VERBAL WORKING-MEMORY COMPONENTS CAN BE SELECTIVELY INFLUENCED BY TRANSCRANIAL MAGNETIC STIMULATION IN PATIENTS WITH LEFT TEMPORAL-LOBE EPILEPSY

The aim of this study was to investigate whether transcranial magnetic stimulation (TMS) can be used for a lateralization of verbal and non- verbal memory functions in candidates for epilepsy surgery by inducing focal, material-specific memory deficits. Twenty patients who underwe nt presurgical epilepsy evaluation with chronically implanted subdural strip electrodes were submitted to focal TMS over the temporal lobes and the vertex while sequences of items of the Digit Span and the Cors i Block test were presented on a computer screen. TMS was applied sync hronously or 200 msec following presentation of each item. The effects of TMS on the memory span and the serial position curve were analysed in comparison to baseline levels. The following results were obtained : the quantitative effects on the verbal (Digit Span) and non-verbal ( Corsi Block) memory span were not significant, but there were signific ant qualitative changes of serial position effects. In the group of si x patients with left temporal epilepsy, TMS over the left temporal lob e induced a significant recency effect in the Digit Span test, while T MS over the vertex significantly increased the recency errors. The abs olute number of errors remained unchanged. No such effects were observ ed in the group of nine patients with right temporal lobe epilepsy. Th ese results suggest that in the presence of a left temporal lobe focus TMS can induce qualitative, material specific changes in verbal worki ng memory (phonological loop) which become apparent in the serial posi tion curve. The dissociation of TMS effects for temporal and vertex st imulation imply that TMS can selectively influence specific phonologic al loop components and that the phonological loop has a functionally a nd neuroanatomically multimodular structure. Copyright (C) 1996 Elsevi er Science Ltd.