Collinear facilitation is largely uncertainty reduction.

When flanked by collinear Gabor patches, detection thresholds for a target Gabor patch improve by up to a factor of 2. This result has been interpreted as evidence for collinear facilitation. However, facilitation has been observed only for targets near detection threshold, where observers seem uncertain about the location and other properties of the stimulus. So the effect of the flankers may be to reduce this uncertainty. If this is true, then other cues to target location should produce a similar improvement in thresholds. To test this hypothesis, we measured contrast detection thresholds for a Gabor target alone, and in the presence of either a faint circle surrounding the target location, or two high-energy flanking Gabor patches. We also used an adaptive procedure to measure the slope of the psychometric function to determine whether the slopes were considerably lower in the presence of location cues or flanking Gabors, as predicted by signal detection theory when uncertainty is reduced. As observed previously, the presence of collinear flankers improved detection thresholds by a factor of two. Yet, on average, the circle alone accounted for the most of the facilitation; for three of our five observers, it improved thresholds as much as the collinear flankers. Other cues that specified target location produced similar improvements in detection thresholds. The slopes of the psychometric functions were much shallower in the presence of these location cues or the collinear flankers compared to the target-alone condition. This change in the slopes indicates that the threshold improvement is largely due to a significant reduction in uncertainty.