Visual motion information passes through several distinct stages including local motion processing in an earlier stage, followed by global motion processing at a later stage. However, the stage at which the perceptual limit of detection arises remains unknown. In order to examine which stage is critical for motion detection, we investigated how vertical illusory motion affected detection performance for physical horizontal motion. We introduced illusory induced motion, such that a central pattern would be perceived as moving in the opposite direction to the surrounding motion, even though the central stimulus was physically stationary. We presented the central Gabor patch, which was barely moving to the left or right, together with a surrounding grating moving vertically. Subjects were asked to judge whether the central stimulus was moving left or right, and thus the illusory vertical motion itself was task-irrelevant. We found that the performance on the horizontal test was enhanced when it was combined with the induced vertical motion, which resulted in the central motion appearing slightly oblique rather than purely horizontal. Our results indicate that the later stage, in which motion integration and centera??surround interaction appears, is critical for determining the perceptual limit of motion detection.
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