Dividing attention across feature dimensions in statistical processing of perceptual groups

Statistical processing has been shown in the perception of several visual dimensions, including size, speed, direction of motion, and orientation. Chong and Treisman (2005) found no decrement when people simultaneously averaged two sets on a single dimension, size. What happens when attention is divided between different dimensions? In two experiments, we investigated judgments of mean size and speed, either within the same objects or in two separate sets. In a third, we examined judgments of mean size and orientation in two different sets. All three experiments suggest a decrement in performance when attention is shared between two dimensions, especially when they are carried by two different sets of objects.