Foreperiod and Response Complexity Effects on Movement-Related Potentials

This study addressed a long-standing controversy in the attention and action literature concerning whether speeding of a voluntary reaction by a neutral warning signal (WS) is due to faster, more efficient preparation and execution of the motor response. In contrast to most previous research, we found that manipulation of foreperiod duration (the interval from onset of the WS to onset of the reaction stimulus) did influence hand-specific activity in the sensorimotor cortex, as measured with the lateralized readiness potential (LRP). We also confirmed that when cortical activity contralateral and ipsilateral to the hand of response is separately measured using localized current source density (surface Laplacian waves), speeding of late motoric processes can be observed. Certain alternative possibilities, such as distortion of the baseline-correction procedure by the contingent negative variation (CNV) or misidentification of latency effects due to what we call the "aperture problem," could not fully account for the findings. Facilitation of late motoric processes was most evident when the response involved a sequence of keypresses, comparable to a short phrase on the piano. Relative merits of conventional LRPs versus surface Laplacians are discussed.