Variability of behavioural responses to transcranial magnetic stimulation: Origins and predictors

Abstract

Transcranial magnetic stimulation (TMS) may modulate the excitability of local cortical stimulation sites and distant functionally interconnected regions for minutes, hours or even days. The effects of TMS suggest that it not only acts on activity of the stimulated area, but also on its connections with remote areas. Due to these properties one of the main rationales for the application of TMS in stroke patients is to improve imbalance in interhemispheric inhibition. However, given that TMS may have excitatory or inhibitory effects the impact of stimulation is not easy to predict. In this review, we discuss the different factors that determine the magnitude and quality of physiological and behavioural responses to TMS. Whether TMS is mainly excitatory or inhibitory not only depends on the parameters of stimulation such as pulse frequency and duration, but also on baseline activity of neural tissue before stimulation, or even on cognitive factors such as attention. A major challenge for the application of TMS as therapy method is to identify predictors of positive effects in individual patients. Neuroimaging studies measuring hemodynamic or electrophysiological responses show that changes in interhemispheric competition or adaptations of functional networks in patients with focal brain lesions may predict the individual response to brain stimulation. Such techniques have the potential to select the most appropriate among different intervention methods for an individual patient.

Neurorehabilitation in Stroke: The Role of Functional Connectivity

“…This functional abnormality predicted cognitive performance, suggesting a role for alpha band connectivity in the processing of cognitive functions [10]. These findings are consistent with the fact that alpha band coherence of clinically dysfunctional areas like the right parietal lobe is able to predict the effects of cortical stimulation on parietal network functions, including visuospatial attention [5].…”

“…A recent review of the use of non-invasive brain stimulation techniques to monitor and modulate the excitability of intracranial neuronal circuits highlights that transcranial magnetic stimulation can produce lasting effects on brain motor function. Of note, functional connectivity between premotor and primary motor cortex assessed with fMRI has been found predictor of good clinical response to transcranial magnetic stimulation [5]. These findings suggest that cortical stimulation may be a promising approach to improve synaptic dysfunction and functional reorganization of motor networks after stroke, enhancing clinical recovery [2].…”

Effects of non-invasive brain stimulation on attention: Current debates, cognitive studies and novel clinical applications

“…However, given that TMS may have excitatory or inhibitory effects the impact of this stimulation is not easy to predict. In their review, Nicolo et al (2015) discuss the different factors that determine magnitude and quality of physiological and behavioural responses to TMS. Whether TMS manifests itself as either excitatory or inhibitory, not only depends on the stimulation parameters, but also on the baseline activity of the neural tissue before stimulation and even on cognitive factors such as attention.…”

Repetitive transcranial magnetic stimulation for the treatment of refractory epilepsy

“…Strong baseline cortical activation, e.g. during physical or cognitive activity, has been found responsible for reversal of the induced effects [70,71]. This could lead to important intersubject as well as intrasubject variability when considering repeated sessions over time [72].…”