Previous studies have found large misperceptions when subjects are reportin
g the perceived angle between two directions of motion moving transparently
at an acute angle, the so called motion repulsion. While these errors have
been assumed to be caused by interactions between the two directions prese
nt, we reassessed these earlier measurements taking into account recent fin
dings about directional misperceptions affecting the perception of single m
otion (reference repulsion). While our measurements confirm that errors in
directional judgements of transparent motions can indeed be as big as 22 de
grees we find that motion repulsion, i.e. the interaction between two direc
tions, contributes at most about 7 degrees to these errors. This value is c
omparable to similar repulsion effects in orientation perception and stereo
scopic depth perception, suggesting that they share a common neural basis.
Our data further suggest that fast time scale adaptation and/or more genera
l interactions between neurons contribute to motion repulsion while trackin
g eye movements play little or no role. These findings should serve as impo
rtant constraints for models of motion perception. (C) 1999 Elsevier Scienc
e Ltd. All rights reserved.