Inclusion Criteria:  Pediatric patients aged between 4 months and 2 years planned to undergo surgery under general anesthesia American Society of Anesthesiologists physical status 1 or 2  Exclusion Criteria:  History of preterm birth (postconceptual age < 32 wks) or history of mechanical ventilation immediately after birth Presence of genetic disease or chromosomal abnormality that can affect brain development Presence of disease in central nervous system History of head trauma or surgery involving head History of hypersensitivity to anesthetic drugs Intubated or sedated state before induction of anesthesia Difficulty in attaching both the bispectral index and patient state index probes to patient's forehead Expected operation time of less than 5 minutes