Fig. 3. Spectral entropy of the electroencephalographic signal corresponding to different Observer’s Assessment of Alertness/Sedation (OAA/S) levels (mean and standard error). The bars of different shades of gray correspond to the patient groups remi0 to remi4: The darkest bar represents patients receiving no remifentanil (group remi0), the middle bar represents patients receiving remifentanil dose of 2 ng/ml (group remi2), and the lightest bar represents patients receiving remifentanil dose of 4 ng/ml (group remi4). Statistical significance ( P < 0.05) of the differences between the results of the patient groups is denoted by an asterisk below and between the corresponding bars. An asterisk at a lower position in the middle of a group of three bars marks significant difference of patient groups remi0 and remi4. The effect of remifentanil is reversed as the frequency band of the signal is shifted toward higher frequencies.