Automatization of electro-oculographic examination

The invention is especially related to a method for performing an electro-oculographic (EOG) examination of the eyes. Alternating optical signals are produced (34, 36) for the stimulation of the eye movement and those changes in the bioelectric potentials are observed (12), which changes are caused by the eye movements corresponding to said optical signals, for the establishment of sample signals. Noise signals are filtered (22) off by forming a moving average from successive sample signal portions, defining from said average signals the potential leaps or transitions in the biopotential signals, which transitions are caused by said eye movements, and calculating (24) values corresponding to said transitions. Possible distorted values are removed from the set of said values and finally the EOG ratio is defined on the basis of values selected from the remaining set of values. More generally the invention relates to a method for determining, from a sample signal comprising potential leaps and spurious noise, a reference value for a potential leap. From the analogue sample signal a digital signal succession is formed from the successive signal portions of which moving averages are calculated. An approximate potential leap point is obtained from said average signals by windowing at an area where a transition is supposed to exist. The invention further relates to an apparatus for the realization of an EOG examination in accordance with a method disclosed above.

The present invention relates to a method as defined in the introductory 
part of claim 1, as well as to an apparatus for the realization of said 
method for carrying out an electro-oculographic (EOG) examination, said 
method being used for detecting certain types of damages in the retina. 
Especially the invention relates to a method for speeding up and 
facilitating to the EOG examination by automatizing it. More generally the 
invention is directed to a method for determining a reference value for a 
potential leap from a sample signal comprising potential leaps and 
spurious noise. 
The invention further is related to an apparatus as defined in the 
introductory part of claim 7, for the realization of the method according 
to the invention. 
Electro-oculography is an electro-physiological examination centrally 
related to opthalmology, said examination being based on the measurement 
of slow fluctuations in the electrical potential differences between the 
retina (- pole) and the cornea (+ pole). The ratio between said 
cornea-retinal potentials is called the EOG-ratio. EOG is used for the 
examination of certain degeneration diseases in the retina. It may further 
be used for ascertaining whether a medication used e.g. for curing 
rheumatism is damaging the retina. Thus, EOG examination is considered to 
be essential in patient examination, but it is also considered to be a 
good examination method in experimental studies and especially when 
carrying out retina toxic examinations for medicines. 
The potential cannot be directly measured at the eye. This could be done by 
arranging an electrode on the cornea, but in this case the eye must be 
anesthetized, which would lead to erroneous results due to the abnormal 
function of the eye. 
This problem is avoided by measuring the EOG ratio indirectly using 
electrodes arranged in the vicinity of the patient's eyes, whereby 
mutually different biopotentials, which can be measured, are formed by the 
movement of the eyes. The biopotential ratio for an eye can then be 
defined by calculation from the mutual ratio of said bio potentials. 
Depending on the test arrangement a varying amount of electrodes are 
arranged on the skin, and further there are numerous different positioning 
alternatives for them. The movement of the eyes is accomplished using e.g. 
blinking light sources. In a widely used test arrangement the patient 
looks at two light sources arranged at opposite sides of his eyes. The 
lights blink in opposite phases at a frequency of e.g. 1 Hz, so that the 
patient's eyes are in a constant movement back and forth. Also such a test 
arrangement is known where a fixed light or some other distinct target is 
moved in front of the patient's eyes. 
The potential always varies when the patient moves his eyes. The wave 
frequency of the potential amplitudes thus initiated is identical with the 
blinking frequency of the light source. 
The size of the potential amplitude is proportional to the EOG ratio, since 
the potential is zero when the eye looks straight ahead and 
correspondingly differs from zero when the eye looks sideways or upwards. 
Since the potential depends on the position of the eye and on the 
cornea-retinal potential it is impossible to define the real 
cornea-retinal potential. This fact, however, is unsignificant, since the 
potential ratio achieved as a result of the examination is more important. 
It is, however, important that during the examination the eye moves 
between the same suitably defined points. 
The examination itself is usually started in darkness. The potential 
difference signal is recorded for about 10 seconds once in every minute 
during a fifteen minute period. The signal is not measured continuously, 
because the continuous moving of the eyes is very exhausting, which would 
distort the results. Also, the changes in this type of cornea-retinal 
potential are very slow. 
First the cornea-retinal potential decreases slowly when the eye adapts to 
the darkness. This should happen in about 8 to 9 minutes from the 
beginning of the examination. After this dark fifteen-minute period the 
light is changed to a very bright one. Then another fifteen-minute period 
is recorded. The cornea-retinal potential should rise as the eye adapts to 
the new lighting. This takes again about 8 to 9 minutes. 
