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The hydro-electroencephalogram, a novel method of EEG recording
Electroencephalography and cfinical Neurophysiology , 1987, 67: 188-190 Elsevier Scientific Publishers Ireland, Ltd.
188
EEG 02016
Short communication
The hydro-electroencephalogram, a novel method of EEG recording D.E. Maynard EEG Department, The London Hospital, London E1 1BB (U.K.) (Accepted for publication: 23 April, 1987)
Summary A novel method is described of recording EEG by means of electrodes in de-ionised water in which the scalp is immersed. It permits large numbers of closely packed electrodes to be connected quickly both in the plane of the scalp and three-dimensionally. A common reference potential can be derived from the whole immersed scalp. The technique should be suitable for the practical application of mathematical methods of source location where large numbers of electrodes are required.
Key words: Hydro-electroencephalography; Source reference; Common reference; Source location; Electrodes
With conventional EEG both the total number of electrodes and the packing density with which they can be placed upon the scalp are subject to practical limitations. The use of toposcopic contour mapping techniques (e.g., Duffy et al. 1979) relies upon interpolations between electrodes that may be separated by distances of the order of 2-4 cm. Such electrode spacings can only have an approximate relationship to underlying anatomy. Techniques of highlighting localised activity, such as 'source reference' (Hjorth 1975), ideally require higher densities of electrode placement. The work described here shows that, by immersing the scalp in a high impedance conductor, de-ionised water, large numbers of electrodes with as little as 4 mm separation may be connected without major problems. The electrodes do not have to be in direct contact with the scalp and three-dimensional measurements of potential are possible. Additionally, an average reference potential may be derived from the whole immersed scalp.
Method For evaluation of the hydro-EEG technique, 4 stainless steel electrodes at a separation of 4 mm on a length of silicon rubber tubing were mounted in a support so that the effect of varying electrode distances vertically from the scalp could be examined. The support was made from a plastic 35 mm film container, with most of its sides cut away to minimise obstruction to three-dimensional current flow (Fig. 1). Two electrode supports placed on the scalp were held in
Correspondence to: Dr. D.E. Maynard, EEG Department, The London Hospital, London E1 1BB (U.K.).
place by means of a loop of wide mesh stockingette bandage placed over them and under the chin. The subject was supine on a couch with the head tipped backwards into a bucket containing de-ionised water. The water level was approximately from the eyebrows to Cv2. Two types of recording bucket were used. The first was plastic (diameter 24 cin, depth 19 cm) with a 40 cm2 stainless steel plate electrode in its base to act as a common average reference; alumininm foil covered the outer surface of the bucket and was grounded to act as an electrical screen. The second bucket was hemispherical (radius 16 cm), of stainless steel, and itself acted as the reference electrode. It was contained within but insulated from a second grounded bucket. De-ionised water was obtained from a Millipore( ~ RQ water de-ionisation apparatus. The water resistivity reading on the Milfipore meter was typically 18 MD-cm, measured at DC. To measure at EEG frequencies the impedance of the water, after transport and storage in containers, an AC bridge was constructed (Fig. 1). Water impedance was found to fall during experiments as solutes from the scalp and hair passed into solution. To reduce this effect the scalp and hair were first rinsed in tap water and then in de-iouised water. Data were digitised in 8' channels into a microprocessor system and were written out via a computer-operated ink jet recorder. Both EEG samples and averaged evoked potentials to photic, auditory and somatosensory stimuli were obtained using both the plastic and metal buckets.
Results The water resistivity measured in the bridge at 15 H_z from a 500 mV peak-to-peak source was of the order of 2 Mf/-cm before use. After use it was of the order of 45 kfl.cm.
0013-4649/87/$03.50 © 1987 Elsevier Scientific Publishers Ireland, Ltd.
THE HYDRO-ELECTROENCEPHALOGRAM
189
02 *2ram
~10mm +14ram
uV
01+2ram +6111111
*10mm ÷It~mm
Fig. 3. Averaged visual evoked potential to a red LED 5 msec stimulus applied to the closed left eye. The electrodes are as for Fig. 2. The stainless steel bucket common reference was used. Positive at the scalp is upwards.
Fig. 1. Top: recording electrodes mounted on silicon rubber t u b i n g and supported in a cut-away 35 n u n f i l m container to enable them to be placed vertically to the scalp. Bottom: bridge used to measure water impedance. R1 = R2 = 820 kQ. Z3 and Z4 are two water columns, Z3 being half the length of
Z4, of 1 cm square cross-section between 1 cm square surface area stainless steel electrodes. A is an oscillator (500 mV peak to peak at 15 Hz), B is a differential high input impedance notch filter tuned to the oscillator frequency, and C is an oscilloscope. At bridge balance, zero output on C, the value of R3 is the impedance of a water column of the length of Z3. The electrode impedances being equal in each arm are balanced out. '
sec '
EO
02
EC
÷lOmm
..A,.. . . . . .
