Patent Publication Number: US-2009234242-A1

Title: Multi-Channel EEG Electrode System

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a multi-channel EEG electrode system. In particular, the invention relates to electrodes of such system, an information indication device for the electrodes and a position localizing system. 
     Electroencephalography is a neurophysiologic measurement of electrical activity of the brain by recording from electrodes placed on the scalp. The resulting traces are known as an electroencephalogram (EEG) and represent an electrical signal (postsynaptic potentials) from a large number of neurons. Electrical currents are not measured, but rather voltage differences between different parts of the brain. 
     In a conventional scalp EEG, recording is obtained by placing electrodes on the scalp with a conductive gel, usually after preparing the scalp area by light abrasion to reduce impedance. Some EEG systems use a fabric cap into which the electrodes are imbedded. 
     Moreover, EEG topography is a neuroimaging technique in which a large number of EEG electrodes are placed onto the head, following a geometrical array of evenly spaced points. A special software plots the impedance of electrodes (electrical conductance) on a computer screen or printer, by coding the values in several tones of color. The spatial points lying between electrodes are calculated by mathematical techniques of interpolation (calculating intermediary values on the basis on the value of its neighbors), and thus a smooth gradation of colors is achieved. 
     BRIEF SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a multi-channel EEG electrode system which overcomes various disadvantages of the heretofore-known devices and methods of this general type and which further improves the prior art devices and methods. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a device, comprising: 
     an indicating unit configured to indicate information; and 
     a connecting unit configured to connect the indicating unit to an electrode operable to sense an EEG signal; 
     wherein the information is indicated at a position at which the electrode is placed. 
     In accordance with an added feature of the invention, the electrode has a circuit board and the connecting unit is configured to connect the indicating unit to the circuit board of the electrode. 
     In accordance with an added feature of the invention, the electrode has a casing and the connecting unit is configured to connect the indicating unit to the casing of the electrode. 
     In accordance with an added feature of the invention, the device further comprises an interfacing unit configured to interface the indicating unit with an external apparatus, and the indicating unit is configured to receive instructions from the external apparatus and indicate the information based on the instructions. 
     In accordance with an added feature of the invention, the indicating unit is configured to indicate the information based on measurement signals output by the electrode. In accordance with a preferred embodiment of the invention, the measurement signals represent impedance measurement results from an impedance measurement. Preferably, the measurement signals represent EEG measurement results. 
     In accordance with again an added feature of the invention, the information is visual display information, audio information, vibration information, and/or radio information. 
     With the above and other objects in view there is also provided, in accordance with the invention, an electrode operable to sense an EEG signal, comprising: 
     a circuit board; 
     a pin connected to the circuit board; 
     an indicating unit configured to indicate information; and 
     a casing enclosing the circuit board and the indicating unit in a water-proof manner and enabling the information to be provided outside of the casing, the casing having a cylindrical hole passing therethrough, the hole being configured to receive an agent and to direct the agent to the pin. 
     In accordance with a concomitant feature of the invention, there is provided a connecting unit configured to detachably connect the electrode to a plug connector. 
     With the above and other objects in view there is also provided, in accordance with the invention, a plug connector, comprising: 
     a plurality of plug connection units each configured to detachably connect to a connecting unit of an electrode; and 
     a multiplexing unit configured to receive input signals from the plurality of plug connection units, and to multiplex the input signals into an output signal. 
     With the above and other objects in view there is also provided, in accordance with the invention, a system, comprising: 
     a plurality of electrodes operable to sense an EEG signal, the electrodes being arranged in a three-dimensional pattern and each including an indicating unit configured to display information at a position at which the respective the electrode is placed; 
     an image sensing device configured to acquire stereoscopic images of the plurality of electrodes; 
     a control device configured to sequentially cause the indicating unit of each electrode to display the information and simultaneously cause the image sensing device to acquire the stereoscopic image of the respective the electrode; and 
