Method and apparatus for measuring the bioelectrical activity under an electrode resting on a patient

Method and apparatus for measuring the bioelectrical activity of a patient, wherein a signal electrode is placed on a patient and a plurality of auxiliary electrodes are placed adjacent the signal electrode. The auxiliary electrodes are connected by a resistor network to provide a mean potential value and the difference between the mean potential value and the potential of the signal electrode is determined and fed to a recorder which indicates this difference value as indicative of the bioelectric activity under and within proximity of the signal electrode. The auxiliary electrodes can be arranged at the corners of a polygon with the signal electrode disposed in the center of the polygon. The entire assembly of electrodes can be mounted on a support device of insulating material.

FIELD OF THE INVENTION 
The present invention relates to a method for measuring the bioelectric 
activity which occurs under a measuring electrode (signal electrode) 
attached to or resting on a patient, in which method a plurality of 
auxiliary electrodes attached to the patient are employed whose potentials 
produce a mean potential value, and wherein the difference between said 
mean potential value and the potential of the signal electrode is measured 
and supplied, for example, to a device for recording the measured value. 
BACKGROUND OF THE INVENTION 
A known method of the above type is employed, for example, for the purpose 
of measuring signals of the central nerve system. Such a measuring process 
is carried out either by means of a number of electrodes which are 
arranged on the skull of the patient in a pattern conforming to 
internationally established standards (electroencephalography -- EEG), or 
by using a number of electrodes which are applied to the exposed cerebral 
cortex, or to the cerebral meninger (electrocortigraphy -- ECOG). The 
electrical activity of the nerve cells and the surrounding medium is 
picked up under the electrodes in the form of respective potential 
variations. The electrodes in both methods, are associated with 
after-connected amplifiers and recording or registering devices. 
The measuring methods may be divided into bipolar and unipolar measuring 
processes. A bipolar measuring method comprises feeding the potential 
differences to the amplifier inputs which are engaged in pairs between the 
electrodes. In the unipolar measuring method, the potential differences 
are picked up between a number of electrodes and a reference point which 
in each case is common for said electrodes. Said reference point may be a 
physical electrode or, for example, the center point of a resistor network 
which is connected with the same resistance value to all electrodes, or 
optionally with the exclusion of those electrodes whose signals, on the 
basis of past experience, are known to adversely affect the results of the 
measurement because they are caused, for example, by the activity of 
muscles. The shortcomings of said two measuring methods are that each 
measured potential is the difference between two electrode potentials, 
which means that a selective pickup of each local change in electrode 
potential does not take place. Accordingly, it is not possible to exactly 
locate the cerebral bioelectric activity. 
Furthermore, both these measuring methods are impaired by an instability 
factor caused by the fact that the electrode potentials are composed of 
the electrical activity both of the cerebral tissue disposed directly 
beneath the electrode and the electrical activity of an adjacent ring 
which is passed laterally on the skull. 
The above-mentioned known measuring methods are described in greater detail 
later with reference to FIGS. 1 to 3. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a measuring method of 
the afore-stated type which will make it possible in a simple manner to 
engage and pick up the electrical activity present locally beneath a 
measuring electrode. 
According to the present invention, this object is achieved by limiting the 
use of auxiliary electrodes exclusively to electrodes which are disposed 
adjacent the signal electrode. 
An advantageous arrangement for carrying out the measuring method comprises 
supporting the signal electrode and the auxiliary electrodes on a holder 
composed of insulating material. 
Additional advantages and details of the present invention will become 
apparent from the description of embodiments thereof with reference to the 
attached drawings.

DETAILED DESCRIPTION 
FIG. 1 shows a bipolar measuring method. Therein, the skull of a patient 1 
carries ten electrodes 2 to 11 placed symmetrically thereon. The amplifier 
inputs of amplifiers A2 to A9 associated with electrodes 2 to 11 receive 
in pairs potential differences existing in each case between the 
associated electrodes. The voltage differences are supplied, after 
amplification, to recording devices 12 to 19 which, for example, may be 
ink jet recorders according to German Pat. Spec. No. 821,065. 
FIG. 2 shows a unipolar measuring method. Therein, 12 electrodes 20 to 31 
are arranged symmetrically on the skull of patient 1. The electrodes 20 to 
29 are respectively connected to one input of amplifiers A20 to A29. To 
the second inputs of amplifiers A20 to A24, there is connected electrode 
30, and electrode 31 is connected to the second inputs of amplifiers A25 
to A29. This means that in this case, electrodes 20 to 24 supply five 
potentials to amplifiers A20 to A24, with the potential of electrode 30 
being the common reference potential, and in the same manner five 
potentials are supplied by electrodes 25 to 29 to amplifiers A25 to A29, 
with the potential of electrode 31 being the common reference potential. 
