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A simple eye movement monitoring system for clinical electroencephalography
182
TECHNICAL
Electroencephalography and Clinical Neurophysiology. 1974, 37:182 184 X, Elsevier Scientific Publishing Company, A m s t e r d a m - Printed in The Netherlands
CONTRIBUTION
A SIMPLE EYE M O V E M E N T M O N I T O R I N G SYSTEM FOR CLINICAL ELECTROENCEPHALOGRAPHY A. L. WINTFR
Burden Neuroloqical Institute, Bristol (Great Britain) (Accepted for publication: January 30, 1974)
Separation of the electrical activity produced by eye movement (Harlan et al. 1958) from similar activity originating from other physiological sources, particularly cerebral ones, continues to present a problem to electrophysiologists. Techniques that have been designed to solve this problem usually involve the spatial separation of the electrical fields of the different sources (Muras and Binnie 1970). An alternative approach is by use of mechanical or other methods which are not dependent upon the electrical field of the eye to detect its movement and display it simultaneously on the EEG record. The channels displaying E E G activity believed to be contaminated with eye movement artefact can then be compared directly with the eye monitor trace. This approach has been explored; Lacey (1963) used a ceramic strain gauge method to detect eye movements during sleep; Sullivan and Weltman (1963) used an electrical impedance method, and photographic techniques have been tried by other workers. These techniques, however, have not gained widespread popularity, probably because they have not satisfied two
Fig. 1. Sensor eye piece and pressure transducer module. (The complete monitoring system as shown is now available from Specialised Laboratory Equipment, Croydon, Surrey, England.)
basic requirements for routine work simplicity of use and the minimum, if any, additional computation of data. The system to be described utilizes the fact that the eyeball is not a perfect sphere. The bulge of the cornea can be easily detected, even through the closed lid, by placing a finger lightly upon it and in this way slight movements of the eyeball eyelid relationship can be recognized. This procedure in which the finger behaves as a pressure transducer is commonly used by L E G technicians to detect eye movement. A pressure sensor replaces the finger in the procedure described and becomes the input of the monitoring system, which is sensitive to very slight changes of pressure, thus detecting very small changes in the eyeball eyelid relationship. The sensor eye piece is housed in a spectacle frame as shown in Fig. 1, providing facilities for monitoring movement of either eye. It consists of a rigid plastic tube approximately 2 cm in diameter, closed at one end by a replaceable thin rubber diaphragm. The other end is connected by means of a flexible plastic tube to the pressure transducer module. Pressure changes at the diaphragm are transmitted by the column of air in the plastic tube to a differential capacitative pressure transducer in the module and cause amplitude modulation of a radio frequency signal. The time constant of the system is not critical when it is used for monitoring eye movements in the presence of LEG activity~ as evidence of no eye movement is the most significant information usually required in those circumstances. Its time constant is dependent upon the care given to avoiding air leaks at the sensor diaphragm and connecting tubes and can be expected to be about 30 sec, without special care being taken. However, in practice this value is reduced considerably by the additional time constant employed at the LEG machine of 1 sec or less, in order to display rapid eye movements of different sizes clearly about a steady baseline. The eye piece can be positioned over the eyeball by sliding it laterally in the frame and then adjusting for depth until it is lightly touching the closed eyelid. This adjustment is not critical and can often be left to the patient to make, for m a x i m u m comfort. The output may be connected to one channel of an L E G machine, the sensitivity of which can be adjusted empirically to display clearly the particular kind of eye m o v e m e n t s to be monitored. Very small movements can
183
EYE MOVEMENT MONITOR FOR CLINICAL EEG
L
Fs -
.
L
L...
l,,,..
L
l
1
l
,9",
Fp2
Fpl" Fpl Fpl - F7 F7 - T3 T3 - Ts
MONITOR I
EOG
,.
I
I "~01
' '
'Cz
C, Fig. 2. This figure shows seven channels of EEG, a recording from tile mechanical eye movement monitor and an electrooculogram (EOG) from electrodes placed above and below the left eye. Points O~, 02 and 03 show eye movements which are clearly shown on both the EOG and mechanical eye monitor channels. Points C 1, C 2 and C3 show cerebral activity from Fp2 and Fpl which is shown to contaminate the EOG, thus making it an unreliable method for distinguishing between cerebral and ocular electrical activity, whilst the mechanical monitor at these times indicates the correct source of the activity.
