Facilitated visual search at low color contrast

The influence of color contrast on visual search behavior was analyzed in young and old observers with normal vision and observers with age-related macular degeneration. A display of a variable number (2 or 8) of 2.7° disks, arrayed on a virtual circle of 12° diameter, was presented to the observer. The task was to indicate as rapidly as possible whether one disk was a different color than the other(s), which was true on 50% of the trials. The color difference was chosen randomly from one of four color axes (achromatic, protan, deutan, or tritan confusions axes) and from one of three contrasts along each of these axes. A significant interaction between stimulus contrast and number of distractors was found in the observers with an intact central visual field. Surprisingly, the source of this variation was a decrease in reaction time with increase in the number of distractors at the lowest contrast levels tested. Though not significant, a similar tendancy was noted throughout the reaction time data in the low-vision group. Because the 8-disk stimuli formed a circle, either proximity or configuration effects might explain this unexpected result. In the normal groups, reaction time as a function of separation of the 2-disk stimuli, however, did not consistently vary as the first of these two hypotheses would predict. In contrast, the average reaction time did increase with separation of the disks in observers with macular degeneration, suggesting that spatial uncertainty constrains their search times.