After this the EOG ratio is defined taking the highest and lowest 
cornea-retinal potential values and calculating their ratio. If this ratio 
is below a defined limit the case usually needs closer investigations. 
However, manually performing this kind of examination has been a very 
tedious task. Different apparatus, like the light source, the measuring 
equipment and the plotter, must be started and stopped a number of times. 
One must know how to select the "good" samples from among the ones 
obtained, and their average values must be calculated, after which a 
graphic presentation is made for the average values of each examination 
period, from which presentation the EOG ratio finally can be defined. It 
is usual to obtain about 1000 "good" potential amplitudes to be defined 
manually, which increases the calculation error possibility at the average 
value calculation. In the examination it is further possible to include 
several special functions which also demand much attention from the 
examiner. 
For said reasons an automatic performance of the examination sequence has 
been suggested, e.g. controlled by a processor or logical circuit. In U.S. 
Pat. No. 4,474,186 there is suggested an apparatus which automatically 
controls an electro-oculographic examination and automatically processes 
the results derived from the examination and presents these results in a 
readable form. The arrangement thus developed is, however, rather 
complicated and the results obtained therefrom have not proven to be exact 
enough. Especially the weakness of the amplitudes obtained from the 
electrodes has caused unexactness, since the relevant signals tend to be 
covered by random electromagnetic and biological background noise, said 
noise distorting the signal obtained and essentially complicating the 
achievement of reliable results. The ability of the electronic error 
filter system in the apparatus according to the cited document to separate 
"good" and "poor" samples from each other has also proven insufficient, 
since it is based on a low and high pass filtering of the noise signals. 
However, the electrodes provides such a weak signal that it is especially 
difficult to select filters capable of providing a sufficient amount of 
reliable results after filtering which still not simultaneously filtering 
away a considerable portion of the signal. Especially due to the high 
error probability in the results given by the apparatus according to the 
cited publication no significant improvement in the EOG examination has 
actually been achieved therewith. 
Further, in the publications DE 3511695 and DE 3511697 complicated systems 
for the automatization of EOG examinations are disclosed. In these known 
systems the filtering of the signals and the poor reliability of the 
results also constitute a problem, since both the systems according to 
said cited publications use low and high pass filtering for the noise 
reduction. 
Further, all the known systems discussed above have been confronted with 
the fact that the computer's pattern recognition ability is insufficient. 
The computer has not been able to define exact enough EOG values from the 
signal graph obtained, and especially the insufficient pattern recognition 
ability has increased the asymmetry and non-continuousness in the graphs. 
The object of the present invention is to overcome the disadvantages in the 
prior art and provide a quite new solution for performing an automatic EOG 
examination exactly and quickly, in a reliable and even very simple 
manner. 
Another object of the invention is to provide a functionally simple and 
easy-to-use method and apparatus for the performance of an EOG 
examination. 
Yet another object of the invention is to provide an apparatus and method 
which give the results of an EOG examination in real-time. 
Yet another object of the invention is to provide such an apparatus for the 
automatic performing of an EOG examination, which apparatus may be 
realized using easily accessible standard components and a micro 
processor. 
The invention is based on the idea that the signal obtained from the 
electrodes is subjected to a computer aided median filtering, i.e. a 
moving average value is defined from the signal's subsets, the signal 
favorably being digitized prior to the median filtering. The filtered 
signal is derivated and, using the peak of the derivative signal and a 
second point defined in accordance with the teaching of the present 
invention, the real level of the potential leap is defined by integration. 
Using the method according to the present invention the system's ability 
to differentiate between "good" and "poor" signals will be essentially 
improved. 
In this connection it should be observed that the digitalized signal is not 
continuous but represents the peak value for samples taken per time unit. 
In a purely mathematical observation such a non-continuous signal cannot 
have any derivative. Since, however, the difference between two adjacent 
samples will behave- like a derivative it will, in this connection, be 
called a derivative, which can be expressed as: 
EQU dx.sub.n =x.sub.n -x.sub.n-1, 
where n is an integer 
In this connection the term integral is used to express the corresponding 
relation between two adjacent samples, which behaves like an integral and 
can be expressed as: 
##EQU1## 
Further, in this connection the term transition will be used to indicate a 
potential leap. 
