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 Department of EC

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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 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 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 5kΩ 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 nonpolarisable 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. Department of EC

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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 es freely across the electrodeelectrolyte 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 ed 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 silversilver (I) chloride electrode on the activity or effective concentration of chloride-ions:

E=E(0)- (RT/F)ln acl-…………………………….………………………… (5.4)

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).

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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 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

The components used for manufacture are non toxic, thus making it for excellent usage in medical applications.

5.4 Metal disk electrodes

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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.

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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 ed 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 filters for the elimination of low frequency noise signals and to high frequency EOG signals. Then the signals are ed to a pair of low filters which are used to eliminate the high frequency noise signals. The function of low and high filters could be performed by using a band filter with cut off frequencies fc1=0.1 Hz and fc2=40 Hz respectively.

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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 openloop 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.

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 Department of EC

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(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 ; 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. 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 and low filters, it is always advantageous to use a pair of band filters to filter the EOG signals.

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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 filtering is done at the first stage near the input terminals and the low filtering is done at the second stage.

6.1.3 DC Drift Elimination scheme

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 filter is used in the path and a subtractor. The DC drift value that is acquired at the output of the low 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. Department of EC

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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 . 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.

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Fig 6.7: Notch filter A Type II Chebyshev low 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 filter was preferred over other filters because it has linear response in its 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 Department of EC

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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.

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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 s to tell the robot where to move in gross 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 . This robotic wheelchair interacts with its , 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. Department of EC

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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 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).

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safety

Filter

VEOG and HEOG Position control

Saccadic movements detector

+

Smooth movements detector

Output control

Speed control

parameters adjustment

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. 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 , 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).

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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 keeps the gaze at the same position), a high 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 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 Department of EC

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EOG Wheel chair Visual

Eye model

FigEye 7.5: Guidance system. position

Command generator

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 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 in the system by means of a tactile screen that the has in front of him.

Fig 7.6: interface Figure 7.6 shows the interface where the commands that the can generate are: Forward, Backwards, Left, Right and Stop.

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Here the direct access guidance of a wheel chair is implemented. In direct access guidance, the can see the different guidance commands in a screen (laptop) and select them directly. In this way, when the looks at somewhere, the cursor is positioned where he is looking, then, the s 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: -wheelchair interface. The figure shows the 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 HumanComputer 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 HumanDepartment of EC

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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 twochannel 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 microprocessor through ZigBee wireless communication technology. The software on the laptop or micro-processor recognizes the direction of eye movement and voluntary eye blinking.

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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 ’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

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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 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- filter (0.01-41 Hz) is provided with two Sallen-Key filters (One second-order high- filter and one fourth-order low- 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-todigital 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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Fig. 7.10: The schematic of a single channel. (a) Power supply. (b) Preamplifiers.

(c)

Band- 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 ’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 s more nodes. Most importantly, the power consumption of ZigBee is very low. Therefore, ZigBee is perfectly suitable in 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

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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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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

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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. interface of the two applications. (a) Typewriter application test. (b) Patient assistant application test The typewriter interface is showed in Fig.7.13 a. s 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, s 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

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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

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[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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