fig2 illustrates one embodiment of an electrode array 10 according to the present invention . the electrode array comprises three measurement electrodes 1 to 3 of which the first and second electrodes are for measuring the eeg signal and the third and second electrodes for measuring the eog signal . the first electrode 1 is positioned onto the hairless front - lateral area of the frontal lobe of the patient , preferably as far as possible from the eye . the measurement electrode 2 is positioned similarly as the first measurement electrode , but on the opposite cortical hemisphere of the patient . furthermore , the first and second electrodes are positioned substantially horizontally ( i . e . in the same horizontal line ) and at substantially equal distances from the vertical center axis 12 of the face , i . e . the second measurement electrode is positioned onto the spot which is the mirror image of the spot of the first measurement electrode , and vice versa , the vertical center axis being the mirror axis . the eeg signal is measured from the measurement electrodes 1 and 2 . due to the symmetrical positions of the electrodes , the potential changes caused by vertical eye movements are substantially the same at both electrodes ( assuming that the eyes move similarly to each other , as is the case normally ). in other words , the potential changes caused by vertical eye movements tend to cancel in the eeg signal representing the voltage difference of the electrodes . the same applies to artifacts caused by blinks of the eyelids . thus , the eeg channel is in this case resistant to artifact caused by vertical eye movements , but remains sensitive to artifacts caused by horizontal eye movements . in the embodiment of fig2 , the third measurement electrode 3 and one of the first and second measurement electrodes is used for measuring the eog signal . for this purpose , the third measurement electrode is positioned so that the eog voltage measured between this electrode and one of the first and second electrodes is as high as possible in case of horizontal movement of the eyes . in the example of fig2 , the said one electrode is measurement electrode 2 , i . e . the eog voltage is measured between electrodes 3 and 2 . therefore , the third measurement electrode is positioned on the temple of the patient , on the hemisphere opposite to that of measurement electrode 2 . the temple here refers to the area between an eye and the ear on the same hemisphere as the eye . as to their internal structure , the electrodes may be similar to each other . fig3 shows an example of the eeg and eog signals measured by the electrode configuration of fig2 . the upper signal is the eeg signal measured between electrodes 1 and 2 and the lower signal is the eog signal measured between electrodes 3 and 2 . as can be seen from the figure , the eeg channel is sensitive to horizontal eye movement , but resistant to vertical eye movement , while the amplitude of the eog signal is high for the horizontal movement , allowing efficient detection of the artifact caused by horizontal eye movement in the eeg signal . fig4 illustrates one embodiment of a method for detecting eye movement artifact in connection with the electrode array of fig2 . as noted above , the eeg signal is measured between electrodes 1 and 2 , whereas the eog signal is measured between electrodes 3 and 2 . first , the eeg and eog channel signals are divided into successive epochs ( step 41 ). it is assumed in this example that the length of each epoch is one second and that each epoch contains 200 samples , i . e . the sampling frequency is 200 hz . for each epoch , the process then compares the two signals for example by determining the difference of the absolute values of simultaneous signal values . this may be done for each sample in an epoch . assuming that the said signal values are s eeg ( t i ) and s eog ( t i ), the process thus determines the difference | s eog ( t i )|−| s eeg ( t i )| for each sample t i within an epoch . the process then examines whether any of the differences fulfills a predetermined criterion indicative of the presence of eye movement ( step 44 ). in this example , the process examines whether any of the differences exceeds a predetermined threshold value , such as 30 μv . if this is the case , the epoch is flagged to indicate that it is contaminated by eye movement artifact ( step 45 ). various other techniques may also be used to test whether at least a predetermined number of the samples of an epoch fulfill the criterion indicative of the presence of artifact . above , the eeg electrode pair is thus positioned so that the eye movement in the vertical direction causes a substantially zero overall potential difference between the eeg electrodes 1 and 2 . additionally , electrode 3 is positioned so that eye movement in the horizontal direction causes a high potential difference between electrodes 3 and 2 , which act as the eog electrodes . however , as discussed below , the electrode array may also be configured so that the roles of the horizontal and vertical directions are reversed . furthermore , a fourth electrode may be provided for measuring the eog channel , and the electrode array may be provided with a ground electrode . the electrode array of the invention comprises at least three electrodes . two electrodes , i . e . the eeg electrodes , are positioned so that that eye movement in one of the vertical or horizontal directions causes a negligible ( i . e . a substantially zero ) overall voltage change in the eeg signal measured between the said electrodes , while the third electrode , i . e . the eog electrode , is positioned so that eye movement in the other of said horizontal and vertical directions causes a maximally high voltage ( potential difference ) between the third electrode and an auxiliary electrode , which is either one of the eeg electrodes or a fourth electrode positioned near the eye of the patient . fig5 illustrates one embodiment of the invention , in which the roles of the principal directions are reversed from those of the embodiment of fig2 . in the embodiment of fig5 , the first electrode 1 is positioned symmetrically with respect to the eyes onto the forehead of the patient , preferably as far as possible from the eyes . the first electrode thus lies on the vertical center axis 12 of the face . the second measurement electrode 2 is positioned on the mastoid behind the ear of the patient , whereas the third electrode 3 is positioned directly above the eyebrow . in this example , the third electrode is positioned on the same hemisphere as the second electrode and in line with the vertical axis 11 of the eye . however , the third electrode may also be positioned on the opposite hemisphere with respect to the second electrode 2 . as the second electrode is behind the ear on the mastoid , it is denoted with a dashed line in the figure . in this case the potential changes caused by horizontal movement of the right and left eye balls tend to cancel each other due to the symmetric location of the electrode . in other words , due to the location of the electrode , the potential change caused by the horizontal movement of one eye at electrode 1 is opposite to the respective potential change caused by the other eye ( assuming that the eye balls move similarly ). furthermore , horizontal eye movement does not cause any substantial potential change at the second electrode located on the mastoid , and therefore the horizontal eye movements tend to get cancelled in the eeg signal representing the voltage between electrodes 1 and 2 . the voltage measured between the third electrode and electrode 2 is in this embodiment maximally high for vertical eye movements and blinks , thus allowing effective detection of the respective artifact in the eeg signal . a method according to fig4 may be used for detecting the artifact caused by eye movements . as discussed above , one of the eog electrodes may be a combined eeg / eog electrode . however , the electrode array may also comprise a dedicated eog electrode pair . fig6 illustrates a four - electrode configuration comprising an additional eog electrode 4 as compared to the embodiment of fig2 . the fourth electrode is positioned directly above the eyebrow on the same hemisphere as electrode 3 and preferentially on the opposite side of the vertical axis of the eye with respect to electrode 3 . in this embodiment , the eeg signal is measured between electrodes 1 and 2 as in the embodiment of fig2 , but the eog signal is now measured between electrodes 3 and 4 . as in the embodiment of fig2 , the eog voltage is high for horizontal eye movements , while the eeg channel is resistant to vertical eye movements and remains sensitive to horizontal eye movements . fig7 illustrates a four - electrode configuration comprising an additional eog electrode 4 as compared to the embodiment of fig5 . the fourth electrode is positioned directly below the same eye above which the third electrode is , so that the electrodes are substantially in line with the vertical axis 12 of the eye . as in the embodiment of fig5 , the eeg signal is measured between electrodes 1 and 2 , whereas the eog signal is now measured between electrodes 3 and 4 . the eog voltage is maximally high in case of vertical eye movements , and the eeg channel is resistant to horizontal eye movements . additionally , a ground electrode ( gnd ) providing a common ground potential ( 0v ) for the channels may be positioned to any location . fig8 illustrates the electrode array of fig2 provided with a ground electrode 5 positioned between the eeg electrodes 1 and 2 . as obvious from the above , the electrode array may comprise distinct electrodes or two or more of the electrodes may be mounted on a thin and flexible substrate made of plastic material , for example . fig2 and 8 show three and respectively four electrodes integrated onto the surface of a strip - like substrate 100 comprising two portions ; a first portion between electrode 3 and electrode 1 and a second portion between electrode 1 and electrode 2 . the first portion may be slightly shorter than or substantially as long as the second portion to enable the electrodes to be positioned in the above - described manner . for example , the length of the first portion may be about 8 cm and that of the second portion about 10 cm . the first portion may be set to an angle relative to the second portion , as is shown in fig2 and 8 . the angle may be , for example , about 130 degrees . furthermore , the second portion may be provided with a mark indicating the center line between electrodes 1 and 2 , whereby the nursing staff may align the said mark with the vertical center line of the face . alternatively , the ground electrode may indicate the center line between electrodes 1 and 2 , as is shown in the embodiment of fig8 . in this case the electrode array may be attached to the patient by first attaching the ground electrode onto the vertical center line of the face . each electrode of the array is further provided with a respective connector connecting the electrode to a terminal ( not shown ) normally manufactured to the free end of the strip - like substrate . the terminal may be connected with a mating terminal at the end of a measurement cable ( not shown ) connected to the amplifier state of the measuring apparatus . the connectors may be printed on the substrate . although the invention was described above with reference to the examples shown in the appended drawings , it is obvious that the invention is not limited to these , but may be modified by those skilled in the art without departing from the scope of the invention .
the invention relates to a method for positioning electrodes in an electrode array comprising a plurality of electrodes for central nervous system monitoring from the forehead of a subject . in order to enable efficient detection of eye movement artifacts with a minimum number of electrodes , first and second electrodes are positioned so that eye movement in one of the horizontal or vertical directions causes a negligible overall voltage change between the first and second electrodes and at least a third electrode is positioned so that eye movement in the other of said horizontal and vertical directions causes a high potential difference between the third electrode and an auxiliary electrode , wherein the auxiliary electrode is one of the electrodes in a set comprising one of the first and second electrodes and a fourth electrode . the first and second electrodes are arranged to record an eeg signal and the third electrode and the auxiliary electrode are arranged to record an eog signal indicative of eye movements of the subject .