012 H \\~ . J / ~~
1
"
0
.14mm\ ~
....
' ~V,,,,Jv
m ..
m
\
~
" ..~.~ ,~.,.~ '
Fig. 2. EEG sample, with plastic bucket and plate common reference electrode, obtained from two electrode arrays of 4 mm pitch (Fig. 1) mounted vertically at occipital derivations. EO ffi eyes open, EC ffi eyes closed. The deflection at the start of the trace is a movement artefact. Some low frequency artefact is present from the ballistocardiogram.
It was found that some attenuation of the signal occurred compared with conventional recording methods. This effect was greater with the metal bucket. Ballistocardiogram and other movement artefacts were marked but could be greatly reduced by a suitable neck and back support. Noise from the stainless steel electrodes was below that of the EEG recorder and could not be measured. Attempts to use silver-silver chloride electrodes proved unsatisfactory, their chloride layer decomposed rapidly. An EEG recording obtained using the plastic bucket and the plate electrode as a common reference is shown in Fig. 2. This was from occipital locations, measured vertically from the scalp, with eyes open and closed. There is little attenuation with height above the scalp in the range measured but there are some small variations of wave morphology. In contrast, Fig. 3 shows the averaged visual evoked potential obtained from the same locations using the metal bucket as a common reference electrode. Here there is noticeable attenuation with height above the scalp as the potentials are divided over distance with respect to the reference.
Discussion
The results show that hydro-EEG is a feasible technique. Its particular advantage is that it permits large numbers of closely spaced electrodes to be placed quickly. Additionally a common reference potential can be derived from the whole immersed scalp. The technique may also be suitable for EMG. Possible applications of hydro-EEG would be in toposcopic displays of derivatives of processed EEG where a high density of electrode placement may be advantageous, and applications which make use of mathematical processing of the signal to localise sources of activity (reviewed by Wood 1982). Additionally the source reference method of recording (Hjorth 1975) is
190 readily applied with the numbers of electrodes possible with hydro-EEG and should be capable of further development. However, before hydro-EEG becomes a routinely practical technique a number of problems have to be solved. Firstly, the buckets used in the work described here, although adequate for proving that the method works, are too small for proper experimental work. In the examples shown the greater attenuation with height above the scalp obtained with the metal bucket is because of the smaller distance between the occiput and the common reference compared with the plastic bucket. A reference electrode should be far enough away from the scalp not only to minimise variations in impedance from scalp to the common reference, caused by variations in head shape, but also, where measurements are in the plane of the scalp, to minimise varying attenuations caused by small variations in electrode distance from the scalp. Secondly, if it were possible to totally immerse the head, with the metal bucket totally surrounding it, then the potential on the bucket should remain constant enabling it to be used as a unipolar reference. The extent to which this is achieved with the hemispheric metal bucket or adaptations of it, with most of the brain volume being below the water surface, will have to be the subject of further experiment. Thirdly, one has to accept that scalp impedance may vary between individuals and that this, in series with the water impedance, will cause some division of the signal. This is an addition to the problem of ascertaining the effects on scalp surface potentials of the impedances of brain, cerebrospinal fluid, and skull of varying thickness. Ideally one would wish to reduce scalp surface impedance to that of the underlying
D.E. MAYNARD tissue, so enabling a low impedance immersion medium to be used. In this case three-dimensional measurements of current flow would give some directional information about possible sources. However, the problem of obtaining low noise electrodes would have to be solved (Flasterstein 1966). Fourthly, the gradual reduction of impedance of de-ionised water as a result of solutes from the subject will require continuous flushing or closed cycle control of water impedance. In conclusion, the hydro-EEG makes possible the acquisition of EEG data at high densities of electrode placement. Although requiring some technological development, it should enable the practical application of source location techniques.
Reterences Duffy, F.H., Burchfiel, J.L. and Lombroso, C.T. Brain electrical activity mapping (BEAM): a new method for extending the clinical utility of EEG and evoked potential data. Ann. Neurol., 1979, 5: 309-321. Flasterstein, A.H. Voltage fluctuations of metal-electrolyte interfaces in electrophysiology. Meal. biol. Engng, 1966, 4: 583-588. Hjorth, B. An on-line transformation of EEG scalp potentials into orthogonal source derivations. Electroenceph. clin. Neurophysiol., 1975, 39: 526-530. Wood, C.C. Application of dipole locali7~tion methods to source identification of human evoked potentials. Ann. N.Y. Acad. Sci., 1982, 388: 139-155.