     a processing device configured to calculate position information of each electrode of the plurality of electrodes from the stereoscopic images. 
     With the above and other objects in view there is also provided, in accordance with the invention, a system, comprising: 
     a plurality of electrodes operable to sense an EEG signal, the electrodes being arranged in a three-dimensional pattern and each including an indicating unit configured to transmit information at a position at which the electrode is placed; 
     a sensing device configured to acquire the information; 
     a control device configured to sequentially cause the indicating unit of each electrode to transmit the information and simultaneously cause the sensing device to acquire the information; and 
     a processing device configured to calculate position information of each electrode of the plurality of electrodes from the information. 
     Once more in sum: The invention provides for a device that indicates information on measurement results derived by using an EEG electrode in a manner such that a testing person can easily be provided with this information. Further, there is provided a water-proof EEG electrode. According to an additional embodiment of the invention, there is provided a system that localizes positions of electrodes placed, say, on a head without requiring intervention of a testing person. In accordance with another embodiment, there is provided a plug connector that enables easy replacement of a damaged electrode. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in multi-channel EEG electrode system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a schematic diagram illustrating an electrode cap worn by a test person; 
         FIG. 2  are perspective views of an electrode operable to sense an EEG signal according to an embodiment of the invention; 
         FIG. 3  shows an exterior view of the electrode operable to sense an EEG signal according to an embodiment of the invention; 
         FIG. 4  shows a schematic block diagram of the electrode according to an embodiment of the invention; 
         FIG. 5  shows a schematic block diagram illustrating an information indication device according to an embodiment of the invention; 
         FIG. 6  shows a plan view of an internal structure of the electrode according to an embodiment of the invention; 
         FIG. 7  shows a schematic block diagram illustrating an EEG system according to an embodiment of the invention; 
         FIG. 8  shows a plug connector according to an embodiment of the invention; 
         FIG. 9  shows a schematic block diagram illustrating an EEG system according to an embodiment of the invention; 
         FIG. 10  shows weak spots of an electrode; 
         FIG. 11  shows a mold including an electrode for melt casting; 
         FIG. 12  shows a schematic block diagram illustrating an 128-channel EEG system; 
         FIG. 13  shows a schematic diagram illustrating impedance measurement; 
         FIG. 14  shows a schematic block diagram illustrating a position detecting system according to an embodiment of the invention; 
         FIG. 15  shows a schematic block diagram illustrating a position detecting system according to an embodiment of the invention; 
         FIG. 16  shows a schematic diagram illustrating an EEG system according to an embodiment of the invention; and 
         FIG. 17  shows a schematic diagram illustrating an EEG system according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to an embodiment of the invention, active electrodes are used in a multi-channel EEG electrode system for measuring electrical activity of the brain. Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, the electrodes may be inserted in a cap worn by a test person as shown in the figure, or attached separately to the subject&#39;s head, whose electrical activity of the brain is to be measured. 
     An active electrode may comprise circuitry for adapting an input impedance of, say, 200 MOhm or more to an impedance working range of, say, 1 to 120 kOhm. By decreasing the output electrode impedance motion artifacts and interferences from external sources such as power lines, etc. are reduced, which results in a higher signal-to-noise ratio. 
     An electrode  10  according to an embodiment of the invention is shown in  FIGS. 2 and 3 .  FIG. 2  show top and side/bottom views of the electrode  10 , and  FIG. 3  shows a more schematic exterior view of the electrode  10  comprising a pin  11  which contacts with a scalp and a hole  12  for inserting an agent such as a conductive gel in order to provide contact between the scalp and the pin  11 . Circuitry of the electrode  10  is included in a casing  13 . 
     The electrode  10  may comprise an information indication device  20  as schematically shown in  FIG. 4 . According to an embodiment of the invention, as schematically shown in  FIG. 5 , the device  20  comprises an indicating unit  21  for indicating information, a connecting unit  22  and an interfacing unit  22 . The connecting unit  22  may connect the indicating unit  21  to the electrode  10  such that the information is indicated at a position at which the electrode  10  is placed. The device  20  may comprise a display device such as a Light Emitting Diode (LED), a Liquid Crystal Device (LCD), etc., or an output device outputting audio signals or vibration signals, or a combination thereof. According to an embodiment, the signals output by the device  20  are receivable by a testing person. 
