- Email: [email protected]
Technique to display topographical evolution of EEG events
Eleetroencephalograpt~; and clinical Neurophysiology, 1984, 58:565-568
565
Elsevier Scientific Publishers Ireland, Ltd.
Technical Section S h o r t communication TECHNIQUE
TO DISPLAY
TOPOGRAPHICAL
EVOLUTION
OF EEG EVENTS
JOHN F. LEMIEUX I ROBERT S. VERA and WARREN T. BLUME
Epilepsy Unit, Department of Clinical Neurological Sciences, University Hospital, London, Ont. N6A 5.45 (Canada) (Accepted for publication: July 13, 1984)
Although it has been several years since the introduction of computers to the task of automating the collection and analysis of the electroencephalogram, the standard paper print-out is still the major tool of the electroencephalographer. With this medium the clinician can gather information about the frequency of voltage variations and the difference in voltage variations between different points on the scalp; however, it is quite difficult in many instances to transform the information recorded as a group of lines on paper into a mental image of the potential field on the scalp. Moreover, in determining the direction and time delays of potential field movement, estimates are prone to error due to the limitations of the recording device. There have been several attempts to represent the spatialtemporal aspects of EEG potentials in a comprehensible fashion. One of the first efforts was by Walter and Shipton (1951) who developed a toposcope: an electronic apparatus demonstrating the spatial aspects of the EEG. R6mond (1968), Estrin et al. (1969), Shaw (1970) and Lehmann (1971) developed systems to create 2-dimensional displays of the isopotential field of the EEG. Petsche et al. (1975) approached the spatialtemporal analysis by not only producing isopotential maps but also maps of their first derivatives to study the time relationships within potential fields. A spatial interpolation procedure described by Harris in 1967 was the basis for the system developed by Harris et al. (1969) and Daube and Lake (1972) which generated 3-dimensional (3-D) displays of the spatial contours of the EEG for each instant in time. We found that 3-D displays more readily projected the information about field distributions than 2-D isopotential maps. Both their system and ours allow the observer to easily orientate the displays in vertical and horizontal positions for a better view of the field distributions. Movie-like displays were created by Harris et al. (1969) by rapidly displaying moment-to-moment spatial maps of the EEG. Dynamics for Daube and Lake's (1972) system were created by
1 Reprint requests to: Mr. John Lemieux, Epilepsy Unit, University Hospital, 339 Windermere Road, London, Ont. N6A 5A5, Canada.
photographing successive plots of the field distributions and creating a motion picture by rapid playback of the photographs. Dynamic displays are more effective in aiding the observer to comprehend the movement of potential field distributions. Applying the same fundamental principles, we have developed a system to sample and generate from the EEG 3-D displays in a dynamic as well as static mode. Our field plotting system (FLD), which is more flexible and simpler to operate than previous systems, provides a comprehensible alternative to analyzing the field development and propagation of EEG potentials. It is a totally interactive analysis program that not only allows one to study the temporal-spatial aspects of the field distributions with a variety of orientations, but also allows one to easily scan the recorded EEG and select the variable size epochs that are to be analyzed in this fashion.
System layout The field plotting program (FLD) currently operates on a computer system with 256 kbytes of memory, two 14 Mbyte disk drives, a digital magnetic tape drive, a graphics display oscilloscope with direct memory access (DMA), a pen plotter, and a powerful laboratory interface consisting of 64 analog-todigital converters (A/Ds), 32 digital input/output channels and associated Schmitt triggers." One disk is capable of holding 35 min of 16-channel EEG sampled at 200 c / s e c per channel. By exchanging disk packs and utilizing the magnetic tape drive, one can greatly increase the storage capacity. Allowable sample rates range from 100 to 1000 c / s e c per channel with sample durations from 1 to 60 sec. The samples are digitized with a 12 bit resolution and stored in separate files. The FLD program can be implemented on several microcomputers of considerably less expense and processing power, but with limitations on data storage capacity, operational flexibility and display processing speed. The basic requirements are 16 analog-to-digital converters, a random access storage device and a graphics display oscilloscope with at least 512 x 512 pixel resolution and light pen option,
0013-4649/84/$03.00 © 1984 Elsevier Scientific Publishers Ireland, Ltd.
566
J.F. L E M I E U X ET AL.
/
Iz
15
J
•
//
/
Fig. 1. Sixteen-channel electrode grid placement in relation to the 10-20 system to study the field potentials of temporal spike discharges.