be recorded but at such a sensitivity gross movements would overload the amplifiers. For most EEG purposes clear indication of movements of 1'~ or more is adequate. A simple procedure for adjustment is to place marks, conveniently on the ceiling above the EEG couch, spaced a measured distance apart to subtend angles of 1~ at the eye, in both the vertical and horizontal planes. The subject is then asked to look at the appropriate marks in turn and the sensitivity of the EEG recorder adjusted until the movement is just visible on the record. This level of sensitivity has been found to be suitable for detecting the type of eye movements that present problems in clinical EEG recording. Fig. 2 demonstrates the limitations of the electro-oculogram for separating electrical activity due to eye movement from cerebral electrical activity and the advantage of the described system over it. The monitor has been in routine clinical EEG use in this department for about 2 years and has become an essential part of the departmental ancillary equipment. SUMMARY A simple eye movement monitoring system for use in clinical EEG is described~ which does not involve recording
of the corneo-retinal potential difference. The system, based upon the mechanical detection of movement by use of a pressure transducer, is shown to be sensitive to the smallest movements of the eye or lid, which cause artefact in EEG recording. The simplicity of both the system and its application makes it a reliable one for use by EEG technicians in clinical EEG. RESUME DISPOSITIF SIMPLE DE CONTR~ILE DES MOUVEMENTS O C U L A I R E S EN EEG C L I N I Q U E On d~crit un dispositif simple, permettant le contr61e des mouvements oculaires en EEG clinique. Ce syst6me n'est pas fond6 sur l'enregistrement des diff6rences de potentiels corneo-r6tiniennes, mais sur une d6tection m6canique des mouvements '5, l'aide d'un transducteur de pression. I1 est sensible au moindre mouvement de Foeil ou de la paupi6re, causes d'artefacts en enregistrement EEG. La simplicit6 du dispositif et de ses applications le rendent ais6ment utilisable par les techniciens EEG.
184 The pressure transducer module was designed originally for monitoring respiration by Dr. L. Kellenyi of the University of Pecs, Hungary, whilst on a British Council Fellowship to this Institute. REFERENCES
HARLAN, W, L., WHITE, P. T. and BICKFORD,R. G. Electrical activity produced by eye flutter simulating frontal elec-
A.L. WINTER troencephalographic rhythms. Electroenceph. olin. Ncurophysiol., 1958, 10: 164-169. LACEY,J. 1. A simplified method of detecting eye movements during dreaming. P©'chosom. Med., 1963, 25: 78. MURAS, J, S. and BINNIE, C. D. The recognition of frontal slow activity in the presence of eye movements. Proc. electrophysiol. Technol. Ass., 1970, 17: 131. SULHVAN, G. H. and WELTMAN, G. Impedance oculograph--a new technique. J. appl. PhysioL. 1963, 18: 215.
TECHNICAL
Electroencephalography and Clinical Neurophysiology. 1974, 37:182 184 X, Elsevier Scientific Publishing Company, A m s t e r d a m - Printed in The Netherlands
CONTRIBUTION
A SIMPLE EYE M O V E M E N T M O N I T O R I N G SYSTEM FOR CLINICAL ELECTROENCEPHALOGRAPHY A. L. WINTFR
Burden Neuroloqical Institute, Bristol (Great Britain) (Accepted for publication: January 30, 1974)
Separation of the electrical activity produced by eye movement (Harlan et al. 1958) from similar activity originating from other physiological sources, particularly cerebral ones, continues to present a problem to electrophysiologists. Techniques that have been designed to solve this problem usually involve the spatial separation of the electrical fields of the different sources (Muras and Binnie 1970). An alternative approach is by use of mechanical or other methods which are not dependent upon the electrical field of the eye to detect its movement and display it simultaneously on the EEG record. The channels displaying E E G activity believed to be contaminated with eye movement artefact can then be compared directly with the eye monitor trace. This approach has been explored; Lacey (1963) used a ceramic strain gauge method to detect eye movements during sleep; Sullivan and Weltman (1963) used an electrical impedance method, and photographic techniques have been tried by other workers. These techniques, however, have not gained widespread popularity, probably because they have not satisfied two
Fig. 1. Sensor eye piece and pressure transducer module. (The complete monitoring system as shown is now available from Specialised Laboratory Equipment, Croydon, Surrey, England.)
