Spatial nonuniformities and velocity of filling-in in dynamic brightness induction

There is converging evidence that brightness induction is neither spatially uniform nor instantaneous. In the temporal domain, multiple studies have shown a relatively low (2-3 Hz) temporal frequency cutoff in brightness induction from temporally modulated surrounds (De Valois, Webster, De Valois, & Lingelbach, 1986; Paradiso & Nakayama, 1991; Rossi & Paradiso, 1996). It was also demonstrated that the temporal frequency cutoff depends on the spatial frequency of the stimulus (Rossi & Paradiso, 1996; see also Robinson & de Sa, 2008). These findings are consistent with the idea that brightness induction is mediated by a filling-in process that begins at the borders and travels at a relatively low velocity. However, in contradiction to observed low temporal cutoff in brightness induction, all earlier attempts to directly estimate the induction velocity resulted in very high estimates (e.g., 100°/s-200°/s). In the present study, using a novel paradigm, we obtained velocity estimates that are in line with predictions based on temporal frequency cutoff. In addition, the use of the same stimulus set to measure spatial nonuniformities, temporal frequency cutoff, and velocity allowed direct comparison of different velocity estimates.