Combining direction and speed for the localisation of visual motion defined contours.

Detecting discontinuities in motion signal distributions is an essential operation of visual systems, contributing to perception and visuo-motor control. Discontinuities can be signalled by a difference in speed, direction or both. We measured how localisation accuracy for a motion defined contour depends on the velocity differences that define it. A vertical motion contour was defined by two fields of random dots with systematically varied combinations of speed and direction. We find that our data is best explained by assuming that localisation precision is inversely proportional to direction and speed differences that are linearly summed and weighted according to reliability, the optimal solution for combining independent estimates.