Rachel Eng¹, Lana Trick¹; ¹University of Guelph

Multiple object tracking (MOT: Pylyshyn & Storm, 1988) is the ability to monitor the positions of a subset of identical items (targets) among identical non-target items (distractors). This ability is believed to be required for everyday activities such as keeping track of children in a crowd or driving a vehicle. However, most real-world situations involve items that differ in surface features (e.g., colour and shape). Our previous research investigated the effect of target similarity and item uniqueness by using displays of 16 items that varied on two feature dimensions: colour (red, blue, green, yellow) and shape (circle, triangle, square, cross), such that each item was a unique combination. Every trial had four targets and 12 distractors. Targets could have the same colour or shape (Colour-share and Shape-share conditions, respectively), or no common features (e.g., the No Share condition: e.g., red circle, blue triangle, green square, yellow cross). We found that performance was significantly better when targets shared a colour or shape than when they did not (target similarity effect), though even performance in the No-share condition was superior to that when items were identical (the uniqueness benefit). To determine whether these two effects were stable across featural change, we compared performance in the four conditions when the items retained their colours and shapes to when they adopted novel colours and shapes during item motion, manipulating whether the items preserved feature grouping (e.g., all red items become purple) or disrupted it (e.g., red items become purple, pink, aqua, brown). In the Colour and Shape Share conditions, performance was significantly impaired when the change disrupted the grouping but not when grouping was preserved. In contrast, colour and shape changes did not affect performance in the No-share or Identical item conditions. This suggests the uniqueness benefit and target similarity effects reflect different mechanisms.

Acknowledgements: We would like to thank NSERC for funding this research.