The Supramarginal Gyrus: a neural storage for order information in Short-Term Memory

In a seminal work, Hurlstone, Hitch and Baddeley (2014) have proposed the existence of serial order mechanisms shared across different Short-Term Memory (STM) domains. However, the neural correlates of these mechanisms are still undiscovered and debated. Recently, in the case of auditory-verbal STM, a cortical area that seems responsible for storing order information has been identified, namely the left Supramarginal Gyrus (SMG). We speculate that the left SMG could retain order information regardless of presentation modality. Our study investigates this hypothesis by modulating the activity of the SMG with inhibitory repetitive Transcranial Magnetic Stimulation (rTMS). In a series of three experiments, we explored the role of the left SMG in the following STM domains: (a) verbal domain, with an auditory-verbal digit span task (Experiment 1); (b) visuo-spatial domain, with a computerized version of the Corsi span task (Experiment 2) and (c) motor domain, with a finger tapping span task (Experiment 3). Furthermore, we tested the selectivity of left SMG function (i.e., maintaining serial order and not a more general attentional role) using a visual pattern span task, a task where no order information has to be retained (Experiment 4). Twenty healthy volunteers took part in each of the three main experiments. The design of these experiments was within-subjects. Each experiment included three randomized sessions. In two stimulation sessions, before the experimental task, ten minutes of 1 Hz rTMS was applied either over the left SMG or over the left Inferior Frontal Gyrus (IFG). This second cortical area served as a control site. The third session was behavioural-only, serving as a baseline measurement. In Experiment 4 (control experiment) fifteen subjects were tested in a two-sessions within-subjects experiment. In this case, rTMS was applied only over the left SMG. Results of the three main experiments show that for all the STM tasks that required serial order maintenance, the inhibition of SMG selectively impaired the proportion of order errors made by the subjects with respect to both the stimulation-free condition and IFG inhibition. Conversely, when the STM task did not require a sequential information to retain, as in Experiment 4, the stimulation of SMG exerted no significant effect on the performance. Interestingly, the inhibition of IFG produced an increased rate of item errors only in the digit span task revealing how, in line with previous literature, IFG stores verbal information. Taken together our results suggest that left SMG retains order information in STM independently from the material type. This evidence not only brings new light to the anatomic bases of STM network and to the functioning of the network itself but could also offer novel insights in clinical disorders where order information is crucial.