     It is to be noted that the arrangement of the functional blocks of the device  20  is not construed to limit the invention. 
     According to an embodiment of the invention, the connecting unit  22  connects the indicating unit  21  to a circuit board  14  of the electrode  10  schematically shown in  FIG. 6 . 
     Alternatively, the connecting unit  22  connects the indicating unit  21  to the casing  13  of the electrode  10 . In this case, commercial electrodes may be used and attached to a subject&#39;s head, which have the indicating unit  21  according to the invention attached. A commercial EEG software may calculate impedance values. According to an embodiment of the invention, based on the calculated impedance values instructions are provided to the indicating unit  21  using a control unit  832  as described below in connection with  FIG. 9 . The indicating unit  21  may be connected to the (commercial) electrode in a permanent manner such that it is not required to remove the indicating unit  21  from the electrode for cleaning, for example. 
     The interfacing unit  23  may interface the indicating unit  21  with an external apparatus  830  shown in  FIG. 9 , such as a Personal Computer, Workstation, etc. The interfacing unit  23  may comprise a Universal Serial Bus (USB). The interfacing unit may also comprise the control unit  832  as shown in  FIG. 9 . 
       FIG. 16  shows a schematic diagram illustrating an EEG system according to an embodiment of the invention, in which an LED  161  serving as indicating unit is mounted on top of an electrode  162 . The LED receives impedance information from a control box (not shown in  FIG. 16 ) which in turn may receive the impedance information from a PC (not shown in  FIG. 16 ). The LED  161  illuminates in accordance with the impedance information. 
       FIG. 17  shows a schematic diagram illustrating an EEG system according to an embodiment of the invention, in which an LED  171  serving as indicating unit is provided in an electrode  172  together with a sensor  173  which is involved in impedance measurement. A control box  174  calculates impedance based on the impedance measurement results from the sensor  173  and transmits impedance information based on stored levels (to be described below) to the LED  171 . The LED  171  illuminates in accordance with the impedance information. 
     As shown in  FIG. 3 , the electrode  10  may further comprise a connecting unit  16 , such as a cable having three lines and a shielding, for detachably connecting the electrode  10  to a plug connector  80  as shown in  FIG. 8 . The plug connector  80  comprises a plurality of plug connection units  81  each detachably connecting to a connecting unit  16  of an electrode  10 , and a multiplexing unit  82  which receives input signals, i.e. EEG signals, from the plurality of plug connection units  81 , and multiplexes the input signals received into an output signal. 
     As shown in  FIG. 7 , the splitter  731  acting as plug connector  80  receives signals from electrodes or channels Ch 1  . . . Chn as well as Gnd and Ref signals from ground and reference electrodes. The splitter comprises a chip (multiplexing unit  82 ) which multiplexes the received signals or lines onto an output unit such as a ribbon cable as shown in  FIG. 8 , comprising lines which are fewer in number than the received lines. 
     With the plug connector  80  shown in  FIG. 8 , the electrodes  10  can be detachably connected to plug connection units  81 . Thus, a damaged electrode can be replaced in an easy manner. 
     As shown in  FIG. 7 , from the splitter  731  the multiplexed lines or signals are fed to a control unit  732  which outputs analogue signals to an EEG amplifier  733  which converts the analogue signals to digital data which are fed to a control and recording entity  730  which may act as the external apparatus. 
     The control and recording entity  730  and the control unit  732  may be connected via a USB line for controlling and/or powering the control unit  732 . The USB line shown in  FIG. 8  may act as interfacing unit  23 . The EEG amplifier  733  may be connected to the control and recording entity  730  via an optical waveguide. 
     The indicating unit  21  may receive instructions from an external apparatus and indicate the information based on these instructions. 
       FIG. 9  shows a system according to an embodiment of the invention in which the indicating unit  21  receives instructions from a recording entity  830  via a control unit  832  which is connected via USB with the recording entity  830 . The recording entity  830  outputs the instructions based on signals provided by an EEG amplifier  833 . In other words, the recording entity  830  comprises a software for calculating impedance values from signals provided by the EEG amplifier  833  which will be described in greater detail below. Based on the calculated impedance values instructions are calculated and, using the USB connection and the control unit  832 , provided to the indicating unit  21 . The instructions may be provided from the control unit  832  to the indicating unit  21  using a wireline or a wireless connection. 
     It is also possible to calculate the impedance values in the control unit  832 . 
