The neural correlates of phonological short-term memory: a repetitive transcranial magnetic stimulation study

Neuropsychological reports and activation studies by means of positron emission tomography and functional magnetic resonance imaging have suggested that the neural correlates of phonological short-term memory are located in the left hemisphere, with Brodmann's area (BA) 40 being responsible for short-term storage, and BA 44 for articulatory rehearsal. However, a careful review of the literature on the role of left BA 40 shows that the data are equivocal. We tested these hypotheses by means of repetitive transcranial magnetic stimulation (rTMS). Participants performed four tasks: two phonological judgements, thought to require only articulatory rehearsal without the contribution of short-term storage; a digit span, which involves both, short-term storage and articulatory rehearsal; and a pattern span, this last being the control task. The sites of stimulation were left BA 40, left BA 44 and the electrode location v(tx), plus a baseline without TMS. Reaction times increased and accuracy decreased in the case of the phonological judgements and digit span after stimulation of both left sites, suggesting that BA 40, in addition to BA 44, is involved in phonological judgements. Possible explanations are discussed, namely, the possibility that (i) the neural correlates of rehearsal are not limited to BA 44 and (ii) phonological judgements involve processes other than rehearsal. We also consider the effects of using different tasks and responses to resolve some of the discrepancies in the literature.