We applied single-pulse transcranial magnetic stimulation (TMS) to the occipital pole of healthy subjects while they performed a forced-choice visual letter-identification task. We found three separate periods when TMS suppressed performance; the first period is best explained by TMS-induced blinking whereas the last two periods are best explained by TMS-induced disruption of letter-processing in the early visual cortex. Unexpectedly, we also found that TMS-induced sup-pression progressively disappeared during three weeks of repeated TMS experiments. However, it was only suppression during the last two periods that disappeared; suppression during the first period remained undiminished. When subjects were then presented with dimmer letters, suppression reappeared. The most likely explanation is a practice-induced increase in neuronal activity in the early visual cortex.
1 Department of Experimental Psychology, South Parks Road, Oxford OX1 3UD, UK
2 Department of Psychology, Oregon State University, Oregon, USA
3 Human Motor Control Section, National Institute of Neurological Disorders and Stroke, NIH, Bethesda, MD, USA
4 Corresponding Author: Erik Corthout
Acknowledgements: This work was supported by EC Biotech ERBCHRXCT 930261, by MRC Programme Grant G7103979, and by MRC ROPA Grant G9711247.
Received 2 February 2000; accepted 5 March 2000