Topographical and temporal stability of the P300

Sixteen normal adults listened to a standard oddball auditory paradigm 3 times, each time separated by 15 min, and this protocol was then repeated 1-2 months later. Stability of the wave forms was measured between 250 and 500 msec for each subject and for each of 16 electrodes by the cross-correlation technique described by Glaser and Ruchkin (1976) and Gevins (1987) in which points on 2 digitized wave forms are paired to produce a correlation coefficient. For each subject, the correlation coefficient was generated for each electrode and then averaged across the 2 parietal and 2 central electrodes to produce a single stability measure for the central electrodes, while the cross-correlation coefficients for the 12 remaining electrodes were averaged for a peripheral stability measure. Three-way repeated measures ANOVAs were performed to determine the significance of cross-correlation coefficient differences. The stability over 15 min for the central electrodes was 0.80, indicating that the P300 was very stable over a short time. The peripheral electrodes were significantly less stable than the central electrodes (P = 0.001). The stability of the wave forms was virtually unchanged when assessed over 1 month (P = 0.9). The target wave forms were significantly more stable than the difference waves (target minus non-target) for both the central and peripheral electrodes (P = 0.04 and 0.01). When the 3 blocks within each session were averaged, there was a significant increase in stability (P less than 0.0005). The wave form cross-correlation coefficients can be used as a measure of the stability of a topographical map over time. The map of the target P300 wave form is very stable, showing no loss in stability from 15 min to 1 month, is more stable than the difference wave form and significantly increases in stability when separate trial blocks are averaged together.