Dichoptic reduction of the direction illusion is not due to binocular rivalry

Simultaneous direction repulsion (the direction illusion) occurs in bidirectional motion displays, typically transparent motion random dot kinematograms. Several laboratories have reported a greatly reduced illusion with dichoptic presentation of the two coherently translating stimuli as compared to monocular or binocular presentation. Some researchers have argued that those results might be due to a confounding factor, namely binocular rivalry occurring between test and inducing stimuli in the dichoptic condition, and so have attributed decisive weight to the results reported by Kim and Wilson (1997, Vision Research, 37, 991-1005) who used centre-surround grating stimuli and found large monocular as well as large dichoptic effects. Here we use centre-surround dot stimuli - with which no binocular rivalry occurs - to confirm a strong monocular contribution to the direction illusion. In addition, we fail to find evidence of a direction illusion with centre-surround grating stimuli, even when seeking to replicate the methods of Kim and Wilson (1997). In light of other evidence that a global motion-sensitive mechanism can determine the magnitude of the direction illusion, we propose that simultaneous direction repulsion can result from activity at multiple stages of the motion processing hierarchy.