Attentional spread in the statistical processing of visual displays.

We tested the hypothesis that distributing attention over an array of similar items makes its statistical properties automatically available. We found that extracting the mean size of sets of circles was easier to combine with tasks requiring distributed or global attention than with tasks requiring focused attention. One explanation may be that extracting the statistical descriptors requires parallel access to all the information in the array. Consistent with this claim, we found an advantage for simultaneous over successive presentation when the total time available was matched. However, the advantage was small; parallel access facilitates statistical processing without being essential. Evidence that statistical processing is automatic when attention is distributed over a display came from the finding that there was no decrement in accuracy relative to single-task performance when mean judgments were made concurrently with another task that required distributed or global attention.