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Cerebral Cortex Advance Access originally published online on October 13, 2004
Cerebral Cortex 2005 15(7):885-887; doi:10.1093/cercor/bhh196
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Cerebral Cortex V 15 N 7 © Oxford University Press 2004; all rights reserved

Feature Article

Suppressing versus Releasing a Habit: Frequency-dependent Effects of Prefrontal Transcranial Magnetic Stimulation

Daria Knoch, Peter Brugger and Marianne Regard

Department of Neurology, University Hospital Zurich, CH-8091 Zurich, Switzerland

Address correspondence to Daria Knoch, Department of Neurology, University Hospital Zurich, Frauenklinikstrasse 26, CH-8091 Zurich, Switzerland. Email: daria.knoch{at}usz.ch.

When subjects are required to generate a random sequence of numbers they typically produce too many forward and backward ‘counts’ (e.g. 5–6, 4–3). This counting bias is interpreted as the consequence of an interference by overlearned tendencies to arrange numbers according to their natural order. Inhibition of such well-learned routines is known to rely on frontal lobe functioning. We examined differential effects of slow (1 Hz) and fast (10 Hz) repetitive transcranial magnetic stimulation (rTMS) over the left or right dorsolateral prefrontal cortex (DLPFC) on random number generation (RNG) performance. Eighteen healthy men performed an RNG task. Those subjects stimulated over the left DLPFC showed a frequency-dependent rTMS effect: counting bias was significantly reduced after the 1 Hz stimulation compared with baseline, but significantly exaggerated after the 10 Hz stimulation compared with 1 Hz stimulation. In contrast, the sequences of the subjects stimulated over the right DLPFC showed the well-known excess of counting in all conditions (i.e. baseline, 1 Hz and 10 Hz). These findings confirm the functional importance of specifically the left DLPFC in sequential response production and show, for the first time, that rTMS affects cognitive processing in a frequency-dependent manner.

Key Words: dorsolateral prefrontal cortex • pulse frequency • random number generation • response suppression • transcranial magnetic stimulation


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