Data collection Data collection resembles a typical EEG recording session except that the amplified EEG from 16 analog outputs of an electroencephalograph is simultaneously routed to the computer for digitization and storage. The program initially requests from the operator identification data about the patient and operating parameters such as sample rate and duration.
The electrical baseline and calibration pulses, typically 50 ~V at sensitivity setting of 5 / ~ V / m m , are sampled from the electroenccphalograph. Calibration constants are calculated for each channel to correct for differences in their baselines and amplification. These constants are used to convert the incoming data values to microvohs. Low and high linear frequency settings of 1 and 70 c / s e c are used for most recordings. Sensitivity settings are adjusted according to the individual's EEG activity to insure a m a x i m u m resolution for digitization and to prevent any high amplitude 'clipping' by the amplifiers. Remote controls allow the technologist to selectively store variable length epochs for analysis. The computer system can store up to 60 sec of the immediately preceding EEG, thus allowing the technologist to first observe the EEG before deciding to store it. The data are referentially recorded by sets of 16 equidistant electrodes arranged in 4 × 4 grid arrays centered over areas of interest. Harris (1967) demonstrated that with interelectrode distances of less than 5 cm, this technique is a reliable means of representing the entire potential field within the grid placement. Our interelectrode distances range from 3 to 4.5 cm for all cases to date. Fig. 1 illustrates an electrode grid placement used in studying temporal lobe spike discharges.
Display method During or after the recording session, the technologists can examine the sampled EEG and display its field distributions either statically or dynamically. The system operates with two
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Fig. 2. a: first display mode of F L D system. Any 4 channels of any EEG epoch can be examined with the aid of the light pen facility. This and subsequent figures are photographs of display oscilloscope, b: second display mode of F L D system constructing the 3-D field potential grid for the instant denoted by the cursor on the EEG tracing at the bottom of the screen. CH 10 = channel 10 = electrode position 10 of Fig. 1 (T3).
T O P O G R A P H I C A L DISPLAY OF T H E EEG display modes. The first mode (Fig. 2a) recreates the stored EEG tracing of all the sampled channels. With the aid of a light pen, one can issue the appropriate c o m m a n d s to examine any portion of any EEG channel and to select the position, length and scaling for the second display mode which generates the 3-D field distributions for that epoch. The patient's identification data as well as the system's parameters are displayed at the top of the screen. 'Plot' initiates the second display mode. • D u m p ' creates a secondary file which contains only the data in the epoch being displayed. This reduced storage requirements by saving only the interesting portions of the patient's EEG. The 'page' and 'step' c o m m a n d s allow a forward and backward scan of the total EEG sample. The 'raise' and 'lower' comm a n d s alter the sensitivity of EEG while the 'compress' and "expand' c o m m a n d s shorten and lengthen the epoch duration being displayed. The c o m m a n d s along the bottom of the display screen control the left and right cursors that select the portion of the epoch that will be used in generating the field potential maps in the second display mode. Fig. 2b is a photograph of the graphics display oscilloscope in its second display mode reconstructing the 16-channel EEG into a 3-D map of the voltage potentials for each instant. The cursor on the EEG tracing denotes the instant in time that is being displayed. The small dots denote the electrode positions. The webbing is constructed from 400 voltage values which are interpolated from the 16 electrode potentials. With an interelectrode distance of 4.0 cm, each interpolated value is equivalent to an electrode spacing of 0.63 cm. Again, light sensitive c o m m a n d s are used to rotate and tilt the grid to desired orientations for the best viewpoints. A simulated motion picture, about 15 frames/sec, is produced by allowing the cursor to advance freely. One can stop and step through the epoch one frame at a time, pausing to change grid orientations. The ' u p ' and 'down' c o m m a n d s control the tilt or elevation of the grid while the 'left' and 'right' c o m m a n d s control the horizontal plane rotation. The 'expand' and 'shrink' c o m m a n d s control the size of the display similar to the zoom lens in photography. The 'plot' c o m m a n d creates a hard-copy output on a 4-pen plotter for documentation of various aspects of the potential fields. The 'solid' c o m m a n d performs a hidden line removal for a more realistic view of a 3-D contour map. The 'return' c o m m a n d allows the user to return to the first display mode without waiting for the total epoch to be displayed.