More exactly expressed, the method according to the invention is mainly 
characterized by the features disclosed in the characterizing parts of 
claims 1 and 6. The apparatus according to the invention is characterized 
by the features disclosed in the characterizing part of claim 7. Other 
characterizing features are found in the dependent claims. 
According to a preferred embodiment of the invention an EOG examination of 
the eyes is performed automatically so that the examiner does not at any 
stage after the initiation of the examination cycle need to interfere with 
the examination procedure, but the system according to the invention will 
independently perform the action cycles of the examination, perform the 
removal of error signals, analyze the obtained measured results as well as 
store and print them in a desired form. 
Reliable and exact results will be obtained using the method according to 
the invention, where signals obtained from electrodes arranged on the skin 
will be digitalized and median filtered for the elimination of erroneous 
signals. The signal obtained is derivated whereby an essentially symmetric 
triangular form is achieved for the obtained signal pattern. 
In the next step the minimum and maximum of the essentially triangular 
derivative signal is localized. In this respect forming of a window may be 
used, the window being applied around that point where a maximum is 
believed to occur. The length of the window is preferably defined as the 
length of one phase of the light source flash. The centre of the window 
will not be exactly where it theoretically should be, since due to the 
reaction time a person will move his eyes with a slight lag with respect 
to the light source and thus also the window will be located slightly 
later than the light source cycle. 
At the setting of the windows it should further be investigated whether a 
minimum or a maximum of the signal will occur first. This is done by 
assuming a maximum to come first and setting the windows accordingly. 
Thereafter the signal values are multiplied with 1 in maximum windows and 
with -1 in minimum windows. If the average of the values thus calculated 
is positive the presumption was correct and if the average value is 
negative the first window was located at a minimum. 
Now the exact positions of the peaks are known. Next the position of the 
edges of the triangular peak area should be found. The remaining noise in 
the signal makes it impossible to define the position of the edges using 
the signal zero points. Further, the last edge of a peak is normally 
further away than the first edge. 
For this procedure the form of the peak is utilized. After the median 
filtering the peak should have straight edges. Finding the position of the 
edges and the peak end is started at the maximum and the samples are 
searched one after the other, moving forwards away from the maximum until 
the average of all the samples between the present position and the 
maximum is half the maximum. Using this position point and the maximum 
point the end point of the peak is obtained for the calculation of the 
area. 
Finally the amplitude size is simply obtained by integrating over the peak 
area of the curve, whereby the real amplitude height is obtained. 
The calculation is preferably performed using a computer and it can 
preferably be made faster by combining the median filter and derivation 
stages. 
The method according to the invention will give several remarkable 
advantages. Not only will the analysis of the signals obtained from the 
electrodes be faster with respect to prior art, but the obtained results 
will also be more exact and reliable. The apparatus needed for performing 
the examination and the method according to the invention is easy to 
realize and favorable regarding the costs, since all the components needed 
are of a type which can be commercially obtained.

More precisely, FIG. 1a discloses the electrical circuit formed by a human 
eye, and its potential when said eye looks straight ahead. Favorably the 
nose base and the temples are used as measuring points for the 
biopotential, i.e. as attachment points of the measuring electrodes 12, 
but other locations may be used too. Said electrodes 12 are connected to 
an amplifier 14, which can be a conventional EKG-apparatus or the like. As 
can be seen from the figure, the potential of the circuit is essentially 
zero when the eye is looking as indicated in the figure. 
In FIG. 1b the eye is shown turned to the left, and it can be observed, 
that the temple electrode potential now is higher than that of the 
electrode 12 at the nose base. 
During the examination procedure it is important that the eye moves between 
the same two points. Further a too excessive angle between light sources 
will cause the patient's eyes to be rapidly wearied and then the signal 
will loose its sharp form. On the other hand, a too narrow angle will give 
too low potential leap values from which it is impossible to define the 
EOG ratio. As a suitable angle would be about 15 from the center line to 
both sides of eyes, as indicated above. 
FIG. 2 schematically shows the basic idea of the present invention in one 
of its simplest embodiments. According to the figure the signal obtained 
from the electrodes 12 is amplified in an EKG-amplifier 14, filtered in a 
median filtering means 22, which is functionally connected to said 
amplifier, the correct signal transition is obtained by a calculation 
means 24, which is functionally connected to said median filter, and said 
signal is favorably stored and displayed in storage and display means 26. 
