Taking attention to task: P300, task response probability, and within-category deviation detection

Previously, we reported enhanced P3 amplitudes to between-category deviations among high and low probability events. Here, we tested the effects of a within-category difference. Subjects performed a go/no-go button press task as they viewed repeated, randomly-ordered presentations of nine double-digit numbers. Eight numbers fell within a prescribed range (42-49, standards); prior to testing, subjects selected one standard number for later recall. A ninth, out-of-range (91, deviant) number was also included. Subjects were tested under two conditions, in which they responded either to one (low response probability) or to seven (high response probability) standard nonselected numbers, designated as targets. Relatively larger P3s were consistently apparent only when the deviant nontarget was associated with a low probability response to a nontarget. There was a selective effect of nontarget response probability on P3 amplitude to the deviant nontarget. Our results indicate that within-category deviation detection is facilitated by "controlled" attention to the structure of the stimulus field.