Speed tuning of optic flow parsing
36.443, Sunday, May 12, 2:45 - 6:45 pm, Orchid Ballroom
Andrew J. Foulkes1, Simon K. Rushton2, Paul A. Warren1; 1School of Psychological Science, University of Manchester, UK, 2School of Psychology, Cardiff University, UK
In order to isolate scene-relative object movement the brain attempts to perform a global subtraction of the optic flow components associated with self movement (Rushton & Warren, Current Biology, 2005). Foulkes et al. (VSS, 2012) provided evidence that this flow parsing mechanism shares some common processing stages with heading mechanisms, suggesting that heading models might provide a starting point for models of the flow parsing process. However, common heading models are speed invariant (they recover direction but not speed of observer movement). Here we investigate speed tuning for the flow parsing mechanism. Stationary participants fixated the centre of a radial flow field (30 degrees FOV) of limited lifetime dots simulating forwards motion of the observer at 0.6 m/s. Median onscreen speed was approximately 2.15 deg/s. Participants judged the direction of motion of a laterally displaced probe moving vertically upwards. To limit the contribution of local motion processing mechanisms, no flow field dots were presented in a patch of radius 3 deg centred on the probe. We manipulated the dot speed by scaling the simulated observer speed by factors of (0.25, 0.5, 1.0, 2.0, 4.0, 8.0). Participants indicated the perceived trajectory of probe motion by setting the orientation of an adjustable paddle. The relative tilt effect (difference between perceived and actual trajectory) was measured. Flow parsing predicts that the relative tilt should be inwards due to subtraction of the outwards radial flow. Results indicate a clear dependence of the relative tilt effect on simulated speed. As speed increases, the relative tilt effect also increases. However, at intermediate speeds the effect stabilises and there is a suggestion of a decrease in relative tilt for the fastest speeds tested. These results suggest that the speed invariance of common heading models makes them inappropriate to underpin models of flow parsing.