Background
The angular gyrus (AG) is involved in numerous cognitive processes, and structural alterations of the AG are reported in many neuropsychiatric diseases. Because abnormal excitability or connectivity of such cortical hubs could precede structural alterations and clinical symptoms, approaches assessing their functional state are needed. The combination of transcranial magnetic stimulation (TMS) with electroencephalography (EEG) can provide such functional readouts by probing how the cortex responds to direct stimulation.

Objective
To characterize TMS-evoked potentials (TEPs) elicited by AG stimulation, determine optimal stimulation parameters, and identify TEP biomarkers of AG function.

Methods
In 19 subjects, we recorded AG-TEPs using four TMS orientations and three intensities and compared their spatiotemporal features using topographic dissimilarity and microstate analyses. We also explored the link between AG-TEPs and TMS-evoked muscle activity.

Results
Early AG TEP components of interest (P25, N45) showed topographic variability dependent on stimulation parameters. The P25 topography was sensitive to TMS orientation and less to intensity, whereas the N45 topography was highly dependent on both coil orientation and intensity. However, TMS-evoked muscular activity was also dependent on coil orientation and the dominant topography of N45 was strongly related to this muscular activity, indicating that the component may reflect somatosensory-evoked responses to this peripheral activation.

Conclusions
The earliest AG TEP component P25 likely reflects neural processes triggered by direct AG activation and could provide an index of local excitability. N45 must be interpreted with caution as it may mostly reflect peripherally evoked activity. Coil orientation can be optimized to minimize muscular contractions.

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