Transcranial Static Magnetic Stimulation Dissociates the Causal Roles of the Parietal Cortex in Spatial and Temporal Processing

Abstract

Accurate time estimation is essential for optimizing our perception and actions. Previous neuroimaging and transcranial magnetic stimulation (TMS) studies have suggested that the right inferior parietal lobule (IPL) and supplementary motor area (SMA) are involved in time perception. However, it remains inconclusive whether the activity in these regions is crucial for time perception, partly due to the possible spread of TMS effects across anatomically connected brain regions. Such a remote effect is less likely to happen with transcranial static magnetic stimulation (tSMS), as the static magnetic field is expected to modulate the firing threshold of neurons rather than directly triggering an action potential. In this study, we aimed to determine the causal relevance of local activities in the right IPL and the SMA for temporal processing using tSMS. 48 human volunteers (26 males and 22 females) participated in the study. We measured duration discrimination thresholds, along with orientation discrimination thresholds, using staircase methods before and during the administration of tSMS over the IPL/SMA. Our results indicated no significant changes in duration discrimination thresholds in either the IPL or SMA conditions. In contrast, we observed an improvement in orientation discrimination thresholds in the IPL condition. This improvement correlated with individual differences in the distance between the scalp and the IPL. Overall, our findings demonstrate a causal involvement of the IPL in orientation processing. The correlation between the effects of tSMS and the scalp-to-target distance suggests that the efficacy of tSMS may be sensitive to the magnetic field strength. Significance StatementAccurate time estimation is essential for optimizing our perception and actions. While prior studies using transcranial magnetic stimulation (TMS) investigated a role of inferior parietal lobule (IPL) and supplementary motor area in spatiotemporal processing, the results have been inconclusive because TMS can affect anatomically connected areas. Here, using transcranial static magnetic stimulation (tSMS), we examined the causal relevance of local activities in these areas. Our findings showed that tSMS over the IPL did not alter duration discrimination but, unexpectedly, significantly enhanced orientation discrimination, with the degree of improvement correlating with individual anatomical differences. These results provide new insights into the neural basis of spatial and temporal processing and emphasize the potential of tSMS as a localized neuromodulation technique of interest.