The role of the right dorsolateral prefrontal cortex in the Tower of London task performance: repetitive transcranial magnetic stimulation study in patients with Parkinson's disease.

We studied whether one session of high-frequency repetitive transcranial magnetic stimulation (rTMS) applied over either the right or left dorsolateral prefrontal cortex would induce any measurable changes in the Tower of London spatial planning task performance in patients with Parkinson's disease (PD). Ten patients with PD (with no dementia and/or depression) entered the randomized, sham-stimulation-controlled study with a crossover design. Active and placebo rTMS were applied over either the left or the right dorsolateral prefrontal cortex (in four separate sessions) in each patient. The order of sessions was randomized. The Tower of London task was performed prior to and immediately after each appropriate session. The "total problem-solving time" was our outcome measure. Only active rTMS of the right dorsolateral prefrontal cortex induced significant enhancement of the total problem-solving time, p = 0.038. Stimulation of the left prefrontal cortex or sham stimulations induced no significant effects. Only rTMS applied over the right dorsolateral prefrontal cortex induced positive changes in the spatial planning task performance in PD, which further supports the results of functional imaging studies indicating the causal engagement of the right-sided hemispheric structures in solving the task in this patient population.

The effect of cortical repetitive transcranial magnetic stimulation on cognitive event-related potentials recorded in the subthalamic nucleus.

We studied whether the cognitive event-related potentials (ERP) in the subthalamic nucleus (STN) are modified by the modulation of the inferior frontal cortex (IFC) and the dorsolateral prefrontal cortex (DLPFC) with repetitive transcranial magnetic stimulation (rTMS). Eighteen patients with Parkinson's disease who had been implanted with a deep brain stimulation (DBS) electrode were included in the study. The ERPs were recorded from the DBS electrode before and after the rTMS (1 Hz, 600 pulses) over either the right IFC (10 patients) or the right DLPFC (8 patients). The ERPs were generated by auditory stimuli. rTMS over the right IFC led to a shortening of ERP latencies from 277 +/- 14 ms (SD) to 252 +/- 19 ms in the standard protocol and from 296 +/- 17 ms to 270 +/- 20 ms in the protocol modified by a higher load of executive functions (both P < 0.01). The application of rTMS over the DLPFC and the sham stimulation over the IFC showed no significant changes. The shortening of ERP latency after rTMS over the right IFC reflected the increase in the speed of the cognitive process. The rTMS modulation of activity of the DLPFC did not influence the ERP. Connections (the IFC-STN hyperdirect pathway) with the cortex that bypass the BG-thalamocortical circuitries could explain the position of the STN in the processing of executive functions.