Dynamics of cross- and iso-surround facilitation suggest distinct mechanisms

Psychophysical studies have found that contrast sensitivity is enhanced by spatially separated flanking stimuli that are collinear with a foveal target. Considerable uncertainty remains, however, about the facilitative effect of other surround configurations. We investigated this by systematically manipulating relative flanker position (target end-zones or side-bands) and orientation (iso-oriented or ortho-oriented targets and flankers) at multiple target–flanker separations. We also examined the effect of a temporal dimension (exposure duration) across combinations of these spatial parameters. We found facilitation in the context of all surround configurations tested, but not at all separations and exposure durations. Interestingly, although the minimum exposure required to induce facilitation (facilitative delay) increased as a function of separation for all configurations (averaged across subjects), the rate at which this occurred depended, not upon flanker position or orientation relative to the target, but the alignment of the flankers relative to each other. By transforming these slopes into striate transmission speeds we estimate that: (i) collinear flanker facilitation matches the slow conduction velocities of long-range (LR) horizontal striate connections and (ii) non-collinear, parallel flanker facilitation correlates with the much faster extra-striate feedforward/feedback connections.