     Alternatively or in addition, the indicating unit  21  may indicate the information based on measurement results provided by the electrode  10 . The measurement results comprise impedance measurement results from an impedance measurement to be described by referring to  FIG. 13 . In other words, the electrode  10  may comprise a circuit for calculating the impedance values inside the electrode and the indicating unit  21  may indicate the information based on the calculated impedance values without feedback from the control unit  832 . 
     Alternatively or in addition, the measurement results comprise EEG measurement results. 
       FIG. 6  shows a plan view of an internal structure of the electrode  10  according to an embodiment of the invention. As shown in  FIG. 6 , the electrode  10  comprises the circuit board  14 , the indicating unit  21 , and the hole  12  which in this embodiment passes through the circuit board  14 . However, it is to be noted that the invention is not limited to an arrangement in which the hole  12  passes through the circuit board  14 . 
     The casing  13  shown in  FIG. 3  may enclose the circuit board  14  and the indicating unit  21  in a water-proof manner and such that the information is provided to the outside of the casing  13 . For example, in case the indicating unit  21  is connected to the circuit board  14  and comprises a display unit providing display signals, the casing  13  should be transparent. The hole  12  passing through the casing  13  is of cylindrical shape in order ensure watertightness of the electrode  10 . When forming the casing  13  to enclose the circuit board  14  e.g. by casting, the circuit board  14  may be dislocated although it is held in a holder during the casting. By using the cylindrical shape of the hole  12  the casing  13  can be formed to completely enclose the circuit board  14 . 
       FIG. 10  shows weak spots of the EEG electrode  10  which may result from forming the casing  13 . In addition to the hole as described above, weak spots may be present at residues of holding pins used during casting, and at material interfaces e.g. between the pin  11  and the cable  16  and the material used for casting. For avoiding the weak spots, according to an embodiment of the invention a melt casting technique is used for forming the casing, in which polyurethane is used which is generated in a mold by polyaddition. 
       FIG. 11  shows a schematic view of the casing of the electrode formed inside the mold. In the melt casting technique adopted according to an embodiment of the invention, two plastic materials are poured into the mold made of tempered steel, in which circuit boards as schematically shown in  FIG. 6  have been inserted. For example, eight circuit boards may be inserted in one mold. After the plastic materials were poured into the mold, the plastic materials are cured inside the mold so that the polyurethane formed by polyaddition of the plastic materials encloses each of the circuit boards in a watertight manner. With the melt casting technique the casting material can be processed without requiring pressure. Moreover, the casting material compounds with the material of the electrode in a better way than done in die casting. In addition, with the melt casting no holding pins are necessary and no air bubbles are generated. Thus, the weak spots shown in  FIG. 10  can be avoided. 
       FIG. 12  shows a schematic block diagram illustrating a 128-channel EEG system  100 . In this system  100  four 32 channels active electrodes blocks  31   a ,  31   b ,  31   c  and  31   d  are shown. To each block  31   a - 31   d    32  electrodes are connected. The system  100  further comprises an active reference (REF) electrode aC-eg 1  with e.g. a 2 m cable, a ground (GND) electrode aC-eg 1  with e.g. a 2 m cable, and a 128 channels control box  32 . The electrodes each may be formed by the electrode  10  described above. 
     The blocks  31 - a - 31   d  are connected to the control box  32  using 1.5 m cables, for example. The electrodes aC-er 1  and aC-eg 1  are also connected to the control box  32  using the 2 m cables. The control box  32  receives EEG signal sensed by the 128 electrodes and outputs analogue EEG signals to an EEG amplifier  33  which converts the analogue EEG signals to digital EEG data which are fed to a PC  30  which may act as the external apparatus. The analogue EEG signals may be guided through an adapter  34  before entering the EEG amplifier  33 , where they are converted into signals which can be processed by the EEG amplifier  33 . 
     The PC  30  and the control box  32  may be connected via a USB line for controlling and/or powering the control box  32 . The USB line shown in  FIG. 5  may act as interfacing unit  23 . 
     The system  100  may comprise the following operation modes: sleep mode, acquisition mode, which can be performed in combination with an active shielding sub-mode, impedance measurement mode, and test signal mode. 
     The sleep mode is equivalent to a system off-state. In this mode the system  100  is waiting for a turn-on command from the PC  30  or can be activated by pressing a “Power” button. 
     The system  100  is going to the acquisition mode after turn-on. In this mode the system  100  transfers the signals from the electrodes  10  attached to a subject head to the external EEG amplifier  33 . The following table shows parameter values of the system  100  for the acquisition mode according to an embodiment of the invention. 
     
