P300 and cognitive ability: Processing demands, equivocation, and speed of processing during simple cognitive tasks.

The hypothesis that individual differences in mental ability depend, in part, on the speed or efficiency of performing elementary cognitive tasks was examined. Performance and event-related potential measures were determined during the performance of a Sternberg memory scanning task with three levels of difficulty, a category matching task, and a synonym-antonym task. In each task two stimuli were presented sequentially on each trial. In the Sternberg task, the first stimulus was one, three, or five letters and is called the memory set. The second stimulus was a probe stimulus. A category name and a category exemplar were presented in each trial of the category matching task. The two stimulus words presented in each trial of the synonym-antonym task were either synonyms or antonyms. Reaction time, movement time, and reaction time standard deviation were all negatively correlated with intelligence at levels consistent with previous research. In the Sternberg task, P300 amplitude to the first stimulus increased as the number of items in the memory set increased, affirming that P300 amplitude is sensitive to demands for processing resources. P300 amplitude to the first stimulus was smaller for higher ability than lower ability subjects. This is consistent with the idea that lower ability subjects require greater processing resources than higher ability subjects. The ERP differences between higher and lower ability subjects to the first stimulus were greater at fronto-central electrode sites than at Pz where P300 is maximal. P300 amplitude to the target stimulus decreased as set size increased. This is consistent with the view that P300 amplitude is sensitive to changes in task difficulty. P300 amplitude to the second stimulus tended to be larger for the higher ability group, an effect that can be understood in terms of equivocation. P300 latency to both the first and second stimulus increased as the set size increased, affirming that P300 latency is a measure of the time required for stimulus evaluation and classification. Higher ability subjects displayed longer P300 latency to the first stimulus than lower ability subjects. The longer P300 latency to the first stimulus suggests that higher ability subjects devote more time to stimulus analysis and planning than lower ability subjects. The performance of higher ability subjects on these tasks was characterised by the more efficient deployment of processing resources and less equivocation than lower ability subjects. Speed of processing and speed of motor response were faster for higher ability than lower ability subjects