Speed-accuracy tradeoff (SAT) refers to the inverse relation between speed
and accuracy found in many tasks. The present study employed reaction times
(RTs) and movement-related brain potentials arising during the RT interval
(lateralized readiness potentials; LRPs) to examine the mechanisms by whic
h people control their position along an SAT continuum. Many models of SAT
postulate that changes in position across conditions (macro-tradeoffs) and
trial-by-trial variations within conditions (micro-tradeoffs) are mediated,
at least in part, by the same mechanisms. These include: (1) all models th
at postulate mixtures of guesses and accurate responses and (2) some models
postulating decision criterions applied to accumulating evidence or respon
se tendencies. Such models would seem to be rejected for conditions under w
hich macro- and micro-tradeoffs can be shown to involve no stages of RT in
common. Under the present conditions, the two types of SAT produced additiv
e effects on RT with the macro-tradeoff involving only that portion of the
RT interval occurring after LRP onset and the micro-tradeoff involving only
that portion before LRP onset. These findings imply that the two types of
SAT arose during different serial stages of RT and that the macro-tradeoff
involved only stages occurring after differential preparation of the two ha
nds had begun. (C) 2000 Elsevier Science B.V. All rights reserved.