567
Discussion This system affords fast presentation and easy manipulation of the EEG data. With its easily controlled selection of grid orientation and dynamics of the display system, the observer may view the data in various ways to better appreciate the often complex field potential relationships of the paroxysmal EEG discharges. It defines the spatial-temporal aspects of the EEG potentials more accurately than can be done in a routine recording and aids the localization of the origin and propagation of epileptiform discharges. Even when the EEG is sampled with simultaneous referential and bipolar montages, new information is easily recognizable by the FLD program that is not immediately apparent on either conventional paper recording method. Since the grid is unlikely to cover the amount of area that the conventional 10-20 system could cover, the interelectrode distance and grid placement should be well thought out. Factors limiting a full appreciation of the field distributions of the recorded discharges would be the amount of high frequency noise and high amplitude background in the original signal. As with most subjective analysis procedures, one must be careful not to let some preconceived notion of the field distribution bias the results. This error can be reduced by accepting only the most obvious events reproduced several times and observed by more than one researcher. An objective analysis method such as cross-correlation can be very helpful in substantiating the visual analysis. This technique is currently being applied to the study of spike and wave complexes, temporal spikes and various normal discharges to better understand their field distributions and propagational aspects.
Summaff We have developed a system to generate from 16 channels of the EEG 3-dimensional sequential topographical displays that facilitate the analysis of the propagation and field distribution of paroxysmal discharges. The ease and flexibility of data storage and analysis is as important as the concept of representing the EEG field potentials in a comprehensible fashion.
Data analysis Resume To better understand the large amount of information displayed with the F L D system and in an attempt to quantify the propagational delay of the spike discharge, cross-correlations of selected electrode pairs are computed. Plots overlapping selected epochs of the EEG tracing of homologous electrode pairs are very helpful in illustrating the interhemispheric delays. These selected EEG segments are then used to create a graph of the cross-correlation coefficients at different time lags for the selected channel pairs. The cross-correlation always supported conclusions drawn from visual analysis of the field plots.
Mbthode de reprksentation de l'bvolution topographique des bvbnements EEG
Un syst6me a 6t~ mis au point pour produire un diagramme topographique s6quentiel en 3 dimensions h partir de 16 canaux EEG. L'analyse de la propagation et celle de la distribution de champs des d6charges paroxystiques sont ainsi rendues ais6es. La flexibilit6 et la facilit6 du stockage des donnees est aussi importante que la possibilit6 de repr6senter, de faqon comprehensible les potentiels de champs de I'EEG.
568 This research was supported by grants from the J.P. Bickell Foundation and the Richard and Jean Ivey Fund. We thank Dr. J. Daube for the valuable information he supplied. Mrs. Maria Raffa carefully typed the manuscript.
References
Daube, J.R. and Lake, R.M. System for computer-generation of 3-D display of electroencephalogram. Proc. San Diego biomed. Symp., 1972, 11: 301-309. Estrin, T. and Uzgalis, R. Computerized display of spatial-temporal EEG patterns. IEEE Trans. biomed. Engng, 1969, 3: 192-196. Harris, J.A. A spatial interpolation procedure for electroencephalography. Thesis, University of Minnesota, Minneapolis, MN, 1967. Harris, J.A., Melby, G.M. and Bickford, R.G. Computer-controlled multidimensional display device for investigation and modeling of physiologic systems. Comput. biomed. Res., 1969, 2: 519-536.
J.F. LEMIEUX ET AL. Lehmann, D. Multichannel topography of human alpha EEG fields. Electroenceph. clin. Neurophysiol., 1971, 31: 439-449. Petsche, H., Nagypal, O., Prohaska, O., Rappelsberger, P. and Vollmer, R. Approaches to the spatio-temporal analysis of seizure patterns. In: G. Dolce and H. Kunkel (Eds.), CEAN Computerized EEG Analysis. Gustav Fischer, Stuttgart, 1975: 111-127. R6mond, A. The importance of topographic data in EEG phenomena, and an electrical model to reproduce them. Electroenceph. clin. Neurophysiol., 1968, Suppl. 27: 29-49. Shaw, J.C. A method for continuously recording characteristics of EEG topography. Electroenceph. clin. Neurophysiol., 1970, 29: 592-601. Walter, W.G. and Shipton, H.W. A new toposcopic display system. Electroenceph. clin. Neurophysiol., 1951, 3: 281-292. Weir, B. The morphology of the spike-wave complex. Electroenceph, clin. Neurophysiol., 1965, 19: 284-290.
