1. A method of displaying the activity of a brain, the method including the steps of: (i) obtaining an electroencephalogram (EEG) signal from the brain; (ii) segmenting said EEG signal into either contiguous or overlapping segments comprised of a sequential number of samples of said EEG signal; (iii) representing said EEG segments as a fixed order 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 and zeros 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) calculating the sum of the number of poles determined in step (vi); (viii) calculating the sum of the number of zeros determined in step (vi); (ix) representing said ARMA signal representation as an infinite order autoregressive (AR) model in z-domain notation; (x) determining autoregressive coefficient data for said infinite AR model from the AR and MA coefficient data generated in step (v) of said fixed order ARMA representation; (xi) determining the sum of the poles for said infinite order AR model for said segments of said EEG signal as either: (a) the first autoregressive coefficient of said infinite order AR model; or (b) the difference of the sum of poles and the sum of zeros as determined respectively in steps (vii) and (viii); (xii) determining an index value representing the activity of the brain for said segments of said EEG signal by applying a discriminating function to the sum of the poles of said infinite AR model for said segments of said EEG signal as determined in step (xi); and (xiii) displaying said index value on display means.