Effects of visual cues on scene-relative object motion judgments and concurrent heading estimation from optic flow

During self-movement, the visual system uses optic flow to identify scene-relative object motion and estimate the direction of self-movement (heading). Although both processes rely on optic flow, their relationship and the conditions under which independent object motion biases heading estimation remain unclear. The causal inference model predicts that misjudging object motion leads to its integration into heading estimation, causing errors in heading estimation, whereas correct judgments reduce these errors. However, most studies have examined these processes independently. Here we used a dual-task paradigm to investigate how visual cues affect the judgment of scene-relative object motion direction and concurrent heading estimation. Participants viewed a 90° × 90° display simulating self-movement through a three-dimensional cloud with a laterally moving object positioned at 8° or 16° from the simulated heading direction. They judged both the object's motion direction in the scene and their heading direction. Results show that increasing an object's speed and reducing its positional offset from the simulated heading direction improved the accuracy of scene-relative object motion direction judgment, but did not consistently improve the accuracy of heading estimation. Surprisingly, visual cues such as binocular disparity and object density improved scene-relative object motion direction judgment but reduced heading estimation accuracy. Furthermore, heading errors mostly peaked at object speeds where observers could reliably judge scene-relative object motion direction, challenging the predictions of the causal inference model. These findings provide strong evidence that scene-relative object motion judgment and heading estimation operate independently and question the generality of the causal inference model in explaining heading biases caused by independent object motion.