The role of the left Supramarginal Gyrus in the Short-Term Memory network: a Transcranial Magnetic Stimulation study

Background. Recently, the existence of serial order retention mechanisms shared across different Short-Term Memory (STM) domains has been hypothesized (1), however, the neural correlates of these mechanisms are still debated (1). Concerning auditory-verbal STM, the left Supramarginal Gyrus (SMG) has been identified as responsible for storing order information (2), but so far there is no study that has investigated whether this area could retain order information regardless of the nature of the stored material (i.e. verbal, spatial or motor). To test this hypothesis, we interfered with the left SMG activity by means of low-frequency repetitive Transcranial Magnetic Stimulation (rTMS), while testing STM in different domains. Methods. In three experiments, we explored the role of the left SMG in verbal (Experiment 1), visuospatial (Experiment 2) and motor (Experiment 3) STM by administering a verbal, a spatial, and a motor version of the span task, respectively, after 10 minutes of offline 1 Hz rTMS. Twenty subjects took part in each experiment, undergoing three sessions: a baseline session without rTMS, and two sessions during which rTMS was applied over the left SMG and the left Inferior Frontal Gyrus (IFG). In a fourth experiment (Control Experiment), a group of 15 subjects was tested in a visual pattern STM task, to assess left SMG selectivity for serial order retention. Number and types of errors were scored. Results. Results from the three experiments show that rTMS applied to the left SMG worsens participants’ accuracy in serial order maintenance, by selectively increasing the proportion of order errors with respect to both the baseline (rTMS-free) performance and the stimulation of the left IFG. Conversely, when the STM task does not require retention of sequential information (Control Experiment), rTMS to the left SMG has no significant effect on performance (i.e., accuracy or reaction times). Interestingly, the stimulation of the left IFG increases the rate of item errors only in the verbal span task (Experiment 1), hence confirming the involvement of this area in storing verbal information. Discussion. Our results show that the left SMG retains order information in STM independently from the information type. This evidence brings new light on the anatomic bases of STM network and to the functioning of the network itself. Furthermore, it also offers novel insights in clinical disorders where order information is prominent.