Perceived global velocity is strongly influenced by motion inside the moving elements

We investigate whether the presence of local movement inside an object influences its perceived global motion. To test this, we did the following psychophysical experiment: Two Gabor patches moved downwards along straight paths on a computer monitor. In addition to moving vertically, they could also move horizontally, either towards or away from each other. The sinusoidal patterns of the Gabor patches could also move relative to their boundaries, either inwards or outwards. Subjects were instructed to detect whether the two Gabor patches were globally moving towards or away from each other. When the sinusoidal pattern of the Gabor patch was stationary relative to its boundary, subjects showed no bias for the perceived direction of horizontal global motion. However, when the sinusoidal pattern of the Gabor patch was moving relative to its boundary, subjects showed large biases. These biases depend solely on the direction and speed of the motion within the Gabor patch; they do not depend on the speed of the Gabor patch as a whole. When the motion within the Gabor patches relative to the boundaries is inwards, the global motion of the Gabor patches needs to be outwards in order to appear vertical, and vice versa. Although they were specifically instructed to look at the motion of the Gabor patches as a whole, subjects appear to track the absolute movement of simple features within the Gabor patch.