Bidirectional orthogonal motion aftereffect

Purpose. Recent modeling and psychophysical research has shown that adaptation to simultaneously presented opposite directions of motion causes a motion aftereffect (MAE) along the orthogonal axis (Nature, 384: 358-360). The observers' subjective report indicated that the orthogonal MAE may be equivalent to low signal-to-noise bidirectional motion. We tested this in an experiment. Methods. The experiment had two phases: adaptation and test. The adaptation conditions were: 1) opposite motion, 2) unbiased noise. During the test phase one of four random dot displays was used: a) no bias, b) 10% bias up, c) 10% bias down, d) 10%bias up and 10% bias down. Subjects indicated whether they saw global motion up, down, or along both directions during the test phase. Results. Subjects perceived the biased test conditions (b-d) veridically. In the unbiased test condition (a) subjects reported seeing motion along both directions following opposite motion adaptation (1). In particular, conditions la (orthogonal MAE) and 2d (bidirectional motion without adaptation) led to similar reports. When subjects saw two directions, they perceived "diffuse transparency" instead of two transparent sheets or locally single directions. Conclusions. The orthogonal MAE following adaptation to opposite directions of motion is bivectorial. However, this bivectorially perceived motion does not give rise to a percept of two coherently moving transparent sheets