Sub-threshold motion influences apparent position

Abstract

Until recently, it was assumed that motion and position coding were independent. However, now it is apparent that these attributes interact. For instance, stationary stimuli that contain internal movement can appear shifted in the same direction as the internal motion. Recently, it has been suggested that this sort of illusory position shift is caused by changes in apparent contrast at the edges of moving objects. This suggestion is consistent with psychophysical observations that the apparent height of static gratings can be influenced by internal motion patterns. Here we show that motion-induced changes in apparent height and position both increase linearly with stimulus presentation time (up to ∼100ms). We also show that apparent position shifts can occur when stimulus presentations are too brief for the direction of internal motion to be identified. This implies that illusory motion-induced position shifts can be a more sensitive direction cue than behavioural responses that are presumably based upon analysis of the motion signal itself.