The capacity limits of mental simulation

People have severe 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 examined the capacity limits of mentally simulating the movement of objects in the mind's eye. In a novel Imagined Objects Tracking task, participants continued the motion of animated objects in their mind up to a pre-defined point. When tracking one object in the imagination (Experiment 1a), participants gave estimations in line with ground truth. But, when imagining two objects (Experiment 1b), behavior altered substantially: responses were 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 and analyses validated that seriality is not due to factors like noise or lack of motivation, and is found also for hyper-simplified physics and naturalistic occlusion (Experiments S1-S5). Altogether, we find that the capacity of moving imagined entities is likely restricted to a single object at a time.