The investigators will record 120 patients who received general anesthesia, their EEG data recorded from Bispectral Index (BIS) machine for emperical mode decomposition (EMD) analysis, the investigators decomposed EEG signal to different oscillations and tried to observe the differences between hypnotic medications through EEG analysis.

The electroencephalography (EEG) is a noninvasive medical technique for monitoring and recording the electrical activity of brain. The Hilbert-Huang transform (HHT) was proposed to decompose EEG signal into intrinsic mode functions (IMF) which can obtain instantaneous frequency data and work well for data that is nonstationary and nonlinear.

The relationship between frontal EEG patterns and hypnotics remain poorly understood. The investigators only can tell the increase in frontal EEG power and shift power to lower frequencies during general anesthesia from publications. The investigators are going to decompose EEG with HHT and find the energy shift during general anesthesia, compare with different anesthetics.

The investigators will record 120 patients who received general anesthesia, their EEG data recorded from BIS machine for EMD analysis, the investigators decomposed EEG signal to different oscillations and tried to observe the differences between hypnotic medications through EEG analysis.

Inclusion Criteria:

• ASA I-II
• scheduled surgery who need general anesthesia

Exclusion Criteria:

• allergy to the BIS patch
• not suitable for general anesthesia

The study population was focus on the patients who need schedule general anesthesia with low anesthesia risk (ASA I-II).