The amplifiers A20 to A29 conduct the difference between the potentials of 
electrodes 20 to 24 and the potential of reference electrode 30, or 
respectively between the potentials of electrodes 25 to 29 and the 
potential of reference electrode 31, to ink jet recorders 32 to 41. 
FIG. 3 illustrates a further unipolar measuring method wherein twelve 
electrodes 42 to 53 are symmetrically arranged and connected in each case 
to one input of amplifiers A42 to A53. To eight electrodes 43, 44, 46, 47, 
48, 49, 51 and 52, there are connected equal resistors 54 to 61, one of 
the ends of said resistors being connected to a common center point 62 
(reference point). All 12 amplifiers A42 to A53 are connected at their 
second inputs to point 62. With respect to the four electrodes 42, 45, 50 
and 53, it is assumed that these electrodes will pick up potentials of a 
non-cerebral type (for example, caused by muscle activity) to an extent 
exceeding the pick up of such non-cerebral potentials of the other 
electrodes, so that the former electrodes (42, 45, 50, 53) are not 
connected to the resistor star. Each of the amplifiers A42 to A53 measures 
the difference between an electrode potential and a mean value potential. 
The difference voltages are supplied to ink jet recorders 63 to 74 for 
recording these values. The mean value potential amounts to 1/8th of the 
sum of the electrode potentials beneath electrodes 43, 44, 46, to 49, 51 
and 52. If the mean value potential at reference point 62 is equal or 
larger than the potential of the individual electrode, the individual 
potentials of the electrodes cannot be picked up. This may be the case if 
some of the electrode potentials are substantially higher than others, 
because the result at point 62 will then be determined substantially by 
these substantially higher electrode potentials. 
In FIG. 4, the actual measuring electrode (signal electrode) is designated 
by reference number 79, the potential of which electrode is to be 
measured, whereas the reference numerals 75 to 78 designate auxiliary 
electrodes. Said auxiliary electrodes are disposed in such a manner that 
they form a regular polygon (a square in the example of the figure), with 
the signal electrode 79 being disposed in the center of the polygon. 
Beneath electrodes 75 to 79, there are present the potentials V.sub.1 to 
V.sub.5. According to the present invention, the following voltage value 
is formed: 
##EQU1## 
This voltage is strongly influenced by the potentials occurring within the 
surface area limited by auxiliary electrodes 75, 76, 77, 78. 
In order to permit an understanding of the spatial limitation of the 
sensitivity distribution, attention is directed to FIGS. 5 to 8 which show 
by way of example different potential values below line II--II in FIG. 4. 
In order to simplify calculations, it is assumed that the potential 
declines in accordance with a linear function in the plane extending 
through electrodes 75, 77, 79, and vertically relative to the surface of 
the body of the patient. The bias or difference in potential formed 
according to the invention, namely 
##EQU2## 
is the highest (100%) if the center of the potential field (potential 
center) is located in said plane precisely under signal electrode 79 (FIG. 
5). The voltage will be reduced to half that value (50%) if the potential 
center is located in the middle between signal electrode 79 and auxiliary 
electrode 77 (FIG. 6), and it will be zero if the potential center is 
located under an auxiliary electrode or outside of the polygon determined 
by the auxiliary electrodes (FIGS. 7 and 8). In reality, the potential 
declines along the skull more in accordance with an exponential function 
whose exponent slightly influences the exactness of the determination of 
the desired potential; however, this has no decisive bearing on the 
essence of the present invention. 
In FIG. 9, wherein 79 is the signal electrode and 75 to 78 designate the 
auxiliary electrodes, the assumption is made that the exponent of 
attenuation of the bioelectric potential, in terms of its magnitude, is 
such that the half-value spacing is equal to the spacing 80 between signal 
electrode 79 and auxiliary electrode 77. Half-value spacing refers, in 
this instance, to the spacing between two points which are radially 
removed from the center of the potential, with the potential declining to 
one-half its value between said two points. With different half-value 
spacings, one of the level lines described below will change; however, the 
principle is maintained. Reference numerals 81 to 85 designate contour 
lines having the following significance, or meaning: if the potential 
center which is to be measured bioelectrically is located on contour line 
81, i.e., exactly under signal electrode 79, the bias voltage 
##EQU3## 
is attributed the summation value of 100%. 
