A method of displaying the activity of a brain, the method including the steps of: (i) obtaining an electroencephalogram signal from the scalp of a subject and converting the EEG signal to a digitised EEG data signal, (ii) segmenting said EEG signal into a plurality of EEG segments each including a predetermined number of sequential samples of said EEG signal and performing artefact rejection on segments, (iii) representing said EEG segments as a fixed order time-invariant autoregressive moving average (ARMA) signal representation with an autoregressive order between 8 and 13 and a moving average order between 5 and 11; (iv) rewriting in z-domain notation said fixed order ARMA signal representation to obtain a z-domain representation; (v) generating AR coefficient data and MA coefficient data for said segments of said EEG signal for said fixed order ARMA signal representation; (vi) determining the poles of said z-domain representation for said segments of said EEG signal by substituting said coefficient data for said segments of said EEG signal into said z-domain representation; (vii) determining a sum of the poles ( z p) for said segments of said EEG signal as either; (a) the sum of the number of poles determined in step (vi); or (b) the first autoregressive AR coefficient of said coefficient data for said fixed order ARMA signal representation; (viii) determining a mean pole location for said segments of said EEG signal by dividing the sum of said poles ( z p) by the number of said poles; (ix) determining an index value representing the activity of the brain for said segments of said EEG signal by applying a scaling function to either the sum of the poles ( z p) or the mean pole location for said segments of said EEG signal; and (x) displaying said index value on display means. (i) obtaining an electroencephalogram signal from the scalp of a subject and converting the EEG signal to a digitised EEG data signal, (ii) segmenting said EEG signal into a plurality of EEG segments each including a predetermined number of sequential samples of said EEG signal and performing artefact rejection on segments, (iii) representing said EEG segments as a fixed order time-invariant autoregressive moving average (ARMA) signal representation with an autoregressive order between 8 and 13 and a moving average order between 5 and 11; (iv) rewriting in z-domain notation said fixed order ARMA signal representation to obtain a z-domain representation; (v) generating AR coefficient data and MA coefficient data for said segments of said EEG signal for said fixed order ARMA signal representation; (vi) determining the poles of said z-domain representation for said segments of said EEG signal by substituting said coefficient data for said segments of said EEG signal into said z-domain representation; (vii) determining a sum of the poles ( z p) for said segments of said EEG signal as either; (a) the sum of the number of poles determined in step (vi); or (b) the first autoregressive AR coefficient of said coefficient data for said fixed order ARMA signal representation; (a) the sum of the number of poles determined in step (vi); or (b) the first autoregressive AR coefficient of said coefficient data for said fixed order ARMA signal representation; (viii) determining a mean pole location for said segments of said EEG signal by dividing the sum of said poles ( z p) by the number of said poles; (ix) determining an index value representing the activity of the brain for said segments of said EEG signal by applying a scaling function to either the sum of the poles ( z p) or the mean pole location for said segments of said EEG signal; and (x) displaying said index value on display means.