Directional bias of illusory stream caused by relative motion adaptation

•Rotating an Enigma figure produced an illusory stream opposing the rotation.•Directional bias of the illusory stream reversed after rotation stopped.•Adapting to opposing motions generates strong directional bias of the stream.•Relative motion detectors may produce the illusory stream of Enigma.

Enigma is an op-art painting that elicits an illusion of rotational streaming motion. In the present study, we tested whether adaptation to various motion configurations that included relative motion components could be reflected in the directional bias of the illusory stream. First, participants viewed the center of a rotating Enigma stimulus for adaptation. There was no physical motion on the ring area. During the adaptation period, the illusory stream on the ring was mainly seen in the direction opposite to that of the physical rotation. After the physical rotation stopped, the illusory stream on the ring was mainly seen in the same direction as that of the preceding physical rotation. Moreover, adapting to strong relative motion induced a strong bias in the illusory motion direction in the subsequently presented static Enigma stimulus. The results suggest that relative motion detectors corresponding to the ring area may produce the illusory stream of Enigma.