Age Differences in the Distractor-Ratio Effect as a Function of Contrast Level

ABSTRACT

Latencies (RT) and eye movement measures were used to examine the effects of age and contrast on the distractor-ratio effect (DRE) in visual search. Younger and older adults performed a contrast × orientation conjunction search task where the ratios of white to black distractors and luminance contrast levels were varied. The distractor-ratio manipulation had similar effects for older and younger adults on both RT and the number of fixations required to find the target. Both measures were largely independent of distractor ratio on target-present trials, while both RTs and the fixation number increased with the number of items sharing the target's contrast polarity on target-absent trials. A more detailed analysis of eye movements suggested that younger adults were a bit more adept at attending to the smaller set of distractors, which presumably facilitated both overt and covert search. Generalized slowing can account for the age differences in RT, but the fixation number data speak to another mechanism, perhaps increased cautiousness on the part of the elderly when signal strength is low.

ACKNOWLEDGMENTS

Many thanks to Geoffrey Ho and David Stewart. Geoffrey Ho provided a great many hours of his time explaining, editing, and providing constructive criticism along every stage of this project. His support and encouragement is most appreciated. David Stewart wrote the software that analyzed the raw data from the Eyegaze program. This saved a great deal of time that otherwise would have been dedicated to labor-intensive data manipulation. He also made himself available in a consulting role regarding aspects of the analysis program, output data and corresponding statistics.