If the center of the potential is located on contour line 82, the bias 
voltage has the summation value of 50%, and if the center of the potential 
is located on contour line 84, the bias or difference in potential is zero 
and remains within the proximity of zero if the center of the potential is 
located further outside of contour line 84. For example, the bias is 
attributed the sum -1% for a potential center located on contour line 85. 
This means that the bias voltage according to the present invention will 
indicate with a high degree of spatial exactness the bioelectrical 
activity prevailing within the polygon formed by the auxiliary electrodes. 
The bias voltage, which is representative of the bioelectrical activity 
present under and within the proximity of the signal electrode, increases 
with the number of auxiliary electrodes used in the system. The embodiment 
shows four symmetrically disposed auxiliary electrodes 75 to 78; however, 
three, five or more electrodes may be used. It is not necessary to arrange 
the auxiliary electrodes symmetrically relative to the signal electrode. 
Furthermore, errors of measurement occurring in connection with auxiliary 
electrodes arranged in an unsymmetrical layout or pattern may be 
eliminated if the auxiliary electrodes are connected to the neutral point 
by way of resistors with values depending on the spacing provided in each 
case between the auxiliary electrode and the signal electrode. 
FIGS. 10 to 12 show electrode-support devices in which the electrodes are 
mounted in such a way that a bias voltage is obtained in accordance with 
the above-described measuring method. 
The electrode mounting device in FIG. 10 comprises a disk 86 composed of 
insulating material; the signal electrode 87 is secured in the center of 
said disk. The auxiliary electrodes 88 to 92 are disposed at the periphery 
of said mounting device and form the corners of a regular polygon 
(pentagon). All electrodes are secured or mounted vertically on their 
support surfaces by means of known elastic means which are not shown in 
the figure. An elastic band 93 is pushed onto or over a pin 94 and presses 
the electrode-support device against the surface of application on the 
patient. 
FIGS. 11 and 12 illustrate an electrode support device of insulating 
material which is formed by two triangles 95, 96, which each support three 
auxiliary electrodes 97 to 99, and 100 to 102. A shaft 103 projects 
through the common center of said triangles 95, 96, and carries a signal 
electrode 104 which is attached to the downwardly facing end of said 
shaft. The other end of face side of shaft 103 supports a disk 105 whose 
diameter is larger than the diameter of said shaft. Between disk 105 and 
triangles 95 and 96, there are arranged coil springs 106, 107, in such a 
way that the triangles 95 and 96 are rendered elastic independently of 
each other. Disk 105 is further provided with an upwardly extending pin 
108 onto which an elastic band 109 is placed for the purpose of pressing 
the electrode-support device against the surface of application. When said 
elastic band 109 urges the signal electrode 104 against the surface of 
application, electrodes 97 to 102 of the two triangles receive a certain 
pressure of application which is dependent on coil springs 106 and 107. 
The electrode-support devices described above are small in size; the 
spacing between the signal electrode and the auxiliary electrodes is about 
1 to 2 cm, so that a number of signal electrodes together with a 
respective number of auxiliary electrodes may be applied which will 
conform to the number of signal electrodes applied customarily in 
connection with known measuring methods. The electrode-support device 
shown in FIGS. 11 and 12 comprise a particularly advantageous construction 
comprising two adjacent electrode holders which may be combined by 
inserting one into the other. 
FIG. 13 illustrates the manner in which the differance voltage is formed 
which is characteristic of the present invention. Namely in the 
aforedescribed electrode arrangements, the difference voltage is formed as 
the difference between the potential of signal electrode 110, which 
potential is supplied to the input of differential amplifier 115, and the 
mean value of the potentials of auxiliary electrodes 111 to 114, said mean 
value being received at reference point 116 and supplied to the second 
input of said differential amplifier 115. 
Between reference point 116 and the auxiliary electrodes 111 to 114, there 
are disposed equal resistors 117 to 120 because the spacing between signal 
electrode 110 and auxiliary electrodes 11 to 114 is also equal in such 
case. The difference voltage between the potential of signal electrode 110 
and the potential of point 116 is amplified in differential amplifier 115 
and further supplied to a recording means 121 for registration. 
The measuring method according to the invention may also be used in an 
electrode arrangement according to FIG. 14 wherein a number of electrodes 
are placed on the skull 1 of a patient in a pattern according to an 
internationally established standard, for example in a so-called 10-20 
system. Each signal electrode is used in this case as an auxiliary 
electrode for adjacent signal electrodes. The bias or difference voltages, 
which are characteristic of the present invention, are formed only after 
one or several steps of amplification, which avoids loading the electrodes 
with a great number of resistors. 
