Visual search for a tilted target: Tests of spatial uncertainty models

We report that spatial cueing of a parafoveal target in the presence of distracters enhances orientational acuity for that target. When no distracters were present, orientation thresholds were in the range 1-4 degrees. For long exposure times, distracters increased threshold by the amount predicted from a winner-takes-all spatial uncertainty model. For short (100-msec) exposures followed by a random dot mask, the rise in threshold with distracters was considerably greater than that predicted from spatial uncertainty. For brief exposures the effect of distracters was greater when the target and distracters were spatially crowded rather than widely spaced. Adding a tilt to the distracters in the opposite direction to the target increased thresholds still further. Cueing the target with a spatial painter decreased the effect of distracters, even when they were crowded. We suggest that when attention cannot be appropriately focused, discrimination is carried out by a relatively coarse texture analyser, which averages over several elements, and that focused attention permits the analysis of the target over a smaller area of space.