Knowing the moment of target occlusion influences time-to-contact estimation strategies

Estimating the time-to-contact (TTC) of moving objects allows us to properly interact with them even if they are momentarily hidden from view. Sometimes, occlusions are unexpected, disrupting the ongoing accumulation of sensory evidence. Other times, the occlusion onset is predictable. The effect that this may have in estimating TTC has never been addressed. To do so, we asked participants (N=12) to press a key when they thought a 1 cm diameter white target moving rightwards would be aligned with the end of a rectangular occluder. In different, randomly interleaved conditions, the occluder could be located either above or below the target’s path. If it was above, it reliably indicated the part of the trajectory during which the target would be occluded. If it was below, the occlusion position could coincide with the occluder or not. Eye movements were recorded to discern whether differences in reliability led to different TTC estimation strategies. In all cases, targets moved at 5, 10 or 15 cm/s, were visible for 150, 300, 600 or 1200 ms, and occluded for 100, 200, 400, 800 or 1600 ms. Participants got feedback about their performance after each trial. TTC estimation errors were more accurate when participants could predict the targets’ disappearance (1.73 ms ± 16.18 ms vs 20.08 ms ± 14.42 ms, mean ± SEM). In both conditions, precision clearly improved as the ratio of target visibility to occlusion time increased. The eye movement patterns differed depending on occlusion cue reliability: the predictable condition prompted fixations on the occluder’s left edge, while in the unpredictable condition, participants tended to smoothly pursue the target until its disappearance. These results suggest that individuals use different strategies to estimate TTC when confronted with varying reliability in occlusion cues.