Abstract
Perceptual load theory has been one of the most influential theories of attentional selection during the past fifteen years, providing a resolution to the early versus late selection debate by arguing for a flexible, load-dependent mechanism of selection. A number of recent behavioral and neurophysiological studies have demonstrated that high perceptual load displays produce inattentional blindness, in which participants are “blind” to task-irrelevant flanking stimuli that appear in the display. Presumably, when the perceptual load of the primary task is high, early selection occurs and participants completely fail to process task-irrelevant information. Such an interpretation argues for a strong, load-dependent early-selection mechanism occurring very early during perceptual processing. However, because inattentional blindness phenomena might be attributed to memory failures, not perceptual failures, we hypothesized that more sensitive measures of flanker identification might provide evidence that participants processed the task-irrelevant flankers to a relatively late level. In the current experiments we measured distractor-related Simon effects and sequential effects to assay interference from flanking stimuli, instead of measuring traditional flanker effects. When distractor processing was measured using traditional flanker effects we replicated the basic load effect: Participants showed no flanker interference under high perceptual load. However, when examining the Simon effect and sequential effects, large interference effects were observed in high-load displays, indicating that the distractors' identities were processed and affected responses. These findings suggest that high perceptual load neither induces ‘blindness’ for task-irrelevant information nor involves early selection.