       
         
           
               
               
             
               
                   
               
               
                 Parameter 
                 Value 
               
               
                   
               
             
            
               
                 Amplification 
                 1 
               
               
                 Tolerance of amplification 
                 &lt;0.001% 
               
               
                 Differential and common input 
                 &gt;200 MOhm 
               
               
                 impedance 
               
               
                 Pass band 
                 0-5000 Hz 
               
               
                 Self noise (include sensors&#39; noise) 
                 &lt;2 μV p.p. for 0.1-35 Hz band 
               
               
                 Dynamic range 
                 ±1000 mV 
               
               
                 Self offset 
                 &lt;20 mV (including sensors&#39; offset) 
               
               
                   
                 measured in 0.9% saline 
               
               
                   
               
            
           
         
       
     
     In the active shielding sub-mode, inverted and gained voltage from the REF electrode aC-er 1  is injected to the GND electrode aC-eg 1  for common-mode noise compensation. In some cases this strongly decreases the common-mode voltage for an external EEG amplifier. 
     According to an embodiment of the invention, the impedance measurement mode can be selected from the acquisition mode, not directly from the sleep mode. Impedance is measured independently for each electrode, including REF and GND electrodes, by using a time separated method of current injection. 
       FIG. 13  shows a schematic diagram illustrating the impedance measurement. In  FIG. 6  a channel  1  corresponding to an electrode  10   1 , a channel N corresponding to an electrode  10   N , and a channel REF corresponding to the reference electrode aC-er 1  are illustrated. It is to be understood that similar channels are provided also for electrodes  10   2  to  10   N-1  of the N-channel EEG system. In the system  100  shown in  FIG. 12  128 channels or electrodes  10   1 - 10   128  are provided. 
     Each channel shown in  FIG. 13  comprises a measuring impedance circuit which includes a 33 kOhm resistor for limiting a patient auxiliary current. The 33 kOhm resistor is a parasitic resistor for the measuring impedance circuit. Moreover, each channel comprises a switch SW controlled by an MCU  30   a . The MCU  30   a  may be part of the PC  30  shown in  FIG. 12 . 
     Before measuring is started, the ground electrode aC-eg 1  is connected. 
     In a first step, the MCU  30   a  closes an electronic switch SW 1  of channel  1  or electrode  10   1 , so that current from the ground electrode aC-eg 1  will flow at this electrode only, as all another channels have high input impedance. 
     In a second step the MCU  30   a  causes a voltage source  35  to generate V sin =1V amplitude (U sin     —     rms =0.7V rms ) positive half-wave of SIN 30 Hz by Digital Direct Synthesis and inject current via an R mes  resistor from the ground electrode aC-eg 1  to a bioimpedance object (patient head). 
     At this moment, in a third step, the MCU  30   a  measures a voltage U mes  on the load (Rx1+RxGND+33 kOhm) and U ref  on the reference electrode REF (as high impedance input). Then, in a fourth step the MCU  30   a  opens the electronic switch SW 1  and in a fifth step calculates Rx1′=U mes /((U sin     —     rms −U mes )/R mes ). 
     If the electrode  10   1  of channel  1  and the REF electrode are connected (Rx1′ and RxREF′ are in valid range from 33 kOhm-15% to 153 kOhm+15%), RxGND′1 is calculated by the MCU  30   a  in a sixth step: 
         RxGND′ 1= Rx 1′−( U   ref /(( U   sin     —     rms   −U   mes )/ R   mes )). 
     In a seventh step, the MCU  30   a  waits for 2-4 msec. 
     The above-described steps 1-7 are repeated for all N channels and the REF electrode. 
     After steps 1-7 have been performed for all N channels and the REF electrode, the MCU  30   a  calculates RxGND=Sum (RxGND′1 . . . RxGND′N)/N, where N is the number of connected electrodes  10   1 - 10   N . Then, the MCU  30   a  calculates Rx1 . . . RxN as Rx=RxN′−RxGND−33 kOhm. 
     The following table shows parameter values for the impedance measurement according to an embodiment of the invention: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
             
