Abstract
Multiple object tracking is the ability to keep track of the positions of a subset of identical items (targets) among equally identical distractors (MOT; Pylyshyn & Storm, 1988). MOT is thought to be an important ability for many real-world activities including playing a sport or driving a vehicle. However, these situations typically involve objects with distinct properties (such as different colours and shapes) rather than identical properties. Previous research has found inconsistent effects of item uniqueness on MOT. One explanation is that item uniqueness is only beneficial when there are sufficient working memory resources. Alternatively, the uniqueness effect may be dependent on perceptual differences between items. The goal of this study was to determine what types of features produce a uniqueness benefit. To investigate this question, we designed an MOT task with 16 unique items - every combination of four colours and four shapes (Experiment1: basic geometric shapes; Experiment 2: digits; Experiment 3: line orientations) - or 16 identical items. Each trial had four targets and 12 distractors. In the colour-share condition, targets shared the same colour but had different shapes. In the shape-share condition, targets shared the same shape but had different colours. In the no-share condition, each target had a different colour and shape. All items were identical in the identical condition. Across all experiments, there was an advantage for unique items over identical items. Furthermore, there was a consistent large benefit of the colour-share condition over the no-share condition. However, there was only a benefit of the shape-share condition over the no-share condition in Experiment 1 (basic shapes) and Experiment 2 (digits). Given that line orientation is a basic feature that requires little working memory resources, these results suggest that the inconsistent effect of shape is driven by perceptual differences between stimuli.