Abstract
“Inattentional blindness” refers to a lack of awareness of stimuli that appear in unattended regions of the visual field. It is typically assessed using “off-line” measures that probe awareness of the stimuli after they are gone. Here, this lack of awareness was used as an operational definition for conditions of inattention. Separate “on-line” measures, taken while the stimuli were still present, were then used to probe processing under those conditions. Results from multiple studies using this strategy provide an emerging picture of the extent to which stimuli in unattended regions of the visual field are and are not processed. In particular, perceptual organization processes, such as grouping by similarity and the completion of occluded (and illusory) surfaces, were engaged without attention. However, spatial codes associated with the positions of very simple stimuli—ones that should have required no complex organizational process to be represented—did not interfere with response-selection processes when they were unattended, but did when they were attended. This interference was assessed using a measure known as the “Simon effect.” Together, the results confirm that focused attention is unnecessary for at least some aspects of the perceptual organization of visual information; rather, the encoding into memory of the representations formed by those processes seems to require focused attention. Further, for perceptual representations of stimuli to influence action, in the manner probed by the Simon effect, stimuli must be attended.