Task decision difficulty: effects on ERPs in a same-different letter classification task.

A "same-different" letter pair choice reaction time task was used to compare reaction time, performance accuracy, and P3 amplitude and latency at three levels of task difficulty. Stimulus set was held constant across tasks, and task difficulty was manipulated by instructions to the subject. Subjects delivered a button-press response to designate whether members of each letter pair matched or mismatched on the basis of their physical identity (low difficulty), name identity (medium difficulty), or category identity (vowels/consonants, high difficulty). Task decision difficulty was confirmed by slower reaction times and reduced response accuracy. P3 amplitude was inversely related to task decision difficulty; relatively larger P3s were associated with matched (vs. mismatched) letter pairs. These findings evidence direct effects of task difficulty on P3 amplitude, possibly as the result of "equivocation" related to more difficult task judgments.