Said median filtering means 22 and calculation means 24 may also favorably 
be combined, as indicated by a line of dots and dashes, whereupon they 
will simultaneously handle the same signal. Further, the operation of the 
blinking lights 36, 38 and the general lights 34 are controlled by 
incentive control means 30. All the sequences performed by these means, 
except for the amplifying phase performed by the amplifier 14, can 
preferably be performed programmatically using a microcomputer. 
FIG. 3 schematically shows a favourable apparatus embodying the invention, 
said apparatus being generally indicated by the reference 10. Said 
apparatus comprises electrodes 12, which are functionally connected to 
EKG-amplifiers 14. Usually one such amplifier is needed for each eye. Said 
EKG-amplifiers 14 are functionally connected to a selector device 16 which 
preferably is connected to an A/D converter device 18. Said apparatus 10 
further comprises a computer 20, which is functionally connected to said 
converter 18 and comprises said median filtering means 22, said transition 
calculator means 24 as well as said storage and display means 26. Said 
computer 20 further comprises said incentive control means 28, which means 
are functionally connected to the actual incentive controller 30. Said 
controller 30 controls the function of said general light 34 which is 
functionally connected to said blinking incentive lights 36, 38 and a 
relay 32. 
Said amplifiers 14 must be completely isolated, i.e. none of the three 
electrodes may have a fixed potential. This is very important with respect 
to the patient security. 
Another important feature is that said amplifier 14 comprises three 
electrodes 12. Two of said electrodes are differential electrodes, between 
which the potential difference is measured. The third electrode is an 
active zero electrode which feeds the measured signal back to the patient, 
which feature will decrease the measured noise level. This especially 
attenuates the strong 50 Hz signal. 
A test arrangement set up according to FIG. 3 comprised two amplifiers, 
both being Kone 521 EKG amplifiers. in order to reduce the number of 
electrodes needed the active zero electrodes of each amplifier were 
interconnected. This is possible since said electrodes are completely 
isolated. Said test arrangement further comprised a notch filter tuned for 
50 Hz. 
The amplification factor for the amplifiers used is 2000. This is quite 
suitable because a typical potential difference in an EOG examination is 
about 1 mV. Thus the amplifier's output voltage is between -2V and +2V. 
The interface card for the computer included in the test apparatus 
comprises several functions when an EOG value is measured. It stores all 
measured results, controls said light sources and acts as voltage supply 
for said EKG amplifiers. 
The used computer interface card had the following properties: 
a two channel A/D converter having high impedance inputs 
a power source (60 V.sub.pp 20 KHz) for the isolation transformers in the 
amplifier 
a TTL level signal output for controlling the light source and 2 outputs 
for controlling the LEDs 
a direct connection to an IBM PC or a PC/AT ISA data bus. 
Since this kind of ready made cards could not be obtained said card was 
constructed and composed according to the above disclosed principles on an 
IBM card having a ready connection to the ISA data bus and an address 
encoding logic. For the person skilled in the art it is evident that the 
processing arrangement according to the invention as such can be performed 
in several different manners. 
The next task comprises the definition of the size of the potential leaps 
from the EKG signal. For human beings this is not any especially difficult 
task, since humans have a rather developed pattern recognition ability and 
thus a slight noise in the measured signal does not significantly disturb 
the recognition process. Unfortunately, a computer cannot perform such a 
pattern recognition, and thus mathematical rules based on statistical 
methods must be deduced for this purpose. 
As already is mentioned, the leaps are quite evident for a human being. 
Unfortunately they cannot be found at standard locations, since each 
patient will move his eyes at a slightly different speed. Thus, each step 
must be found independently. 
FIG. 4b discloses the signal after derivation, which signal in FIG. 4a is 
disclosed without a median filtering. The derivation is performed in order 
to define the points where the patient moves his eyes. As evident from 
FIG. 4b a random noise will produce a derivative which is undulating to 
quite a high extent and which has very little regularity, except for a 
rather clear initial signal. From this curve it is difficult even for the 
human pattern recognition ability to find similarities with the actual 
signal curve. Thus, it is important to reduce the disturbing amount of 
noise. 
In FIG. 5 one derivated signal peak is disclosed. As is evident from this 
figure the median filtering broadens the peak producing a clearly 
triangular form. The processing of said peak starts with the defining of 
the maximum of said peak. Here the problem is to find a local maximum 
x.sub.p. The task is not difficult if the signal is as clear and clean as 
the signal shown in FIG. 5, but in practice the patient's eyes sometimes 
"get lost", i.e. they look somewhere else and not at the light source, 
and-this produces small local maxima. These error maxima are excluded from 
the real maxima in a manner to be disclosed hereafter. 
