Abstract
Lateral masking experiments indicate greater sensitivity to a foveal Gabor target when presented with flanking Gabors whose local carrier orientations match the target orientation. We examined the spatial and temporal determinants of this surround facilitation by measuring target contrast detection thresholds at multiple exposure durations, target-flanker SOAs and separations. Consistent with previous findings, we found maximum facilitation with synchronous collinear target-flanker onsets and offsets. Critically, the minimum exposure required to elicit facilitation increased with target-flanker separation. Transforming the visual field to human striate co-ordinates, rates of increase in facilitative delay across cortical space corresponded with the conduction velocities of long-range intra-striate connections (0.1 m/s). Contrary to previous findings (Polat & Sagi, 2004), robust suppression occurred when flanker offsets preceded target onsets, but not when the target preceded flankers. This temporally contingent suppression was evident 20ms following flanker offset at all target-flanker separations (0.6 – 1.7 degrees of visual angle), implying very fast suppressive transmission velocities, possibly due to striate-extrastriate interaction. Using circular Gaussian targets, facilitation was also maximal for synchronous presentation, but with no evidence of suppression at any SOA. These data suggest temporally dependent suppressive effects may be contingent upon local carrier information.