Abstract
To examine how local velocities from different regions of the visual
field combine to form a coherent motion percept, we subjected a sinusoidal
line stimulus to translational motion. Horizontal movement of a sinewave
line along its axial direction is perceived as nonrigid if the angle
at the zero crossing is smaller than a critical angle of about 15
deg. This angle is independent of spatial scale and the number of
sinusoidal cycles. To extend the applicability of this concept of
angle, we developed a mathematical model to predict an observer's
sensitivity to small changes in motion direction based on two assumptions:
(1) the computed velocity signal is obtained from the intersection
of constraint lines defined by local velocity components,(2) local
velocity components are contaminated by noise. Measurement of directional
discrimination thresholds of moving targets confirmed our expectations.
Thresholds varied as a function of the angle of the local contour
independent of spatial scale and in quantitative accord with our
assumptions.
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