A method for positioning electrodes in an electrode array to minimise the number of electrodes required in the array to identify time periods in an electroencephalographic signal produced by the array that are contaminated wit artifacts resulting from eye movements of a subject, the method comprising the steps of: - positioning first and second electrodes (1, 2) on the head of the subject with the first electrode being positioned on the fronto-lateral area of the frontal lobe and the second electrode being positioned similarly but on the opposite cortical hemisphere such that the first and second electrodes (1, 2) are positioned substantially horizontally and at substantially equal distances from a vertical center axis of a face and the locations being arranged in positions at which the electrodes (1, 2) obtain both the EEG signal and an electrooculographic signal, the EEG signal comprising a voltage obtained at the first electrode and a voltage obtained at the second electrode and wherein the locations of the first and second electrodes is such that eye movement in one of the horizontal or vertical directions will result in a minimal overall change in the voltage difference between the first electrode and the second electrode to provide the EEG signal in which artifacts resulting from eye movement in the one direction are minimized or eliminated, - positioning a third electrode on the temple of the subject on the hemisphere opposite to that of the second electrode at a third location to provide an electrooculographic signal comprising a voltage difference between a voltage obtained at the third electrode and a voltage obtained at an auxiliary electrode being an additional electrode or one of the first and second electrodes (1, 2), the location of the third electrode being such as to maximize voltage changes occurring in the electrooculographic signal as a result of eye movement in the other of said horizontal and vertical directions; and identifying time periods in which the maximized voltage changes occur in the EOG signal as artifact contaminated time periods in the EEG signal wherein the maximized voltage changes result from eye movement in the other direction. - positioning first and second electrodes (1, 2) on the head of the subject with the first electrode being positioned on the fronto-lateral area of the frontal lobe and the second electrode being positioned similarly but on the opposite cortical hemisphere such that the first and second electrodes (1, 2) are positioned substantially horizontally and at substantially equal distances from a vertical center axis of a face and the locations being arranged in positions at which the electrodes (1, 2) obtain both the EEG signal and an electrooculographic signal, the EEG signal comprising a voltage obtained at the first electrode and a voltage obtained at the second electrode and wherein the locations of the first and second electrodes is such that eye movement in one of the horizontal or vertical directions will result in a minimal overall change in the voltage difference between the first electrode and the second electrode to provide the EEG signal in which artifacts resulting from eye movement in the one direction are minimized or eliminated, - positioning a third electrode on the temple of the subject on the hemisphere opposite to that of the second electrode at a third location to provide an electrooculographic signal comprising a voltage difference between a voltage obtained at the third electrode and a voltage obtained at an auxiliary electrode being an additional electrode or one of the first and second electrodes (1, 2), the location of the third electrode being such as to maximize voltage changes occurring in the electrooculographic signal as a result of eye movement in the other of said horizontal and vertical directions; and identifying time periods in which the maximized voltage changes occur in the EOG signal as artifact contaminated time periods in the EEG signal wherein the maximized voltage changes result from eye movement in the other direction.