Measuring device for measuring the bioelectrical activity of the central nervous system

In an exemplary embodiment, a plurality of electrodes to be applied to the patient are employed and potential differences between the electrodes and a reference potential derived from the electrode potentials are formed. Weighting resistors are interposed which weight the potential differences, and the weighted values are combined in such a manner that the potential components proceeding from a specific point under the electrode system and especially from a point below the scalp are comprehended according to their magnitude while signal components proceeding from other points are suppressed.

BACKGROUND OF THE INVENTION 
The invention relates to a measuring device for measuring the bioelectrical 
activity of the central nervous system in which a plurality of electrodes 
which can be placed on the patient are employed and in which the potential 
differences between the electrodes and a reference potential derived from 
the electrode potentials are formed. 
In known measuring devices of this type, the measurement ensues either by 
means of a plurality of electrodes arranged according to an international 
standardization on the scalp (electroencephalograph EEG) or by means of a 
plurality of electrodes which are applied to the exposed cerebral cortex 
or to the meninx (electrocortigraphy, ECoG). The electrical activity of 
the nerve cells and of the surrounding medium is comprehended under the 
electrodes as corresponding potential changes. In both cases, amplifiers 
and registering devices are post-connected to the electrodes. 
The known measuring processes which are carried out with such measuring 
devices are divided into bipolar and unipolar measuring processes as they 
are illustrated and described in the "Handbook of Electroencephalography", 
Elsevier Scientific Publishing Company, Amsterdam, 1974, Volume 3, Part 3, 
particularly in FIGS. 10 and 13 with the appertaining description. In a 
bipolar measuring process, differential voltages are conducted to the 
amplifier inputs which are realized in pairs between the electrodes. 
Thereby, each measured voltage is the difference between two electrode 
potentials. A selective covering of each localized electrode potential 
change does not ensue. Accordingly, it is difficult to precisely localize 
the cerebral bioelectrical activity. In a unipolar measuring process, 
differential voltages are comprehended between a plurality of electrodes 
and a respective reference point provided in common for these electrodes. 
This reference point can be a physical electrode or, for example, the 
midpoint of a resistance star which is connected to all electrodes with 
the same amount of resistance, possibly with the exception of those 
electrodes whose signals could falsify the measuring result because, for 
example, they are caused by means of muscle activity. In the majority of 
electrodes employed, it has up to now resulted by necessity that not all 
reference-forming electrodes are arranged neighboring the signal 
electrode, but that, rather, they were distributed over the entire scalp 
of the patient. For this reason, it was not possible to comprehend a 
potential which was a measure for the bioelectrical activity in the direct 
area of the signal electrode. On the contrary, only potential differences 
were measured which allowed of no precise conclusions concerning the 
location of the measured bioelectrical activity. 
In the German AS No. 25 18269 (corresponds to the U.S. Pat. No. 4,084,583), 
a measuring process is described in which only such electrodes are used as 
the auxiliary electrodes for a signal electrode which neighbor the signal 
electrode. In the registration of a cerebral electrical activity with the 
assistance of surface electrodes on the scalp, however, a spreading effect 
is present in contrast to the registration of potential distributions in 
tissues which lie deeper, for example, on the cortex. There is a desire, 
however, to know the potentials prevailing on the cortex. 
SUMMARY OF THE INVENTION 
Accordingly, the object of the invention is to create a measuring device of 
the type initially cited in which the bioelectrical activity of the 
central nervous system can be determined at a location which lies as close 
as possible to the cortex. 
This object is inventively achieved in that means are present which weight 
the potential differences with a weighting factor in such manner and 
subsequently sum them so that the potential components proceeding from a 
specific point beneath the electrode system are comprehended according to 
their magnitude, and signal components proceeding from other points are 
suppressed. Based on the fact that a point potential below the scalp 
surface on or near the cortex can be determined from the potential field 
of the scalp surface which has a distribution similar to a gauss bell 
curve for a point potential on the cortex, it is possible to determine the 
potential distribution in a plane close to the cortex even given the 
employment of surface electrodes. 
The weighting factors are selected in such manner that those potential 
components which proceed from the interesting locations are comprehended, 
and the potential components proceeding from the other locations are 
suppressed. By means of summing the weighted signals, one then receives a 
signal which very precisely reproduces the potential at the interesting 
location on the cortex. 
Further details of the invention derive from the subclaims. In the 
following, the invention is described in greater detail on the basis of an 
exemplary embodiment illustrated on the accompanying sheets of drawings; 
and other objects, features and advantages will be apparent from this 
detailed disclosure and from the appended claims.

