Abstract
People have capacity limits when tracking objects in direct perception. But how many objects can people track in their imagination? In nine pre-registered experiments (N = 313 total), we examine the capacity limits of mentally simulating the movement of objects in the mind's eye. In a novel Imagined Objects Tracking task, participants continue the motion of animated objects in their mind up to a pre-defined point. When tracking one object in the imagination (Experiment 1a), participants give estimations in line with ground truth. But, when imagining two objects (Experiment 1b), behavior alters substantially: responses are fit best by the predictions of a Serial Model that simulates only one object at a time, as opposed to a Parallel Model that simulates objects in tandem. The serial bottleneck is not due to response/motor limitations (Experiment 2), and is reduced - but not eliminated - by adding extremely strong grouping cues (Experiment 3). Additional studies validate that seriality is found for naturalistic occlusion (Experiment 4) and hyper-simplified physics (Experiment 5), and is not due to factors like noise or lack of motivation (Experiments S1-S3). Altogether, we find that the capacity of moving imagined entities is likely restricted to a single object at a time.