565
Elsevier Scientific Publishers Ireland, Ltd.
Technical Section S h o r t communication TECHNIQUE
TO DISPLAY
TOPOGRAPHICAL
EVOLUTION
OF EEG EVENTS
JOHN F. LEMIEUX I ROBERT S. VERA and WARREN T. BLUME
Epilepsy Unit, Department of Clinical Neurological Sciences, University Hospital, London, Ont. N6A 5.45 (Canada) (Accepted for publication: July 13, 1984)
Although it has been several years since the introduction of computers to the task of automating the collection and analysis of the electroencephalogram, the standard paper print-out is still the major tool of the electroencephalographer. With this medium the clinician can gather information about the frequency of voltage variations and the difference in voltage variations between different points on the scalp; however, it is quite difficult in many instances to transform the information recorded as a group of lines on paper into a mental image of the potential field on the scalp. Moreover, in determining the direction and time delays of potential field movement, estimates are prone to error due to the limitations of the recording device. There have been several attempts to represent the spatialtemporal aspects of EEG potentials in a comprehensible fashion. One of the first efforts was by Walter and Shipton (1951) who developed a toposcope: an electronic apparatus demonstrating the spatial aspects of the EEG. R6mond (1968), Estrin et al. (1969), Shaw (1970) and Lehmann (1971) developed systems to create 2-dimensional displays of the isopotential field of the EEG. Petsche et al. (1975) approached the spatialtemporal analysis by not only producing isopotential maps but also maps of their first derivatives to study the time relationships within potential fields. A spatial interpolation procedure described by Harris in 1967 was the basis for the system developed by Harris et al. (1969) and Daube and Lake (1972) which generated 3-dimensional (3-D) displays of the spatial contours of the EEG for each instant in time. We found that 3-D displays more readily projected the information about field distributions than 2-D isopotential maps. Both their system and ours allow the observer to easily orientate the displays in vertical and horizontal positions for a better view of the field distributions. Movie-like displays were created by Harris et al. (1969) by rapidly displaying moment-to-moment spatial maps of the EEG. Dynamics for Daube and Lake's (1972) system were created by
1 Reprint requests to: Mr. John Lemieux, Epilepsy Unit, University Hospital, 339 Windermere Road, London, Ont. N6A 5A5, Canada.
photographing successive plots of the field distributions and creating a motion picture by rapid playback of the photographs. Dynamic displays are more effective in aiding the observer to comprehend the movement of potential field distributions. Applying the same fundamental principles, we have developed a system to sample and generate from the EEG 3-D displays in a dynamic as well as static mode. Our field plotting system (FLD), which is more flexible and simpler to operate than previous systems, provides a comprehensible alternative to analyzing the field development and propagation of EEG potentials. It is a totally interactive analysis program that not only allows one to study the temporal-spatial aspects of the field distributions with a variety of orientations, but also allows one to easily scan the recorded EEG and select the variable size epochs that are to be analyzed in this fashion.
System layout The field plotting program (FLD) currently operates on a computer system with 256 kbytes of memory, two 14 Mbyte disk drives, a digital magnetic tape drive, a graphics display oscilloscope with direct memory access (DMA), a pen plotter, and a powerful laboratory interface consisting of 64 analog-todigital converters (A/Ds), 32 digital input/output channels and associated Schmitt triggers." One disk is capable of holding 35 min of 16-channel EEG sampled at 200 c / s e c per channel. By exchanging disk packs and utilizing the magnetic tape drive, one can greatly increase the storage capacity. Allowable sample rates range from 100 to 1000 c / s e c per channel with sample durations from 1 to 60 sec. The samples are digitized with a 12 bit resolution and stored in separate files. The FLD program can be implemented on several microcomputers of considerably less expense and processing power, but with limitations on data storage capacity, operational flexibility and display processing speed. The basic requirements are 16 analog-to-digital converters, a random access storage device and a graphics display oscilloscope with at least 512 x 512 pixel resolution and light pen option,
0013-4649/84/$03.00 © 1984 Elsevier Scientific Publishers Ireland, Ltd.
566
J.F. L E M I E U X ET AL.
/
Iz
15
J
•
//
/
Fig. 1. Sixteen-channel electrode grid placement in relation to the 10-20 system to study the field potentials of temporal spike discharges.
