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Christopher Shooner, J. Anthony Movshon; Two forms of directional bias revealed by multistable motion stimuli. Journal of Vision 2011;11(11):752. doi: https://doi.org/10.1167/11.11.752.
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Two superimposed sinusoidal gratings drifting in different directions can combine to appear as a single rigidly moving pattern whose motion is unambiguous. Adding a third grating can create a multistable motion stimulus. We superimposed three gratings with evenly-spaced directions of motion, 120 deg apart. Any two of these can combine and be perceived as a plaid, while the third grating is seen in isolation, moving in the opposite direction. Only one plaid is perceived at a time, and the symmetry of the stimulus suggests that each of the three possible percepts should be seen equally often. In reality, some percepts are strongly favored, so we measured and characterized this unexpected perceptual bias.
Five subjects fixated the center of a display on which we presented ‘tri-plaid’ stimuli for 500 ms; this was too brief for multistability to become evident within a single trial. Stimuli were placed at one of four randomly chosen locations symmetrically disposed around the center of gaze; after each presentation the observers reported the perceived direction of plaid motion. The triplaids were oriented so that no motion or orientation was closer than 15 deg to a cardinal. Across all locations, our observers reported near-vertical motions more often than near-horizontal, and either cardinal direction more often than oblique. They also had an independent bias to report motion toward fixation from any location.
These two forms of bias may have different origins. Bias for motion in cardinal directions could be related to similar biases in the orientation domain (e.g. Andrews & Schluppeck, 2000), but our effects seem to be tied to motion rather than orientation. The centripetal bias might reflect the action of mechanisms tuned to complex global motion patterns associated with optic flow, which could influence the integration of local motion signals.
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