Does assessment of scene-relative object movement rely upon recovery of heading?

We have provided compelling evidence that the recovery of scene-relative object movement is aided by neural flow parsing mechanisms which discount (subtract) global optic flow consistent with observer movement [Warren and Rushton, 2009, Current Biology, 19, 1555–1560]. Here, we examine whether flow parsing can occur independently from heading recovery. Stimuli comprised two simultaneously presented limited lifetime optic flow fields: 1. Expanding radial flow with focus of expansion (FOE) at the centre of the display; 2. Rightwards laminar flow. This stimulus gives rise to the optic flow illusion (OFI) under which illusory shifts in heading are perceived in the direction of laminar flow. Experiment 1: Observers viewed the OFI stimulus together with a probe moving upwards (∼0.6°/s) from the centre of the display. After 2 s stimulus presentation, observers reported the perceived probe motion direction by adjusting the orientation of a virtual paddle. If flow parsing depended on heading recovery under the OFI then perceived probe trajectory should tilt rightwards towards the illusory FOE. Instead, perceived motion was tilted leftwards. Experiment 2: To test that the results of experiment 1 were due to optic flow parsing we manipulated the configuration of the OFI stimulus (full field, left hemi-field only, right hemi-field only) and the start position of the probe (3 deg left/right of central fixation). When the probe was to the right, perceived trajectory tilted by 15–20° leftwards towards the centre. When the probe was to the left perceived trajectory was close to veridical. Crucially, the effects persisted when the probe was in the opposite hemi-field to the OFI stimulus - a key feature of global flow parsing. Again, these results are inconsistent with discounting of optic flow based on perceived heading under the OFI, suggesting that flow parsing does not necessarily depend on heading recovery.