Effect of object discriminability on multiple object tracking

Abstract

In the Multiple Object Tracking (MOT) paradigm observers track a designated set of (typically 4) objects that move independently and unpredictably among an equal number of identical distractors. It has been shown (Scholl, Pylyshyn & Franconeri, 1999) that, consistent with Pylyshyn's Visual Indexing Theory, a change in the color or shape of a tracked objects is not encoded during MOT. Even if properties are not encoded, it might still be easier to track targets if some properties distinguished targets from nontargets. However, if targets were consistently different from nontargets, selecting the targets at the end of a trial could be accomplished without actually tracking. Consequently, we developed a paradigm for investigating whether a distinctive property of objects (e.g., their color) would improve tracking even when the property could not be used directly to pick out targets without tracking them. In this paradigm the color of each object was always distinct from the color of every other object, but it did not remain fixed, and therefore targets could not be tracked merely by recalling their color. The method involved selecting 8 equispaced points on a color circle and rotating all 8 selections around the circle in a continuous manner. Three conditions were compared: (1) Colors of objects were always distinct but continuously changing, (2) colors of all objects were always the same but changed continuously in synchrony, and (3) colors of objects were identical and fixed. Results showed that that performance in these three conditions were significantly different, with (1) being best and (3) being worst. The improvement in tracking arising from maintaining distinct colors is discussed in terms of two possible explanations: Reduced confusability of target-nontarget pairs that came close to one another during a trial, and use of color differences in an “error recovery” stage.