Role of onset asynchrony in contour integration

Evidence that visual grouping is facilitated when elements comprising a foreground figure are presented simultaneously, and are temporally separated from elements comprising the background, has suggested cortical synchronous oscillations as a possible neural substrate. Supporting this theory, Usher and Donnelly (Nature 394 (1998) 179) showed in one of their experiments that contour integration is facilitated when path and background elements alternate with an asynchrony below the integration time of the visual system, suggesting that these flickering stimuli interact with this hypothetical binding mechanism. I replicated this experiment and report that the effect depends in fact on the order of asynchrony between path and background elements in the first cycle of stimuli presented for more than 100 ms: facilitation in visual grouping only occurs when path elements are presented before background elements. A second experiment, exploring the effect of onset delays between path and background elements, demonstrates a strong priming effect of path elements. I conclude that Usher and Donnelly's result is likely due to the high sensitivity of the visual system to stimulus onset, and that simple flickering stimuli are inadequate for revealing the neural code for binding in figure-ground segregation without controlling for the effect of stimulus onset.