Investigating serial order retention mechanisms in Short-Term Memory with repetitive Transcranial Magnetic Stimulation

Recently, the existence of serial order retention mechanisms shared across different Short-Term Memory (STM) domains has been hypothesized; however, the neural correlates of these mechanisms are debated and partially undiscovered. In auditory-verbal STM, one cortical area that seems responsible for storing order information is the left Supramarginal Gyrus (SMG) but so far, it’s unknown whether this area could retain order information regardless of the nature of the stored material (i.e. verbal, spatial or motor). To investigate this hypothesis, in three experiments, we interfered with the left SMG activity by means of low-frequency (1 Hz) repetitive Transcranial Magnetic Stimulation (rTMS), while testing STM in different domains, namely in the verbal, visuospatial and motor domains, using different versions of the span task. 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), serving as a control site. Results show that in all the STM domains tested, rTMS applied to the left SMG worsens subjects’ accuracy in serial order maintenance by selectively increasing the proportion of order errors with respect to both the baseline performance and the stimulation of the left IFG. Our finding is the first causal demonstration that left SMG retains order information in STM independently from the information type. This evidence brings new light on the anatomo-functional bases of STM network and could also offer novel insights in clinical disorders where order information is crucial.