Introduction: Bipolar disorder is a chronic illness and included functional impairment, disability or lost work productivity, increased health care costs, and high risk of suicide. Recently some reports showed cognitive dysfunction in bipolar disorder. In neurophysiologically, steady state response (SSR) is one of index of the neural circuitry, and might be contributed to cognitive integration. Though previously there were some reports about low gamma oscillations in bipolar disorder, there was no report about high gamma oscillations in bipolar disorder as far as we know. In the current study, we examined high and low gamma SSR in bipolar disorder.
 Methods: 14 bipolar disorder patients and 25 healthy controls participated. Auditory steady state response (ASSR) was recorded by presenting 20 Hz, 30 Hz, 40 Hz and 80 Hz click trains using a whole-head 306-channel magnetoencephalography. We calculated ASSR power and phase locking factor (PLF). The mean ASSR power and PLF were submitted a repeated measures analysis of variance.
 Results: Bipolar disorder patients showed significantly reduced mean ASSR power and PLF bilaterally, specific to the 30, 40, and 80 Hz frequencies.
 Conclusions: Bipolar disorder patients are characterized by deficits in gamma band oscillations, which may be associated with γ-amino butyric acid (GABA) inhibitory interneuronal activity dysfunction.
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