Visual Space Orientation and the Onset Repulsion Effect: The Role of Visual Gravitational Motion in Modulating Spatial Mislocalizations

The perceived onset position of a moving target has been found to be systematically displaced backwards, in a direction opposite to motion direction, a phenomenon coined as the Onset Repulsion Effect (ORE). Of particular relevance to the present work, the ORE has been found to be increased for ascending, in comparison with descending targets, a pattern putatively due to the greater effort required to launch an object upwards against gravitational acceleration. Although this account remains speculative, it raises the possibility that the effect may reflect a ‘natural history’ to dynamic events, akin to the windup which precedes throwing an object, resulting in a backward overcompensation of the target’s onset location. To further explore the role played by visual gravitational motion on the ORE, two experiments were conducted in which participants were required to indicate the onset location of targets moving along one out of sixteen possible trajectories. Differences between the actual and the perceived motion onset were measured and subjected to a discrete Fourier decomposition. Results disclosed an enhanced ORE for targets moving upwards (Gravity Related Component; Experiment 1), an effect that increased with longer retention intervals. Experiment 2 further demonstrated that the Gravity Related Component is tilted in congruence with the orientation of a visual background context. These findings are discussed within the framework of internal models of gravity, human spatial orientation, and their influence on visual motion perception.