Motion Parallax

Motion parallax is the relative motion of objects at different depths due to the motion of the observer. This is the kind of information that is available when we look out of the window of a moving vehicle or when we look around while on a moving bicycle. Objects farther than our point of fixation move with us; objects that are closer move in the opposite direction (Fig. 9). This is indeed a powerful monocular cue to depth. Brian Rogers and Maureen Graham demonstrated the effect of motion parallax in the laboratory. Observers viewed a flat screen with randomly positioned dots. They were asked to move their heads from side to side. This lateral motion was instantaneously measured and used to compute differential motion of the random dots for each head position. The dot motion was calculated so that it was consistent with motion parallax from a 3D surface corrugated sinusoidally in depth. This imposed motion was similar to motion that would have impinged on the retina if the observer were moving with respect to a real corrugated surface. Observers had the compelling percept of a surface modulated in depth, despite the fact that there were no stereo cues to the depth.

It is possible that neurons in MT can encode depth from motion parallax. It is known that populations of MT neurons have different preferred directions in the center and surround. A subset of these neurons have centers and surrounds with opposite preferred directions of motion. These cells could potentially encode a motion parallax signal generated by a moving observer fixating a point between two objects separated in depth. Furthermore, we also know that some cells in MT are disparity selective. A cell that combined relative motion specificity with disparity selectivity could potentially encode depth from both motion parallax and stereo information.

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