Thirty three (10 males, 23 females) patients in clinically remitted state diagnosed with bipolar disorder based on DSM-IV criteria were enrolled at one outpatient psychiatric clinic and 59 healthy controls (23 males, 36 females) from health promotion center in a university-affiliated general hospital. We defined an individual patient as being in subsyndromal depressive phase when the patient had a Montgomery-Åsberg depression rating scale (MADRS) score of 10 or less and Clinical Global Impression severity (CGI-S) of 3 or less for last one month. Healthy controls had no history of psychiatric disorder and had no psychiatric symptom when interviewed by a board-certified psychiatrist.