Data collection Data collection resembles a typical EEG recording session except that the amplified EEG from 16 analog outputs of an electroencephalograph is simultaneously routed to the computer for digitization and storage. The program initially requests from the operator identification data about the patient and operating parameters such as sample rate and duration.
The electrical baseline and calibration pulses, typically 50 ~V at sensitivity setting of 5 / ~ V / m m , are sampled from the electroenccphalograph. Calibration constants are calculated for each channel to correct for differences in their baselines and amplification. These constants are used to convert the incoming data values to microvohs. Low and high linear frequency settings of 1 and 70 c / s e c are used for most recordings. Sensitivity settings are adjusted according to the individual's EEG activity to insure a m a x i m u m resolution for digitization and to prevent any high amplitude 'clipping' by the amplifiers. Remote controls allow the technologist to selectively store variable length epochs for analysis. The computer system can store up to 60 sec of the immediately preceding EEG, thus allowing the technologist to first observe the EEG before deciding to store it. The data are referentially recorded by sets of 16 equidistant electrodes arranged in 4 × 4 grid arrays centered over areas of interest. Harris (1967) demonstrated that with interelectrode distances of less than 5 cm, this technique is a reliable means of representing the entire potential field within the grid placement. Our interelectrode distances range from 3 to 4.5 cm for all cases to date. Fig. 1 illustrates an electrode grid placement used in studying temporal lobe spike discharges.
Display method During or after the recording session, the technologists can examine the sampled EEG and display its field distributions either statically or dynamically. The system operates with two
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COHTII~CT
Fig. 2. a: first display mode of F L D system. Any 4 channels of any EEG epoch can be examined with the aid of the light pen facility. This and subsequent figures are photographs of display oscilloscope, b: second display mode of F L D system constructing the 3-D field potential grid for the instant denoted by the cursor on the EEG tracing at the bottom of the screen. CH 10 = channel 10 = electrode position 10 of Fig. 1 (T3).
T O P O G R A P H I C A L DISPLAY OF T H E EEG display modes. The first mode (Fig. 2a) recreates the stored EEG tracing of all the sampled channels. With the aid of a light pen, one can issue the appropriate c o m m a n d s to examine any portion of any EEG channel and to select the position, length and scaling for the second display mode which generates the 3-D field distributions for that epoch. The patient's identification data as well as the system's parameters are displayed at the top of the screen. 'Plot' initiates the second display mode. • D u m p ' creates a secondary file which contains only the data in the epoch being displayed. This reduced storage requirements by saving only the interesting portions of the patient's EEG. The 'page' and 'step' c o m m a n d s allow a forward and backward scan of the total EEG sample. The 'raise' and 'lower' comm a n d s alter the sensitivity of EEG while the 'compress' and "expand' c o m m a n d s shorten and lengthen the epoch duration being displayed. The c o m m a n d s along the bottom of the display screen control the left and right cursors that select the portion of the epoch that will be used in generating the field potential maps in the second display mode. Fig. 2b is a photograph of the graphics display oscilloscope in its second display mode reconstructing the 16-channel EEG into a 3-D map of the voltage potentials for each instant. The cursor on the EEG tracing denotes the instant in time that is being displayed. The small dots denote the electrode positions. The webbing is constructed from 400 voltage values which are interpolated from the 16 electrode potentials. With an interelectrode distance of 4.0 cm, each interpolated value is equivalent to an electrode spacing of 0.63 cm. Again, light sensitive c o m m a n d s are used to rotate and tilt the grid to desired orientations for the best viewpoints. A simulated motion picture, about 15 frames/sec, is produced by allowing the cursor to advance freely. One can stop and step through the epoch one frame at a time, pausing to change grid orientations. The ' u p ' and 'down' c o m m a n d s control the tilt or elevation of the grid while the 'left' and 'right' c o m m a n d s control the horizontal plane rotation. The 'expand' and 'shrink' c o m m a n d s control the size of the display similar to the zoom lens in photography. The 'plot' c o m m a n d creates a hard-copy output on a 4-pen plotter for documentation of various aspects of the potential fields. The 'solid' c o m m a n d performs a hidden line removal for a more realistic view of a 3-D contour map. The 'return' c o m m a n d allows the user to return to the first display mode without waiting for the total epoch to be displayed.
