Pushing the limits of transparent-motion detection with binocular disparity

When transparent motion is defined purely by direction differences, observers fail to detect more than two signal directions simultaneously [Edwards, M., & Greenwood, J.A. (2005). The perception of motion transparency: A signal-to-noise limit. Vision Research, 45, 1877-1884]. This limit is strongly related to signal-detection thresholds for transparent motion, which are several times higher than uni-directional thresholds. When the effective signal intensities are elevated by speed differences that drive independent global-motion systems, the transparent-motion limit can be extended to allow detection of three signals [Greenwood, J.A., & Edwards, M. (2006). An extension of transparent-motion detection limit using speed-tuned global-motion systems. Vision Research, 46, 1440-1449]. Because there are independent disparity-tuned global-motion systems, distributing transparent-motion signals across distinct depth planes also allows an increase in their effective signal intensity. In the present study, the addition of depth differences enabled the simultaneous detection of three signals. However, as with the addition of speed differences, observers were not able to detect four signals, which would be predicted if signal intensity were the sole constraint on transparent-motion detection. The combination of depth and speed produced similar results, suggesting that there is a strict higher-order limit, possibly related to attention, restricting the maximum number of signals that can be detected simultaneously to three.