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In 4 Simon experiments the authors examined control over 2 routes of sensorimotor processing: response priming in the unconditional route and response selection via the conditional route. The Simon effect diminished as the frequency of noncorresponding trials increased. Location-based response priming was observed only when the stimulus followed a corresponding event but not after a noncorresponding trial. Therefore, the unconditional route appears to be suppressed whenever the task context indicates priming as potentially disadvantageous. Moreover, the task-irrelevant stimulus location was used for response selection as a function of correspondence probability. Although exact repetitions of stimulus-response sequences caused a marked speed-up of responses, this 3rd mechanism is independent of unconditional route suppression and frequency-based adjustments in the conditional route.

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Response speed to a signal is faster when advance information about the forthcoming movement is provided before signal onset. Although this precuing effect is well established, the location of this saving in reaction time (RT) in the information-processing system is controversial. Some authors have claimed that the precuing effect resides at a motoric level, whereas others have suggested a nonmotoric locus. The present experiments used onset latencies of the lateralized readiness potential (LRP) to locate the precuing effect. The results of 2 experiments with a highly compatible (Experiment 1) and with an incompatible (Experiment 2) stimulus-response mapping indicate that this effect resides, at least partially, in the motoric portion of RT. In addition, the LRP amplitude before signal appearance increased with the amount of advance information, supporting a muscle-specific preparation hypothesis.

The execution of efficient motor actions is often preceded by preparation in the central nervous system. Although this kind of preparation cannot be observed directly, it, nevertheless, shortens reaction time. In this review we focus on two types of action preparation, namely event and temporal preparation. In particular, we show how modern event-related brain potential techniques can be employed to determine both the covert processes underlying such preparatory effects as well as their locus within the processing chain between stimulus input and action output.