Recently, it has been shown that increasing size and luminance contrast of a motion stimulus makes it more difficult to perceive its direction of motion (Tadin et al. 2003). It was argued that this was the result of surround inhibition in motion selective neurons in the human brain. The neural response to motion presented to a neuron's classical receptive field can be inhibited by presenting motion to its surround. Question: does increasing size and contrast of a motion stimulus have a similar effect on perception of motion in the center as decreasing the center's luminance contrast?

In binocular rivalry, dissimilar targets compete for perceptual dominance. Decreasing one target's contrast increases the dominance duration of the other target, leaving its own duration unaffected. We investigated whether increasing size and contrast of surround motion has the same effect on rival targets as decreasing one target's contrast.

Rivalry was instigated between a moving and stationary target. Surround motion was either in the opposite direction of the moving target, or in the same direction. Both contrast of center and surround and width of the surround were varied. Observers continuously indicated which target was perceptually dominant.

A same direction surround modulated rivalry: at high contrast (100 and 50% Michelson), increasing the size of surround motion gradually decreased the dominance of the moving target. At low contrast (5%), the decrease in dominance was constant for various widths of the surround. Interestingly, as dominance durations of the static target increased, durations of the moving target where relatively unaffected. An opposite direction surround did not modulate dominance.

This study shows that increasing size and contrast of a moving stimulus has a similar effect on perception of center motion as decreasing the center's contrast. It appears as though surround inhibition decreases the signal for center motion in a manner similar to lowering the center's contrast.