MOVEMENT AFTEREFFECT OF BI-VECTORIAL TRANSPARENT MOTION

Two moving random-pixel arrays (RPAs) were presented simultaneously in the same target field. These RPAs are perceived as two superimposed t ransparent moving sheets. Although two directions are perceived simult aneously during stimulus presentation, the movement aftereffect (MAE) is unidirectional. The visual system averages both motion signals in t he MAE. For motion vectors of equal magnitude and perpendicular direct ion the MAE direction is the inverse of the sum of both vectors. In th e first experiment we measured perceived direction of the MAE of trans parent motion for a range of speed combinations. Results indicate that vector summation only predicts the correct MAE direction for combinat ions of equal speeds. It is suggested that the direction of the MAE of transparent motion is a resultant of the weighted summation of the co mponent inducing vectors. The question then arises what determines the weighting factors. Directional sensitivity and MAE duration of the in dividual vectors under transparent conditions were measured and used t o weigh the vectors and predict the MAE direction of transparent motio n. Statistical analyses showed that MAE duration is a better basis to determine the weighting factors predicting the direction of the MAE of transparent motion than component sensitivity. The direction of the M AE of transparent motion thus seems to be determined by the amount of adaptation to the component vectors as reflected by MAE duration. The results suggest that this gain control cannot be located in the indivi dual motion detectors and must be situated at or after some subsequent cooperation stage of the human motion analysis system.