EVIDENCE THAT LUMINANT AND EQUILUMINANT MOTION SIGNALS ARE INTEGRATEDBY DIRECTIONALLY SELECTIVE MECHANISMS

Three experiments tested whether motion information for nonequiluminan t (luminant) and equiluminant dots affects direction judgments when bo th types of stimuli are moving simultaneously in the same display. The motion directions for the two sets of dots were manipulated to produc e four direction differences (0 degrees, 30 degrees, 60 degrees, and 9 0 degrees). The equiluminant dots were moved in a perfectly correlated fashion, but the percentage of correlated motion for the luminant dot s was varied. When subjects judged whether the directions of the equil uminant and luminant dots were the same or different, performance for the conditions with 0 degrees, 60 degrees, and 90 degrees difference i mproved as the percentage of correlated luminant motion increased. The same result occurred for a control display that contained two sets of luminant dots. However, for the 30 degrees difference, performance wa s at chance level for the control display, but dropped below chance fo r the equiluminant-luminant display. When subjects indicated just the direction of the luminant dots, judgments were not affected by equilum inant motion. Judgments for the equiluminant dots also were accurate, except for the conditions with 30 degrees difference; these responses were biased by the luminant motion, indicating some form of motion cap ture. The interactive effects are discussed in terms of a directionall y selective mechanism that combines equiluminant and luminant motion s ignals.