It is known that the real peaks come in a regular order, i.e. a maximum 
must follow a minimum and vice versa. Further, it is clear that even if 
the exact eye movement interval is unknown the number of maxima and minima 
during the light blinking is in the range from n-1 to n+1. In other words 
it is known where the peaks should be located, i.e. they should be at the 
same location where the light source flashes. It must of course be 
understood that the peaks cannot be exactly at that location since due to 
the human reaction time the patients eyes react slightly behind the light 
source. This problem is removed by assuming a window around the location 
of the peak, the length 1 of said window being identical to the 
illumination time for one light source. Further, it is preferable to 
locate the center of the window slightly behind the theoretical maximum 
since it is much more probable that the patient will move his eyes 
slightly after the light source than before it. 
Here it should further be analyzed whether the first window comprises a 
maximum or a minimum. The signal phase can be investigated by modulating 
it with a "window signal". This is accomplished by first assuming the 
coming signal to be a maximum, and setting the windows accordingly. 
Thereafter the signal sample peak values are multiplied with the value 1 
in the maximum windows and with the value -1 in the minimum windows. If 
the average of the values thus calculated is positive the assumption of 
the first coming maximum was correct, and if the average becomes negative 
the first window was set at the location of a minimum. 
Thus, the exact positions of the peaks and their maxima x.sub.p are 
obtained. The next step comprises the exact position of the edges of the 
triangular area of the peak. The rest noise in the signal makes the 
definition of the signal's edges using the zero points impossible. 
Moreover, the last edge of the peak will usually be located farther from 
the zero point than the first edge. 
Here the form of the peak is used as a help. After the median filtering the 
peak should have straight edges. In FIG. 5 the edges of such a theoretical 
ideal peak is indicated in phantom line between said maximum x.sub.p and 
zero points x.sub.e. From said figure it is evident that if the average of 
a local maximum x.sub.p and a point x.sub.e is half of the maximum value 
said point x.sub.e is a zero point for said peak. In practice, finding the 
edges and zero points starts from a maximum value and the samples are 
analyzed one after the other proceeding away from said maximum, until the 
average of the investigated samples is half the maximum value. A straight 
line through this point and said maximum point defines the zero point 
x.sub.e for the peak, which point is needed in the calculations. It should 
be observed that even if it usually is favorable to calculate said average 
point for only one of the triangle's sides and thereafter assume the 
triangle to be an isosceles due to signal symmetry, also the average value 
for the first edge can be calculated and then the triangle need not 
necessarily be an isosceles. 
Using the maximum points x.sub.p and the zero points x.sub.e obtained in 
accordance with the above, the area of the triangle peak can easily be 
calculated. Since the peak represents the derivative for the measured 
signal the potential leap height can be obtained by integrating over the 
peak area. This can preferably be done by summing the signal values 
between said points x.sub.e and then the sum obtained corresponds to the 
height of one leap. This equation can be expressed as: 
##EQU2## 
where a=the leap size 
x=the derivative signal 
s and e are the edges of the peak. 
FIG. 6 shows some measured signals (FIGS. 6A, 6B, 6C and 6D) and 
derivatives (FIGS. 6A', 6B', 6C', and 6D') obtained therefrom using 
different median filtering lengths 1. As can be seen the obtained 
derivatives have a very symmetrical form. There is also considerably less 
rounding of the edges than can be achieved by conventional low pass 
filtering. It is easier to observe peaks having a regular form and regular 
edges than it would be in case of a low pass filtered signal. In the 
example case a median filter length of 1=5 was chosen since it will 
eliminate most of the noise but hardly distorts the form of the signal at 
all. 
Now there is obtained the size of about 2*30*10 peaks, which will be formed 
for two eyes during 30 minutes from values taken 10 times a minute. Now a 
representative for each minute should be calculated from these ten values. 
FIG. 7 shows as an example the sizes of potential leaps calculated from 
values for one minute as measured during an actual test series. It is 
probable that the first value is not reliable. If the measuring series 
comprise one or more clearly erroneous results like that above, one cannot 
simply take an average value of the measured series and define it to 
represent the whole one minute measuring series. 
Such error values cannot be eliminated by set threshold values, since the 
real values vary a lot. One favorable method that has proven to be 
sufficiently exact is to search for n samples which are as close to each 
other as possible. The integer n should be sufficiently small in order to 
eliminate erroneous samples. In tests a value for n corresponding to half 
the amount of the measured values has proven advantageous. If the number 
of erroneous values is more than half the amount it is rather impossible 
with any method to find the real values. 
