Light-adapted foveal luminance increment thresholds were measured for white photopic targets of 1.5-arc min diameter and 220-ms duration. We aimed to learn about the properties of mechanisms that subserve the detection of these targets. To study this subject we developed a noise probe technique that inserts noise close to the site of the stimulus. Threshold is more than doubled when zero-mean luminance noise is placed at a pair of flanking spots in the horizontal meridian centered on the test spot and 1.5 arc min distant. The detection mechanism thus has a broad field, since noise effects persist at 5-arc min separation. The masking effect increases when the noise is in antiphase at the two flanking spots. Neither even- nor odd-symmetric mechanisms are able to explain these findings, regardless of whether linear or nonlinear processing is employed. The target detection may be mediated in part by a motion-sensitive mechanism.
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