Deep brain stimulation (DBS) has emerged as an invasive neuromodulation technique for the treatment of several neurological disorders, but the mechanisms underlying its effects remain partially elusive. In this context, the application of Transcranial Magnetic Stimulation (TMS) in patients treated with DBS represents an intriguing approach to investigate the neurophysiology of cortico-basal networks. Experimental studies combining TMS and DBS that have been performed so far have mainly aimed to evaluate the effects of DBS on the cerebral cortex and thus to provide insights into DBS’s mechanisms of action. The modulation of cortical excitability and plasticity by DBS is emerging as a potential contributor to its therapeutic effects. Moreover, pairing DBS and TMS stimuli could represent a method to induce cortical synaptic plasticity, the therapeutic potential of which is still unexplored. Furthermore, the advent of new DBS technologies and novel treatment targets will present new research opportunities and prospects to investigate brain networks. However, the application of the combined TMS-DBS approach is currently limited by safety concerns. In this review, we sought to present an overview of studies performed by combining TMS and DBS in neurological disorders, as well as available evidence and recommendations on the safety of their combination. Additionally, we outline perspectives for future research by highlighting knowledge gaps and possible novel applications of this approach.

Transcranial alternating current stimulation (tACS) mechanisms remain poorly understood. While tACS is expected to modulate periodic electroencephalography (EEG) activity due to the imposition of oscillatory alternating currents, it may also modulate the aperiodic EEG component, suggesting alternative tACS mechanisms that have not yet been explored. This study examined whether 20-minute tACS session delivered at 40 Hz over C3/C4 can selectively modulate periodic and aperiodic components. We implemented a paired, counterbalanced, and sham-controlled design consisting of two sessions during which tACS was administered over the sensorimotor regions. EEG was recorded before and after tACS under three EEG conditions: resting state, active task (i.e., the serial reaction time task), and passive task (i.e., 40-Hz auditory stimulation). We found that tACS led to selective modulation of the aperiodic component, specifically a broadband downward shift in the power spectrum (i.e., reduction in the offset), independent of the EEG conditions and with no evidence of an effect on periodic activity. This result may reflect a general effect on neural spiking rather than direct modulation of oscillatory activity. Disentangling the contribution of the aperiodic component from periodic activity represents an opportunity to identify alternative neurophysiological mechanisms underlying neural modulation and provides a clearer interpretation of tACS mechanisms.

Background: The ability to perceive two tactile stimuli as asynchronous can be measured using the somatosensory temporal discrimination threshold (STDT). In healthy humans, the execution of a voluntary movement determines an increase in STDT values, while the integration of STDT and movement execution is abnormal in patients with basal ganglia disorders. Sensorimotor integration can be modulated using focal muscle vibration (fMV), a neurophysiological approach that selectively activates proprioceptive afferents from the vibrated muscle. Method: In this study, we investigated whether fMV was able to modulate STDT or STDT–movement integration in healthy subjects by measuring them before, during and after fMV applied over the first dorsalis interosseous, abductor pollicis brevis and flexor radialis carpi muscles. Results: The results showed that fMV modulated STDT–movement integration only when applied over the first dorsalis interosseous, namely, the muscle performing the motor task involved in STDT–movement integration. These changes occurred during and up to 10 min after fMV. Differently, fMV did not influence STDT at rest. We suggest that that fMV interferes with the STDT–movement task processing, possibly disrupting the physiological processing of sensory information. Conclusions: This study showed that FMV is able to modulate STDT–movement integration when applied over the muscle involved in the motor task. This result provides further information on the mechanisms underlying fMV, and has potential future implications in basal ganglia disorders characterized by altered sensorimotor integration.