Dorsolateral prefrontal cortex and motor preparation: a TMS investigation of the foreperiod effect

When Foreperiods (FPs) of different duration alternate on a trial-by-trial basis equiprobably but randomly, the reaction time (RT) decreases as the FP increases (FP effect). Another phenomenon observed in that paradigm consists of the sequential effects: RT is slower as the FP in the preceding trial gets longer. It is not clear if the two effects are due to different underlying mechanisms. Patients with lesions on the right dorsolateral prefrontal cortex do not show the typical FP effect [Stuss et al. (2005). Neuropsychologia, 43, 396-417]. Aim of the present study was to replicate this result in healthy adults, through an off-line TMS paradigm (Theta Burst Stimulation). Another purpose was to investigate whether any change of the sequential effects would follow a possible variation of the FP effect. Results of two experiments (with simple and choice RT task, respectively) indicate a significant reduction of the FP effect, selectively after stimulation of the right dorsolateral prefrontal cortex with respect to a pre-TMS baseline, and to the stimulation of a contralateral site and of another site in the right Angular Girus. The dissociation between the two effects supports a dual-process model [Vallesi & Shallice, subm.], according to which they are due to two distinct but interacting processes. In particular, the FP effect is attributed to a strategic process monitoring the conditional probability of the stimulus occurrence, whereas the sequential effects would originate by an automatic mechanism of arousal regulation by the length of the preceding FP.