Disambiguation of motion direction by first-order and second-order motion mechanisms

The display comprised 12 black dots arranged evenly around a virtual circle on a grey background. Half of these dots (every second dot) were displayed on one frame, followed by the other half after a suitable interval. This caused a sensation of ambiguous motion that could be either clockwise or counterclockwise on a particular trial. A continuous version of the display can be constructed by repeated cycling of the stimuli. We then interspersed lines between every second pair of points on intermediate frames. When the streaks were the same polarity as the dots (black), they disambiguated the direction of motion in the direction of the added streaks, as may be expected. However, for a wide range of dot-spacing and frame-rates, lines of opposite polarity disambiguated the motion in the opposite direction, against the positioning of the lines. For low frame-rates and large dot-spacing the white lines disambiguated in the direction of the motion. We explain the effects of the black lines and the inversion of motion direction caused by the white lines with a simple first-order model of motion detection: inverting the contrast produces motion energy of opposite direction. Facilitation of motion in the same direction by opposite-polarity stimuli (at longer distances and times) is consistent with a feature-tracking or second-order motion detection mechanism. A detailed model incorporating these two factors is presented