DETAILED DESCRIPTION 
FIG. 1 shows a schematic illustration of a cross section through the tissue 
of a scalp from the scalp surface to the cortex. The cerebral electrical 
activity changes on the path from the cortex to the scalp surface in that 
a spreading occurs in the tissue lying between, so that each potential 
component of the cortex occurs at the scalp surface with a potential 
distribution of a specific width. The potential at each point in the 
potential field of the scalp surface is therefore composed of various 
potential components which come from various locations of the cortex. A 
voltage source 11 is illustrated on the cortex 10 which is meant to 
represent the cerebral electrical activity at one point. The magnitude of 
the activity at the point is designated with 12. The spread of the 
potential at the Niveau 13 is represented by means of the curve 13' which 
is approximately bell shaped. At the Niveau 14, the spread of this 
potential is even greater, as the curve 14' shows. The potential spread is 
greatest at the scalp surface 15 as the bell-shaped curve 15' shows. 
FIG. 2 shows sixty-one (61) electrodes placed on the scalp of a patient in 
a specific geometrical configuration. Here, the magnitude of the cerebral 
electrical activity at the cortex is to be measured precisely below the 
signal electrode 1. Reference electrodes have been designated with 0 and 2 
through 6, whereby the electrodes 0 are the boundary electrodes. The 
interval between the electrodes is approximately 10 mm. 
The signals which are obtained between these electrodes 0, 1, 2 through 6 
and a reference point are weighted and summed. The weighting and summation 
ensues in such manner that the effect under the point in question is 
suppressed as to components occurring at the surface with a potential 
spread which is not centered under the interesting point or which occur 
with a potential spread which is larger than the spread of a point 
potential in a plane in the proximity of the cortex at the surface of the 
scalp. 
The weighting coefficients are selected in such manner that the measuring 
plane lies as close as possible to the cortex in order to achieve the 
greatest possible precision in the registration of individual potential 
components. 
The weighting coefficients are selected for a measuring plane at a depth 
which corresponds to a gauss spread of a point potential with a width 
which is equal to 1.6 times the electrode interval. 
The calculation of a weighting coefficient can be carried out with the 
assistance of a matrix algebra and is a known process in image processing 
technology (Harry C. Andrews, Computer Techniques in Image Processing, 
Academic Press, 1970, New York). 
When the weighting coefficient is designated with b and the electrodes with 
1 through 6, the weighting coefficients are as follows: 
b.sub.1 =+0.38; b.sub.2 =0.72 (for each of six electrodes 2); b.sub.3 
=+0.51 (for each of six electrodes 3); b.sub.4 =+0.02 (for each of six 
electrodes 4); b.sub.5 =-0.27 (for each of twelve electrodes 5); b.sub.6 
=+0.06 (for each of six electrodes 6). 
The potential differences between the potentials of the thirty-seven (37) 
electrodes 1 through 6 multiplied by the corresponding weighting 
coefficients, and a reference potential (Ref.) which is the mean potential 
of all sixty-one (61) electrode potentials, are processed by means of a 
circuit arrangement according to FIG. 3. It is shown in this figure that 
resistances 1' through 6' whose sizes correspond inversely to the 
weighting factors b.sub.1 through b.sub.6 lie between the electrodes 1 
through 6 and a summation circuit 16. 
A signal corresponding to the sum of the weighted signals of the electrodes 
1, 3, 4, and 6 is obtained at the summation point 25 and is supplied to 
the negative input of an operational amplifier 26. The reference potential 
(Ref.) is supplied to the positive input of this operational amplifier 26. 
A potential which corresponds to the summation current at point 25 is 
obtained at the output 27 of the operational amplifier 26. A current 
corresponding to the sum of the weighted signals of the electrodes 2 and 5 
is obtained at the summation point 28. The current at point 28 is 
subracted from the current at point 25 and the differential current is 
supplied to the negative input of a further operational amplifier 29. The 
reference potential (Ref.) likewise lies at the positive input of this 
operational amplifier 29. A signal thus lies at the output 30 of the 
amplifier 29 which is obtained from the electrode potentials in the manner 
described in conjunction with FIG. 2. This final potential is now supplied 
to the one input 31 of a differential amplifier 32. At the same time, the 
reference potential is supplied to a second input 33 of the differential 
amplifier 32. The difference between these signals is supplied to a 
storage or recording device 34. The output signal of the differential 
amplifier 32 corresponds to the point potential at the cortex directly 
below the electrode 1. 
FIG. 4 shows a fastening arrangement 17 with a concaveshaped plate 18 with 
a raised edge 19. The plate 18 has holes which correspond to the electrode 
configuration according to FIG. 2. The center hole 20, corresponding to 
the position of the electrode 1, serves as the point of reference, i.e., 
that the operating personnel apply the fastening arrangement 17 to the 
scalp of the patient in such manner that the point of reference lies on 
the point to be investigated. The fastening arrangement 17 can be secured 
to the scalp of the patient by means of a strap 26 which is arranged on 
the raised edge 19 at the fastening locations 21 and can be applied under 
the chin of the patient. 
FIG. 5 shows that an electrode mount 23 of insulating material can be 
placed on the fastening arrangement 17 and be aligned by means of two 
nipples 24, 25. The electrodes 1 through 6 are spring-seated in the mount 
23 approximately perpendicular to the application surface, of which only 
three (3) electrodes are illustrated here. The electrodes, whose ends can 
be pointed or round, are approximately 2 mm in diameter. The electrodes 0, 
1 through 6 are connected with a differential amplifier according to FIG. 
3 via lines. 
The measuring device with a circuit arrangement for the weighting and 
summation of the electrode signal is, according to the invention, a device 
for the focusing of scalp potential registrations on a measuring plane 
lying below the scalp and thus renders possible an improved diagnosis of 
localized phenomena in the electrical activity of the brain. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts and teachings of 
the present invention.