Contextual influences on interactive processing: effects of discriminability, quantity, and uncertainty.

Three contextual factors--(1) the discriminability of stimuli in pitch, (2) the number of stimuli differing in pitch, and (3) the uncertainty regarding which stimuli or tasks would appear--were manipulated as subjects performed speeded loudness classifications in each of six experiments. The magnitude of Garner interference and effects of congruity were used to gauge the degree of interactive processing. Enhancing pitch discriminability caused monotonic increases in interference and congruity. Stimulus-task uncertainty mediated the changes in Garner interference wrought by increased discriminability. Uncertainty also caused a surprising shift in congruity from strongly positive to strongly negative as uncertainty grew. Increasing stimulus quantity lowered interference, but had inconsistent effects on congruity. Regression analyses suggested that, collectively, these three contextual variables underlie most failures of selective attention in speeded classification.