AbstractThis paper investigates the existence of stochastic resonance (SR) in the human brain, based on the noise effect of the α‐wave. In order to show clear evidence of the SR phenomenon in the central nervous system, the measurement was carried out under the following conditions; the periodic and noisy light stimuli are respectively applied to the right and left eye of a subject separately. When the periodic stimulus is applied with a subthreshold condition and the amplitude of noise stimulus of which the frequency bandwidth ranges from 15 to 60 Hz is varied, strong entrainment occurs for a certain range of the noise amplitude. Then the curve for the amplitude of the peak versus the noise amplitude shows a bell‐type shape, that is, the maximum signal amplitude at an optimum noise. Thus, the SR phenomenon occurs not on the retina but on the visual processing area after the optic chiasm—no mixing occurs before primary visual cortex in this manner, because the periodic and random stimuli are separately applied to the right and left eyes, respectively. This result suggests that the human brain may exploit internal and/or external noise for the higher‐order activities such as perception and cognition. © 2004 Wiley Periodicals, Inc. Syst Comp Jpn, 35(11): 39–47, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.10398