Delayed flanker effects on lateralized readiness potentials.

When participants were required to respond to a relevant central target and ignore irrelevant flanking stimuli, the flankers produced a response compatibility effect. Electrophysiological studies have shown that irrelevant flanker stimuli can affect the motor system. The present experiments further examined the characteristics of flanker effects on the motor system. Sixty participants responded in the flanker task to arrows (experiment 1) or letters (experiment 2). To examine time and extent of flanker effects on the motor system, target onset was delayed with blocked or random stimulus-onset asynchronies (SOA). With SOA of 0 and 100 ms, flanker effects on behavioral measures were reduced in random as compared to blocked conditions, but enhanced with SOA of 400 ms. With SOA of 400 ms, flanker effects on the early lateralized readiness potentials (LRP) were reduced in blocked as compared to random conditions, indicating that the onset of flanker effects on the LRP was delayed. Response-locked LRPs suggest that flanker and target stimuli activate the motor system successively. Findings challenge current theories of the flanker compatibility effect.