Purpose: The most prevalent current view of the functional role of the P300
component of the event-related potential (ERP) is that it indexes strategi
c processing related to context updating. Using independent-components anal
ysis (ICA), the present study examined the role of P300 in the tactical pro
cess of response selection. Methods: Ln a task crossing manipulations of pe
rceptual difficulty (PD) and response-selection difficulty (R-SD), ICA was
employed to measure not only P300 latency, but its onset and duration as we
ll. Results: Increased PD delayed P300 latency and onset in parallel, while
increased R-SD lengthened P300 duration. Conclusions: The latency and onse
t results suggest that the often-cited covaration of P300 latency with stim
ulus-evaluation time is secondary to effects on processing stages preceding
P300. The results for duration indicate that P300 is involved in response
selection, suggesting that it is not a unitary phenomenon. While P300's wel
l-known relation to stimulus probability indicates a strategic role, our fi
ndings indicate a tactical role as well.