Electrooculography

ELECTRO-OCULOGRAPHY

PRESENTED BY : TOWFEEQ UMARENROLL : 2551

CONTENTS Introduction Principle Electrooculographic Potential Arden Ratio Eye Movements Guidance of a Wheelchair Using EOG Controlling Robotic Arm Using EOG Applications Advantages Disadvantages Conclusion

INTRODUCTION

An eye controlled based on electro-oculography is designed to develop a system for assisted mobility.

Assistive robotics can improve the Quality of life for disable people. Now a days, there are many help systems to control and guide autonomous mobile robots.

The system consists of a standard electric wheel chair, an onboard computer, sensors and a graphical user interface.

DEFINITION Electrooculography (EOG) is a technique for measuring the corneo-retinal

standing potential that exists between the front and the back of the human eye. The resulting signal is called the ELECTROOCULOGRAM.

Measurement of eye movements is done by placing pairs of electrodes either above and below the eye or to the left and right of the eye.

If the eye moves from center position toward one of the two electrodes, this electrode sees the positive side of the Retina and the opposite electrode sees the negative side of the retina. potential difference occurs between the electrodes.

The recorded potential is a measure of the eye's position.

ELECTROOCULOGRAPHIC POTENTIAL

Measurement of electric potential between electrodes placed at points close

to eye used to investigate eye movements especially in psychology

research.

The EOG ranges from 0.05 to 3.5 mV in humans and is linearly

proportional to eye displacement.

The human eye is an electrical dipole with a negative pole at the Fundus

and a positive pole at the Cornea.

PRINCIPLE

The eye acts as a dipole in which the anterior pole is positive and the

posterior pole is negative.

1. Left gaze: The Cornea approaches the electrode near the outer Canthus of

the left eye, resulting in a negative-trending change in the recorded

potential difference.

2. Right gaze: the Cornea approaches the electrode near the inner Canthus of

the left eye, resulting in a positive - trending change in the recorded

potential difference.

ARDEN RATIO

The Electro-Oculogram makes an indirect measurement of the minimum

amplitude of the standing potential in the dark and then again at its peak

after the light rise. This is usually expressed ratio of "light peak to dark

trough" and is referred to as the Arden ratio.

EYE MODEL BASED IN EOG (BIDIM-EOG)

Fig 1 : Bi Dimensional Bi Polar Model (BiDiMEOG)

EYE MODEL BASED IN EOG

AIM:

To design a system capable of obtaining the gaze direction detecting the

eye movements.

For this, a model of the ocular motor system based on electrooculography

is proposed (figure 1) (Bi Dimensional dipolar model EOG, BiDiM-EOG).

This model allows us to spare saccadic and smooth eye movements and

calculate the eye position into its orbit with good accuracy.

EYE MODEL BASED IN EOG FUNCTION OF EACH BLOCK:

FILTER eliminates the effects due to other biopotentials, just as the blinks over to the EOG signal. Its a band pass filter with a very small Cut off frequency (0.05 Hz – 30Hz)

SECURITY BLOCK detects when the eyes are closed and in this case, the output is disabled.

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.

The process followed can be observed in figure 2 where the results of a process in which the user made a sequence of saccadic movements of ±10º - ±40º in horizontal.

EYE MODEL BASED IN EOG

Fig 2 : Process Results To Detect The Eye Gaze Angle

EYE MOVEMENTS SACCADES : Rapid eye movement (a jump) which is usually

conjugate and under voluntary control.

When we make a saccadic movement towards a specific object, then

the saccade can accurately land on the object or, commonly, either

overshoot or undershoot it, so giving rise to a subsequent small

corrective saccade to the object.

During a saccadic movement our vision is not completely eliminated

but it is considerably reduced (this is known as SACCADIC

SUPPRESSION).

EYE MOVEMENTS

PURSUIT EYE MOVEMENTS

These are conjugate eye movements which smoothly track slowly moving

objects in the visual field.

They typically require a moving object to elicit them and are not usually

under voluntary control.

Their purpose, partly, is to stabilize moving objects on the retina thereby

enabling us to perceive the object in detail.

EYE MOVEMENTS

VERGENCE EYE MOVEMENTS

These are movements where both eyes move in opposite horizontal

directions to permit the acquisition of a near or far object.

With an object coming towards us then our two eyes move together slightly

to maintain binocular vision of it, as the object recedes away from us then

the two eyes diverge again.

EYE MOVEMENTS

VESTIBULAR EYE MOVEMENTS:

The vestibulo-ocular reflex (VOR), is a reflex eye movement that stabilizes

images on the retina during head movement by producing an eye movement in

the direction opposite to head movement, thus preserving the image on the

center of the visual field.

The VOR has both rotational and translational aspects.

 When the head translates, for example during walking, the visual fixation

point is maintained by rotating gaze direction in the opposite direction, by an

amount that depends on distance.

GUIDANCE OF A WHEELCHAIR

Fig 3 : Wheel Chair Guidance System

GUIDANCE OF WHEEL CHAIR The aim of this control system is to guide an autonomous mobile robot

using the positioning of the eye into its orbit by means of EOG signal. Figure shows a diagram of the guidance 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.

To control the robot movements there are three methods:

a) Direct access guidance

b) Scan guidance

c) Guidance based on ocular commands.

CONTROLLING ROBOTIC ARM

Fig 4 : System Architecture And EOG Electrodes Placement

CONTROLLING ROBOTIC ARM Implemented solution consist in recording EEG and EOG

The recording block represents an interface between physical device, used for signal acquisition, and computer, used for signal processing.

For eye movement events, a real-time pattern recognition algorithm was implemented. The algorithm recognizes values for amplitude and width during a requested action.

EOG based control model assumes eye saccades for object selection and double eye blinking events for gripper closure. An eye saccade represents a potential variation of around 20μV for each degree of eye movement.

APPLICATIONS Guidance of a wheelchair

A portable wireless eye movement controlled Human Controller

Interface for the disabled

It has four parts: Surface electrodes.Two-channel amplifier.Laptop.Zigbee wireless module.

Used for people suffering from the following disorders:Amyotrophic lateral sclerosis (ALS).Brainstem stroke.Brain or spinal cord injury.

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ADVANTAGES

The EOG is easy to use.

The signals are measured with respect to the head.

Registration of eye movements with closed eyelids and during sleep are possible.

Eye movement data are analog and the sampling rate of a following analog-

digital-converter (ADC) can be chosen freely.

The EOG is the most inexpensive eye movement recording system.

DISADVANTAGES

Superposition of signals from mimic or chewing muscles (EMG)

Dependency on changes in CRP

DC-recordings always have the problem of drift superimposed on the

signal of interest

CONCLUSION Eye movements require minimal effort and allow direct selection

techniques, this increases the response time and the rate of

information flow.

There are a lot of applications can be developed using EOG because

this technique allows to users an free degree in the environment.

Many applications can be developed using EOG because this

technique provides the users with a degree of independence in the

environment.

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