            
               
                   
                 Impedance measurement 
                 30 Hz 
               
               
                   
                 frequency 
               
               
                   
                 Range 
                 0 to 120 kOhm 
               
               
                   
                 absolute tolerance 
                 &lt;±15% (in 1 to 120 kOhm range) 
               
               
                   
                 Injected current 
                 &lt;7.5 μA 
               
               
                   
                 Time of measuring cycle 
                 &lt;4 sec. for 128 channels 
               
               
                   
                   
               
            
           
         
       
     
     According to an embodiment of the invention, the indicating unit  21  comprises LEDs which are connected to the circuit board of each of the electrodes  10   1 - 10   N , or are attached to the casing of each of the electrodes  10   1 - 10   N . During the above-described impedance measurement the impedance values may be read via a USB-port by the external apparatus  30  and shown by illuminating the LEDs with different colors depending on measured values. After turn-on of the system  100  the default threshold levels and corresponding colors may be set by default to: 
     Green Color—impedance less than 10 kOhm 
     Yellow Color—impedance 10-50 kOhm 
     Red Color—impedance greater than 50 kOhm 
     According to an embodiment of the invention, these thresholds can be set by a command from the PC  30 . This setting may be stored into a nonvolatile memory. The LEDs may also be disabled by the PC  30 . 
     Illumination of the LEDs may be performed based on a command from the PC  30 , i.e. the PC  30  causes illumination of an LED of a corresponding electrode  10  with a specific color depending on the measured impedance of the corresponding electrode  10 . Alternatively, it is also possible to have an illumination control circuit in the electrode  10 , which causes the LED to illuminate in the specific color. The impedance values and corresponding color of each electrode  10  can be stored in the PC  30  for further processing. 
     The duration of the impedance measuring mode may be limited to 3 min. After this time-out the system  100  should switch to the acquisition mode. This duration can be changed by a command from the PC  30  and stored in the nonvolatile memory. 
     In the test signal generation mode a meander signal of 200 μV±2% amplitude and 1 sec duration is applied between the ground electrode aC-eg 1  and each electrode  10   1 - 10   N . 
     This mode may be used for testing the functionality of the system  100 , checking the system connection to the external EEG amplifier  33  and for testing/calibration of the external EEG amplifier  33 . For this purpose it is necessary to short-connect all electrodes by water immersion and set a monopolar acquisition scheme in the external EEG amplifier  33 . 
     By using the indicating unit  21  in connection with each electrode  10  of the system  100 , a testing person can easily recognize which electrode  10  has which impedance value, and is not required to search for the electrode on the patient&#39;s head by referring to a screen only on which the patient&#39;s head with the electrodes attached may be schematically displayed. 
     Moreover, the indicating unit  21 , e.g. the LEDs, may be driven by the external apparatus  30  in reaction to EEG signals acquired in the acquisition mode in order to indicate regions in the patient&#39;s head where the EEG signals have been generated. 
     Information indication by the indicating unit  21  using LEDs is not restricted to different colors. It is also possible to cause blinking of the LEDs with different frequencies depending on the impedance values measured by the respective electrodes. The indicating unit  21  also comprises any kind of display device including an LCD, a plasma display, etc. 
     As described above, the indicating unit  21  also is not restricted to displaying information. The indicating unit  21  may comprise any kind of output device which outputs signals which can—by a test person or a testing person—be associated with a position at which the signals are output. 
     Moreover, the indicating unit  21  comprises any kind of output device which outputs signals which can be recognized by an image sensing device. The image sensing device may comprise a digital camera. 
     According to a further embodiment of the invention, position of the electrodes of the system  100  is detected using a position detecting system  700  as shown in  FIG. 14 . The system  700  may comprise a plurality of electrodes  10  operable to sense an EEG signal, arranged in a three-dimensional pattern and each comprising the indicating unit  21  which, according to this embodiment, displays information at a position at which the electrode is placed. The plurality of electrodes  10  may be positioned on a patient&#39;s head  7 . The system  700  further comprises an image sensing device  70  which acquires stereoscopic images of the plurality of electrodes  10 , a control device  71  which sequentially causes the indicating unit  21  of each one of the plurality of electrodes  10  to display the information and simultaneously cause the image sensing device  70  to acquire the stereoscopic images of each one of the plurality of electrodes  10 , and a processing device  72  which calculates position information of each one of the plurality of electrodes  10  from the stereoscopic images. 
