tDCS to temporoparietal cortex during familiarisation enhances the subsequent phonological coherence of nonwords in immediate serial recall

Research has shown that direct current stimulation (tDCS) over left temporoparietal cortex - a region implicated in phonological processing - aids new word learning. The locus of this effect remains unclear since (i) experiments have not empirically separated the acquisition of phonological forms from lexical-semantic links and (ii) outcome measures have focused on learnt associations with a referent rather than phonological stability. We tested the hypothesis that left temporoparietal tDCS would strengthen the acquisition of phonological forms, even in the absence of the opportunity to acquire lexical-semantic associations. Participants were familiarised with nonwords paired with (i) photographs of concrete referents or (ii) blurred images where no clear features were visible. Nonword familiarisation proceeded under conditions of anodal tDCS and sham stimulation in different sessions. We examined the impact of these manipulations on the stability of the phonological trace in an immediate serial recall (ISR) task the following day, ensuring that any effects were due to the influence of tDCS on long-term learning and not a direct consequence of short-term changes in neural excitability. We found that only a few exposures to the phonological forms of nonwords were sufficient to enhance nonword ISR overall compared to entirely novel items. Anodal tDCS during familiarisation further enhanced the acquisition of phonological forms, producing a specific reduction in the frequency of phoneme migrations when sequences of nonwords were maintained in verbal short-term memory. More of the phonemes that were recalled were bound together as a whole correct nonword following tDCS. These data show that tDCS to left temporoparietal cortex can facilitate word learning by strengthening the acquisition of long-term phonological forms, irrespective of the availability of a concrete referent, and that the consequences of this learning can be seen beyond the learning task as strengthened phonological coherence in verbal short-term memory.