Each signal electrode 122 to 140 is connected in each case to an input of a 
differential amplifier A122 to A140 having a high input and a low output 
impedance and two inputs and two respective outputs. The other inputs of 
differential amplifiers A122 to A140 are connected to a reference 
electrode 141 placed on skull 1 of the patient. 
It is intended, by way of example, to measure the potential occurring 
locally under and within the proximity of signal electrode 125. This may 
involve the use of auxiliary electrodes 124, 122, 126 and 130 which form a 
polygon. The picked-up potentials of electrodes 122, 124, 125, 126 and 130 
are supplied to one input of the associated amplifiers A122, A124, A125, 
A126 and A130. The corresponding outputs of amplifiers A122, A124, A126 
and A130 are connected to a common point 146 by way of resistors 142 to 
145. The potential under reference electrode 141 is supplied to the second 
inputs of said amplifiers. The second outputs of said amplifiers A122, 
A124, A125, A126 and A130, like the respective outputs of all amplifiers 
A122 to A140, are connected to a common point 147 (output point). The 
differential amplifier B125 measures the difference between the potential 
at output 148 of amplifier A125 and the potential at point 146 of the 
resistor network. The potential of said point 146 is the mean value of the 
output voltages of amplifiers A122, A124, A126, A130, because the outputs 
of all amplifiers A122 to A140 are lower ohmic outputs. The output voltage 
of differential amplifier B125 is supplied to an ink jet recorder 156 for 
recording purposes and this output voltage corresponds to the potential 
present under signal electrode 125 and within an area defined by auxiliary 
electrodes 122, 126, 130, 124. 
The potentials under electrodes 122 to 141 are designated in the present 
case by V.sub.122 to V.sub.141. The amplification in amplifiers A.sub.122 
to A.sub.140, for the sake of simplification, is assumed to be equal 1. 
The output voltage of amplifier A.sub.122 is V.sub.122 - V.sub.141. 
Between point 146 and output point 147, said voltage reduces to: 
##EQU4## 
Between points 146 and 147, the output voltages are added by amplifiers 
A122, A124, A126 and A130 as follows: 
##EQU5## 
The output voltage of amplifier A125 between points 148 and 147 is 
V.sub.125 -V.sub.141. The differential amplifier B125 measures the voltage 
between points 148 and 146, i.e.: 
##EQU6## 
Therefore, the output voltage of differential amplifier B125 corresponds 
to the difference between the potential under signal electrode 125 and the 
mean value of the potentials of auxiliary electrodes 122, 126, 124 and 
130. The voltage under reference electrode 141 has been eliminated. 
If the locally occurring potential is to be measured within the range of 
electrode 130, the adjacent electrodes 125, 129, 131 and 135 will be used 
as auxiliary electrodes. The potentials picked up from these electrodes 
are supplied to the inputs of the associated amplifiers A125, A129, A130, 
A131 and A135. The respective outputs of said amplifiers are connected to 
point 153 by way of resistors 149 to 152 shown by dash lines in FIG. 13. 
The voltage within the range of reference electrode 141 is supplied to the 
other inputs of amplifiers A125, A129, A130, A131 and A135. The 
corresponding outputs are in this case also combined at output point 147. 
Differential amplifier B130, shown in FIG. 13 in dash lines, picks up the 
difference between output 154 of amplifier A130 and point 153 of the 
resistor network. The output voltage of differential amplifier B130 is 
conducted to ink jet recording device 155. This voltage corresponds to the 
potential which occurs under signal electrode 130 and within the polygon 
formed by auxiliary electrodes 125, 129, 131 and 135. The locally 
occurring potentials are similarly picked up under the other electrodes. 
The measuring method described above may be employed also in connection 
with the perimetral electrodes 122, 123, 128, 133, 138, 140, 139, 134, 129 
and 124. The two adjacent perimetral electrodes are, in each case, used as 
auxiliary electrodes for each of said perimetral electrodes. For example, 
if electrode 129 is a signal electrode, electrodes 124 and 134 are used as 
the mean value-forming auxiliary electrodes. The differential amplifier 
associated with signal electrode 129 thus picks up the difference between 
the one output of amplifier A129 and the center point of the two resistors 
connected to the outputs of amplifiers A124 and A134. 
As distinct from known measuring methods, it is possible in this way to 
achieve also for the perimetral electrodes superior exactness in 
determining the local electrode potential. 
The measuring method described in the aforegoing and the arrangements for 
carrying out this method are also applicable to the measuring of other 
bioelectrical signals, for example, in connection with an 
electrocardiogram (EGG) of a fetus.