basic requirements for routine work simplicity of use and the minimum, if any, additional computation of data. The system to be described utilizes the fact that the eyeball is not a perfect sphere. The bulge of the cornea can be easily detected, even through the closed lid, by placing a finger lightly upon it and in this way slight movements of the eyeball eyelid relationship can be recognized. This procedure in which the finger behaves as a pressure transducer is commonly used by L E G technicians to detect eye movement. A pressure sensor replaces the finger in the procedure described and becomes the input of the monitoring system, which is sensitive to very slight changes of pressure, thus detecting very small changes in the eyeball eyelid relationship. The sensor eye piece is housed in a spectacle frame as shown in Fig. 1, providing facilities for monitoring movement of either eye. It consists of a rigid plastic tube approximately 2 cm in diameter, closed at one end by a replaceable thin rubber diaphragm. The other end is connected by means of a flexible plastic tube to the pressure transducer module. Pressure changes at the diaphragm are transmitted by the column of air in the plastic tube to a differential capacitative pressure transducer in the module and cause amplitude modulation of a radio frequency signal. The time constant of the system is not critical when it is used for monitoring eye movements in the presence of LEG activity~ as evidence of no eye movement is the most significant information usually required in those circumstances. Its time constant is dependent upon the care given to avoiding air leaks at the sensor diaphragm and connecting tubes and can be expected to be about 30 sec, without special care being taken. However, in practice this value is reduced considerably by the additional time constant employed at the LEG machine of 1 sec or less, in order to display rapid eye movements of different sizes clearly about a steady baseline. The eye piece can be positioned over the eyeball by sliding it laterally in the frame and then adjusting for depth until it is lightly touching the closed eyelid. This adjustment is not critical and can often be left to the patient to make, for m a x i m u m comfort. The output may be connected to one channel of an L E G machine, the sensitivity of which can be adjusted empirically to display clearly the particular kind of eye m o v e m e n t s to be monitored. Very small movements can
183
EYE MOVEMENT MONITOR FOR CLINICAL EEG
L
Fs -
.
L
L...
l,,,..
L
l
1
l
,9",
Fp2
Fpl" Fpl Fpl - F7 F7 - T3 T3 - Ts
MONITOR I
EOG
,.
I
I "~01
' '
'Cz
C, Fig. 2. This figure shows seven channels of EEG, a recording from tile mechanical eye movement monitor and an electrooculogram (EOG) from electrodes placed above and below the left eye. Points O~, 02 and 03 show eye movements which are clearly shown on both the EOG and mechanical eye monitor channels. Points C 1, C 2 and C3 show cerebral activity from Fp2 and Fpl which is shown to contaminate the EOG, thus making it an unreliable method for distinguishing between cerebral and ocular electrical activity, whilst the mechanical monitor at these times indicates the correct source of the activity.
be recorded but at such a sensitivity gross movements would overload the amplifiers. For most EEG purposes clear indication of movements of 1'~ or more is adequate. A simple procedure for adjustment is to place marks, conveniently on the ceiling above the EEG couch, spaced a measured distance apart to subtend angles of 1~ at the eye, in both the vertical and horizontal planes. The subject is then asked to look at the appropriate marks in turn and the sensitivity of the EEG recorder adjusted until the movement is just visible on the record. This level of sensitivity has been found to be suitable for detecting the type of eye movements that present problems in clinical EEG recording. Fig. 2 demonstrates the limitations of the electro-oculogram for separating electrical activity due to eye movement from cerebral electrical activity and the advantage of the described system over it. The monitor has been in routine clinical EEG use in this department for about 2 years and has become an essential part of the departmental ancillary equipment. SUMMARY A simple eye movement monitoring system for use in clinical EEG is described~ which does not involve recording
of the corneo-retinal potential difference. The system, based upon the mechanical detection of movement by use of a pressure transducer, is shown to be sensitive to the smallest movements of the eye or lid, which cause artefact in EEG recording. The simplicity of both the system and its application makes it a reliable one for use by EEG technicians in clinical EEG. RESUME DISPOSITIF SIMPLE DE CONTR~ILE DES MOUVEMENTS O C U L A I R E S EN EEG C L I N I Q U E On d~crit un dispositif simple, permettant le contr61e des mouvements oculaires en EEG clinique. Ce syst6me n'est pas fond6 sur l'enregistrement des diff6rences de potentiels corneo-r6tiniennes, mais sur une d6tection m6canique des mouvements '5, l'aide d'un transducteur de pression. I1 est sensible au moindre mouvement de Foeil ou de la paupi6re, causes d'artefacts en enregistrement EEG. La simplicit6 du dispositif et de ses applications le rendent ais6ment utilisable par les techniciens EEG.
184 The pressure transducer module was designed originally for monitoring respiration by Dr. L. Kellenyi of the University of Pecs, Hungary, whilst on a British Council Fellowship to this Institute. REFERENCES
HARLAN, W, L., WHITE, P. T. and BICKFORD,R. G. Electrical activity produced by eye flutter simulating frontal elec-
A.L. WINTER troencephalographic rhythms. Electroenceph. olin. Ncurophysiol., 1958, 10: 164-169. LACEY,J. 1. A simplified method of detecting eye movements during dreaming. P©'chosom. Med., 1963, 25: 78. MURAS, J, S. and BINNIE, C. D. The recognition of frontal slow activity in the presence of eye movements. Proc. electrophysiol. Technol. Ass., 1970, 17: 131. SULHVAN, G. H. and WELTMAN, G. Impedance oculograph--a new technique. J. appl. PhysioL. 1963, 18: 215.