567
Discussion This system affords fast presentation and easy manipulation of the EEG data. With its easily controlled selection of grid orientation and dynamics of the display system, the observer may view the data in various ways to better appreciate the often complex field potential relationships of the paroxysmal EEG discharges. It defines the spatial-temporal aspects of the EEG potentials more accurately than can be done in a routine recording and aids the localization of the origin and propagation of epileptiform discharges. Even when the EEG is sampled with simultaneous referential and bipolar montages, new information is easily recognizable by the FLD program that is not immediately apparent on either conventional paper recording method. Since the grid is unlikely to cover the amount of area that the conventional 10-20 system could cover, the interelectrode distance and grid placement should be well thought out. Factors limiting a full appreciation of the field distributions of the recorded discharges would be the amount of high frequency noise and high amplitude background in the original signal. As with most subjective analysis procedures, one must be careful not to let some preconceived notion of the field distribution bias the results. This error can be reduced by accepting only the most obvious events reproduced several times and observed by more than one researcher. An objective analysis method such as cross-correlation can be very helpful in substantiating the visual analysis. This technique is currently being applied to the study of spike and wave complexes, temporal spikes and various normal discharges to better understand their field distributions and propagational aspects.
Summaff We have developed a system to generate from 16 channels of the EEG 3-dimensional sequential topographical displays that facilitate the analysis of the propagation and field distribution of paroxysmal discharges. The ease and flexibility of data storage and analysis is as important as the concept of representing the EEG field potentials in a comprehensible fashion.
Data analysis Resume To better understand the large amount of information displayed with the F L D system and in an attempt to quantify the propagational delay of the spike discharge, cross-correlations of selected electrode pairs are computed. Plots overlapping selected epochs of the EEG tracing of homologous electrode pairs are very helpful in illustrating the interhemispheric delays. These selected EEG segments are then used to create a graph of the cross-correlation coefficients at different time lags for the selected channel pairs. The cross-correlation always supported conclusions drawn from visual analysis of the field plots.
Mbthode de reprksentation de l'bvolution topographique des bvbnements EEG
Un syst6me a 6t~ mis au point pour produire un diagramme topographique s6quentiel en 3 dimensions h partir de 16 canaux EEG. L'analyse de la propagation et celle de la distribution de champs des d6charges paroxystiques sont ainsi rendues ais6es. La flexibilit6 et la facilit6 du stockage des donnees est aussi importante que la possibilit6 de repr6senter, de faqon comprehensible les potentiels de champs de I'EEG.
568 This research was supported by grants from the J.P. Bickell Foundation and the Richard and Jean Ivey Fund. We thank Dr. J. Daube for the valuable information he supplied. Mrs. Maria Raffa carefully typed the manuscript.
References
Daube, J.R. and Lake, R.M. System for computer-generation of 3-D display of electroencephalogram. Proc. San Diego biomed. Symp., 1972, 11: 301-309. Estrin, T. and Uzgalis, R. Computerized display of spatial-temporal EEG patterns. IEEE Trans. biomed. Engng, 1969, 3: 192-196. Harris, J.A. A spatial interpolation procedure for electroencephalography. Thesis, University of Minnesota, Minneapolis, MN, 1967. Harris, J.A., Melby, G.M. and Bickford, R.G. Computer-controlled multidimensional display device for investigation and modeling of physiologic systems. Comput. biomed. Res., 1969, 2: 519-536.
J.F. LEMIEUX ET AL. Lehmann, D. Multichannel topography of human alpha EEG fields. Electroenceph. clin. Neurophysiol., 1971, 31: 439-449. Petsche, H., Nagypal, O., Prohaska, O., Rappelsberger, P. and Vollmer, R. Approaches to the spatio-temporal analysis of seizure patterns. In: G. Dolce and H. Kunkel (Eds.), CEAN Computerized EEG Analysis. Gustav Fischer, Stuttgart, 1975: 111-127. R6mond, A. The importance of topographic data in EEG phenomena, and an electrical model to reproduce them. Electroenceph. clin. Neurophysiol., 1968, Suppl. 27: 29-49. Shaw, J.C. A method for continuously recording characteristics of EEG topography. Electroenceph. clin. Neurophysiol., 1970, 29: 592-601. Walter, W.G. and Shipton, H.W. A new toposcopic display system. Electroenceph. clin. Neurophysiol., 1951, 3: 281-292. Weir, B. The morphology of the spike-wave complex. Electroenceph, clin. Neurophysiol., 1965, 19: 284-290.