and published as:
Beaudot, W.H.A. (2001). Temporal asynchrony in contour integration: A retelling of the fable of "The Hare and the Tortoise" [Abstract]. Journal of Vision, 1(3), 147a, http://journalofvision.org/1/3/147
Full paper published as:
Beaudot W.H.A. (2002), Role of onset asynchrony in contour integration, Vision Research 42(1):1-9, January 2002. 
TEMPORAL ASYNCHRONY IN CONTOUR INTEGRATION: A RETELLING OF THE FABLE OF "THE HARE AND THE TORTOISE"
McGill Vision Research, Department of Ophthalmology, McGill University, Montreal, Canada.
Purpose. 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 & Donnelly (Nature, 1998) showed 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. We investigated this puzzling result using a standard contour integration paradigm (Field et al, Vis. Res., 1993).
Methods. The task required the linking of orientation across space to detect a path in a stimulus composed of randomly oriented Gabor elements (1.5 cpd, sigma = 0.17 deg), measured using a temporal 2AFC method. In a first experiment, we explored the effect of onset delays between path and background elements. In a second experiment, we replicated Usher & Donnelly's experiment with a cyclic presentation and controlled for stimulus order.
Results. The first experiment demonstrated a strong priming effect of path elements. The second experiment demonstrated that the effect reported by Usher & Donnelly depends, in fact, on the order of asynchrony between path and background elements in the first cycle of the stimuli presentation: facilitation in visual grouping only occurred when path elements were presented before background elements.
Conclusions. We conclude that Usher & 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.
Research supported by CIHR Grants to K.T. Mullen (MOP-10819) and R.F. Hess (MT-10818)