     It is to be noted that the arrangement of the functional blocks of the system  700  is not construed to limit the invention. For example, the functions of the control device  71  and the processing device  72  can be included in one apparatus. Moreover, the control device may be formed by the external apparatus  30 . 
     After acquiring the position information for each electrode  10 , the processing device  72  may compare the position information with reference position information and decide whether the acquired position information deviates. In case the acquired position information deviates, the electrode concerned may be re-positioned. Alternatively, the deviation is taken into account when electrodes measuring brain activity and locations of the activity in the brain are correlated. 
     According to an embodiment of the invention, the image sensing device  70  may comprise two or more cameras for taking two or more stereoscopic images from different positions. In case of fixed cameras it is preferred that four cameras are used to be able to take three images of each of the electrodes positioned over the patient&#39;s head  7  at different positions. 
     According to an alternative embodiment, the image sensing device  70  comprises one camera which is placed at different positions for taking the stereoscopic images. 
     The processing device  72  recognizes the information displayed by the indicating unit  21  in each stereoscopic image and identifies it as common point. A line of sight (or ray) can be constructed from the camera location to this common point. It is the intersection of these rays (triangulation) that determines the three-dimensional location of the common point and, thus, the position of the electrode whose indicating unit  21  displays the information. More sophisticated algorithms can exploit other information about the scene that is known a priori, for example symmetries, in some cases allowing reconstructions of 3D coordinates from only one camera position. 
     The position detection system  700  can be used with electrodes  10  comprising the indicating unit  21  inside or with electrodes  10  having the indicating unit  21  fixed to the casing after manufacture of the electrode. 
       FIG. 15  shows a position detection system  1500  according to an embodiment of the invention. 
     The system  1500  comprises six video cameras  150   a - 150   f  which are mounted on a rotatable and vertically adjustable stand (not shown). Calibration is performed using a calibration cube by means of software for adjusting position of the video cameras. After calibration, only common movement of the video cameras  150   a - 150   f  is allowed. 
     The video cameras  150   a - 150   f  are arranged such that at least two of the video cameras  150   a - 150   f  sense an electrode positioned at any position on a head. This is achieved by arranging the video cameras  150   a - 150   f  on the stand. The head has attached a plurality of electrodes, each comprising an LED as indicating unit  21 . For a photogrammetric survey each electrode on the head (the electrodes are shown as small circles on the head in  FIG. 15 ) is driven using a control unit  151  and a recording entity  152 . Driving an electrode means that the LED of this electrode is turned on to illuminate. 
     At first, four reference electrodes are surveyed. After survey of the four reference electrodes, these are kept in an on-state, i.e. in an illumination state. Thus, the head may be moved without impacting the result of the further survey. 
     The video cameras  150   a - 150   f  are synchronized e.g. using a cable. The video cameras  150   a - 150   f  simultaneously pick up images of a driven electrode from different perspectives and fed the images via the control unit  151  to the recording entity  152 . Each electrode is driven about 300 ms. 
     After conduction of the survey of all of the electrodes, which may be done automatically, position data of the electrodes are converted to a standardized sphere model using a least mean square fitting algorithm in order to obtain a scaling of the position data. The conversion may take place in the recording entity  152 . The result may be exported into an ASCII file and fed to some analysing programs performing e.g. source localization. 
     According to an alternative embodiment of the invention, position of an electrode on the head may be measured using GPS. In this case, the indicating unit of the electrode may be a sender transmitting radio signals. 
     It is to be understood that the above description of the embodiments of the invention is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.