Asymmetrical masking between radial and parallel motion flow in transparent displays

Human spatial behavior remains surprisingly accurate and stable, despite the complexities of the concurrent retinal image flow. How does visual motion analysis contribute to behavioral stability? To approach this question, we rely on current knowledge about heading perception and coherent motion extraction from animated random dot displays. This chapter presents psychophysical evidence for an asymmetry in the global visual decomposition of the retinal motion flow. We examined the ability of human observers to identify the radial and the parallel flow component in displays where the two motions were transparently superimposed. Whereas sensitivity to the radial motion remained unaffected by the presence of parallel flow, sensitivity to the latter was greatly degraded in the presence of radial flow. This imbalance was observed within the first 200 ms of motion exposure, prior to the perception of heading (430 ms; Hooge et al. (1999) Exp. Brain Res., 129: 615-628). Although such an asymmetrical masking is well suited to minimize heading errors, whether it really contributes to the extraction of behavioral direction remains an open question.