Different anesthetic drugs can produce different electroencephalographic (EEG) patterns. Nondrug induced influences, such as surgical stimulation, may also alter the EEG in anesthetized patients. Increases of delta activity are interpreted as signs of deepening of anesthesia. These EEG patterns are sometimes also found in frontal leads after surgical stimulation during isoflurane anesthesia. However, to avoid incorrect anesthetic management under EEG monitoring, this phenomenon of "paradoxical arousal" must be taken into consideration. This study demonstrates that, after surgical stimulation, frontal activity was enhanced in the 2-3 Hz and 3-4 Hz frequency bands. On the other hand, in the subdelta range (the 0.05-1 Hz and 1-2 Hz frequency bands), there was an opposite reaction. This significant (p<0.001, according to ANOVA and Dunn's method) reduction of EEG power as a result of the surgical stimuli was found in 25 patients (mean age, 49.1+/-16.3 years) scheduled for elective neurosurgical intervention in the lumbosacral region. This report shows some of the problems arising from the common practice of arbitrarily subdividing a power spectrum in frequency bands.