Contrast as seen in visual search reaction times

Three pigeons searched arrays of alphabetic letters displayed on computer monitors. On each trial, either an A or an E appeared, and the reaction time and accuracy with which the bird pecked at this target were measured. In each block of trials, each target (A or E) was displayed alone, or together with a number of distractor letters (2 or 18) that varied in their similarity to the target. During a baseline series of sessions, responses to the A and to the E each yielded food reinforcement on 10% of the trials. In the next series of sessions, reinforcement continued at 10% for A, but rose to 30% for E. In a final series, these reinforcement conditions were reversed. As expected, reaction times increased with target-distractor similarity and (for similar distractors) with the number of distractors. Increased reinforcement of E had no effect on reaction times to E, but produced a very consistent increase in reaction times to A; the average increase was constant across the various display conditions. Reversal of the differential reinforcement conditions reversed this contrast effect. Analysis of the reaction time distributions indicated that increased reinforcement to E decreased the momentary probability of response to A by a constant amount, regardless of display conditions. These results are discussed in relation to theories of contrast, memory, and of the search image.