188
EEG 02016
Short communication
The hydro-electroencephalogram, a novel method of EEG recording D.E. Maynard EEG Department, The London Hospital, London E1 1BB (U.K.) (Accepted for publication: 23 April, 1987)
Summary A novel method is described of recording EEG by means of electrodes in de-ionised water in which the scalp is immersed. It permits large numbers of closely packed electrodes to be connected quickly both in the plane of the scalp and three-dimensionally. A common reference potential can be derived from the whole immersed scalp. The technique should be suitable for the practical application of mathematical methods of source location where large numbers of electrodes are required.
Key words: Hydro-electroencephalography; Source reference; Common reference; Source location; Electrodes
With conventional EEG both the total number of electrodes and the packing density with which they can be placed upon the scalp are subject to practical limitations. The use of toposcopic contour mapping techniques (e.g., Duffy et al. 1979) relies upon interpolations between electrodes that may be separated by distances of the order of 2-4 cm. Such electrode spacings can only have an approximate relationship to underlying anatomy. Techniques of highlighting localised activity, such as 'source reference' (Hjorth 1975), ideally require higher densities of electrode placement. The work described here shows that, by immersing the scalp in a high impedance conductor, de-ionised water, large numbers of electrodes with as little as 4 mm separation may be connected without major problems. The electrodes do not have to be in direct contact with the scalp and three-dimensional measurements of potential are possible. Additionally, an average reference potential may be derived from the whole immersed scalp.
Method For evaluation of the hydro-EEG technique, 4 stainless steel electrodes at a separation of 4 mm on a length of silicon rubber tubing were mounted in a support so that the effect of varying electrode distances vertically from the scalp could be examined. The support was made from a plastic 35 mm film container, with most of its sides cut away to minimise obstruction to three-dimensional current flow (Fig. 1). Two electrode supports placed on the scalp were held in
Correspondence to: Dr. D.E. Maynard, EEG Department, The London Hospital, London E1 1BB (U.K.).
place by means of a loop of wide mesh stockingette bandage placed over them and under the chin. The subject was supine on a couch with the head tipped backwards into a bucket containing de-ionised water. The water level was approximately from the eyebrows to Cv2. Two types of recording bucket were used. The first was plastic (diameter 24 cin, depth 19 cm) with a 40 cm2 stainless steel plate electrode in its base to act as a common average reference; alumininm foil covered the outer surface of the bucket and was grounded to act as an electrical screen. The second bucket was hemispherical (radius 16 cm), of stainless steel, and itself acted as the reference electrode. It was contained within but insulated from a second grounded bucket. De-ionised water was obtained from a Millipore( ~ RQ water de-ionisation apparatus. The water resistivity reading on the Milfipore meter was typically 18 MD-cm, measured at DC. To measure at EEG frequencies the impedance of the water, after transport and storage in containers, an AC bridge was constructed (Fig. 1). Water impedance was found to fall during experiments as solutes from the scalp and hair passed into solution. To reduce this effect the scalp and hair were first rinsed in tap water and then in de-iouised water. Data were digitised in 8' channels into a microprocessor system and were written out via a computer-operated ink jet recorder. Both EEG samples and averaged evoked potentials to photic, auditory and somatosensory stimuli were obtained using both the plastic and metal buckets.
Results The water resistivity measured in the bridge at 15 H_z from a 500 mV peak-to-peak source was of the order of 2 Mf/-cm before use. After use it was of the order of 45 kfl.cm.
0013-4649/87/$03.50 © 1987 Elsevier Scientific Publishers Ireland, Ltd.
THE HYDRO-ELECTROENCEPHALOGRAM
189
02 *2ram
~10mm +14ram
uV
01+2ram +6111111
*10mm ÷It~mm
Fig. 3. Averaged visual evoked potential to a red LED 5 msec stimulus applied to the closed left eye. The electrodes are as for Fig. 2. The stainless steel bucket common reference was used. Positive at the scalp is upwards.
Fig. 1. Top: recording electrodes mounted on silicon rubber t u b i n g and supported in a cut-away 35 n u n f i l m container to enable them to be placed vertically to the scalp. Bottom: bridge used to measure water impedance. R1 = R2 = 820 kQ. Z3 and Z4 are two water columns, Z3 being half the length of
Z4, of 1 cm square cross-section between 1 cm square surface area stainless steel electrodes. A is an oscillator (500 mV peak to peak at 15 Hz), B is a differential high input impedance notch filter tuned to the oscillator frequency, and C is an oscilloscope. At bridge balance, zero output on C, the value of R3 is the impedance of a water column of the length of Z3. The electrode impedances being equal in each arm are balanced out. '
sec '
EO
02
EC
÷lOmm
..A,.. . . . . .