The n specimen of values to be obtained are the points which have the 
smallest standard deviation and they can be found as follows: 
First the average value of the whole set of points is calculated. 
Thereafter the most remote-point is removed. After this the average value 
is calculated for the remaining set of points and again the remotest point 
is removed. This procedure is repeated until there are n points left. 
Hereafter the average of this set of points represents the whole set of 
points. 
In this manner the number of values needed for the calculation of the 
coronea-retinal potential is reduced to 2*30 values. In the next step the 
EOG ratio is calculated from these values. FIG. 8 shows a curve 
representing such values in a test examination. 
Often it is favorable to define the EOG ratio by looking for the curve 
minimum in the dark interval between 5 and 12 minutes. A minimum found 
should be checked with respect to its neighbor values in order to 
ascertain its reliability. A maximum value is obtained in a corresponding 
manner but from the light test period area. From the maxima and minima 
obtained the EOG ratio is finally calculated, printed and stored at some 
preferred means. 
Until now an automatic determining of clearly expressable quantities from 
such signals has proven to be very difficult and unexact. However, the 
method according to the invention brings about an essential improvement. 
According to the invention there is now utilized a median filtering, where 
the signals firstly are suitably digitized in order to exploit all the 
benefits of the median filtering. Median filtering is namely a very 
effective way to reduce such random deficiencies which depend on the 
environment and even on the examined patient's brain functions, which 
deficiencies in the prior art have constituted a real problem. 
The median filtering or calculating the moving average value of the signals 
is performed by defining a new value for each new point utilizing the 
average of its neighboring points. This filtering method reduces the noise 
very efficiently, since the average value for the random noise is zero 
over the infinite interval. Of course, the interval used in the practical 
solutions is not infinite, but the filter still attenuates the noise very 
well. The noise amplitude will roughly be attenuated by a factor 1 (i.e. 
the length of the filter). 
The formula of the median filter is 
##EQU3## 
All filters that eliminate noise also destroy some significant information. 
Also this filter rounds the signal edges, but the length of this rounding 
is rather short since only the points in near proximity affect the 
filtered point. This is because the points further than 1 from the point 
to be filtered cannot have any effect on the result. In this respect a 
median filter decisively differs from e.g. a low pass filter where all 
preceding points affect the filtered point. 
In order to obtain a signal having clear peaks the signal now must be 
processed. In a computer this is preferably done in real time so that the 
number of operations is minimized, and thus the effect simultaneously is 
maximized. An effective method for reducing the number of operations is 
often to perform them simultaneously. In this case this is favorably done 
by combining the median filtering and the differentiating. In the 
following formulae x is the original signal, y is the median filtered 
signal and dy is the derivative of the median filtered signal: 
##EQU4## 
When these formulae are combined the following formula is obtained: 
##EQU5## 
which for most of the practical embodiments can be reduced to: 
EQU z.sub.1 =x.sub.1+l -x.sub.i-l-1 (D) 
The next step in the signal processing is to find the maxima and minima of 
the derivative signal, and the first problem then is to find local maxima. 
The task would not be difficult if the signal were completely clean but in 
the signal in practice there usually are small false maxima, due to the 
fact that the patient's eye sometimes "gets lost", i.e. it does not 
actually look directly at the light source 36, 38. However, the correct 
maxima should be found, and this is performed in the manner described 
above. When the maxima are found according to the above the edges of the 
peaks are defined and finally the amplitude size is formed using 
integration, as above, has been described in greater detail. As the signal 
in question is a digitized one this can normally be easily performed by 
summing all signal values between the beginning and end points of a peak 
value. 
Thereafter the EOG ratio must still be calculated on the basis of a set of 
values consisting of several peak values. In this set there probably still 
will exist also such values which for some reason do not represent typical 
values but rather should be considered as errors. For this reason those 
values which are considered to best correspond to the des-red properties 
are separated for the calculation. According to a simple solution this 
elimination is performed using always smaller subsets so that the average 
of the remaining set is calculated and the farthest value is removed 
therefrom until suitably about the half of the original values are left. 
From this subset of values the EOG ratio is now calculated in a manner 
known per se. 
Above a preferred embodiment of the invention has been disclosed, but for 
the person skilled in the art it is clear that the invention can be varied 
and adapted in many other ways within the scope of the appended claims. 
In addition to the above the enclosed program utilized in the example 
embodiment is included within this presentation. 
##SPC1##