012 H \\~ . J / ~~
1
"
0
.14mm\ ~
....
' ~V,,,,Jv
m ..
m
\
~
" ..~.~ ,~.,.~ '
Fig. 2. EEG sample, with plastic bucket and plate common reference electrode, obtained from two electrode arrays of 4 mm pitch (Fig. 1) mounted vertically at occipital derivations. EO ffi eyes open, EC ffi eyes closed. The deflection at the start of the trace is a movement artefact. Some low frequency artefact is present from the ballistocardiogram.
It was found that some attenuation of the signal occurred compared with conventional recording methods. This effect was greater with the metal bucket. Ballistocardiogram and other movement artefacts were marked but could be greatly reduced by a suitable neck and back support. Noise from the stainless steel electrodes was below that of the EEG recorder and could not be measured. Attempts to use silver-silver chloride electrodes proved unsatisfactory, their chloride layer decomposed rapidly. An EEG recording obtained using the plastic bucket and the plate electrode as a common reference is shown in Fig. 2. This was from occipital locations, measured vertically from the scalp, with eyes open and closed. There is little attenuation with height above the scalp in the range measured but there are some small variations of wave morphology. In contrast, Fig. 3 shows the averaged visual evoked potential obtained from the same locations using the metal bucket as a common reference electrode. Here there is noticeable attenuation with height above the scalp as the potentials are divided over distance with respect to the reference.
Discussion
The results show that hydro-EEG is a feasible technique. Its particular advantage is that it permits large numbers of closely spaced electrodes to be placed quickly. Additionally a common reference potential can be derived from the whole immersed scalp. The technique may also be suitable for EMG. Possible applications of hydro-EEG would be in toposcopic displays of derivatives of processed EEG where a high density of electrode placement may be advantageous, and applications which make use of mathematical processing of the signal to localise sources of activity (reviewed by Wood 1982). Additionally the source reference method of recording (Hjorth 1975) is
190 readily applied with the numbers of electrodes possible with hydro-EEG and should be capable of further development. However, before hydro-EEG becomes a routinely practical technique a number of problems have to be solved. Firstly, the buckets used in the work described here, although adequate for proving that the method works, are too small for proper experimental work. In the examples shown the greater attenuation with height above the scalp obtained with the metal bucket is because of the smaller distance between the occiput and the common reference compared with the plastic bucket. A reference electrode should be far enough away from the scalp not only to minimise variations in impedance from scalp to the common reference, caused by variations in head shape, but also, where measurements are in the plane of the scalp, to minimise varying attenuations caused by small variations in electrode distance from the scalp. Secondly, if it were possible to totally immerse the head, with the metal bucket totally surrounding it, then the potential on the bucket should remain constant enabling it to be used as a unipolar reference. The extent to which this is achieved with the hemispheric metal bucket or adaptations of it, with most of the brain volume being below the water surface, will have to be the subject of further experiment. Thirdly, one has to accept that scalp impedance may vary between individuals and that this, in series with the water impedance, will cause some division of the signal. This is an addition to the problem of ascertaining the effects on scalp surface potentials of the impedances of brain, cerebrospinal fluid, and skull of varying thickness. Ideally one would wish to reduce scalp surface impedance to that of the underlying
D.E. MAYNARD tissue, so enabling a low impedance immersion medium to be used. In this case three-dimensional measurements of current flow would give some directional information about possible sources. However, the problem of obtaining low noise electrodes would have to be solved (Flasterstein 1966). Fourthly, the gradual reduction of impedance of de-ionised water as a result of solutes from the subject will require continuous flushing or closed cycle control of water impedance. In conclusion, the hydro-EEG makes possible the acquisition of EEG data at high densities of electrode placement. Although requiring some technological development, it should enable the practical application of source location techniques.
Reterences Duffy, F.H., Burchfiel, J.L. and Lombroso, C.T. Brain electrical activity mapping (BEAM): a new method for extending the clinical utility of EEG and evoked potential data. Ann. Neurol., 1979, 5: 309-321. Flasterstein, A.H. Voltage fluctuations of metal-electrolyte interfaces in electrophysiology. Meal. biol. Engng, 1966, 4: 583-588. Hjorth, B. An on-line transformation of EEG scalp potentials into orthogonal source derivations. Electroenceph. clin. Neurophysiol., 1975, 39: 526-530. Wood, C.C. Application of dipole locali7~tion methods to source identification of human evoked potentials. Ann. N.Y. Acad. Sci., 1982, 388: 139-155.