On the Mechanisms of Transcranial Magnetic Stimulation (TMS): How Brain State and Baseline Performance Level Determine Behavioral Effects of TMS

Abstract

The behavioral effects of Transcranial Magnetic Stimulation (TMS) can change qualitatively when stimulation is preceded by initial state manipulations such as priming or adaptation. In addition, baseline performance level of the participant has been shown to play a role in modulating the impact of TMS. Here we examined the link between these two factors. This was done using data from a previous study using a TMS-priming paradigm, in which, at group level, TMS selectively facilitated targets incongruent with the prime while having no statistically significant effects on other prime-target congruencies. Correlation and linear mixed-effects analyses indicated that, for all prime-target congruencies, a significant linear relationship between baseline performance and the magnitude of the induced TMS effect was present: low levels of baseline performance were associated with TMS-induced facilitations and high baseline performance with impairments. Thus as performance level increased, TMS effects turned from facilitation to impairment. The key finding was that priming shifted the transition from facilitatory to disruptive effects for targets incongruent with the prime, such that TMS-induced facilitations were obtained until a higher level of performance than for other prime-target congruencies. Given that brain state manipulations such as priming operate via modulations of neural excitability, this result is consistent with the view that neural excitability, coupled with non-linear neural effects, underlie behavioral effects of TMS.

Effects of transcranial magnetic stimulation over the left posterior superior temporal gyrus on picture-word interference

“…We argue that the high specificity of our facilitation effect, selectively observed with increasing task demands in the congruent condition, argues against the unspecificity hypothesis. Rather, we believe that our findings indicate a specific facilitation effect best explained within the state-dependency framework [ 74 , 75 ]. Within this framework, it is argued that the direction and strength of an rTMS-induced effect strongly depends on the task-induced brain state and might thus either result in inhibition or facilitation of task performance.…”

“…The induced activity might be synchronized with the ongoing relevant signal, thereby rendering the signal stronger and providing an “optimal” level of noise for a specific task or process, which might explain the observed facilitation effect in several studies of cognition [ 73 ], including the present one. Moreover, factors like stimulation intensity, time point of stimulation, and task difficulty have been shown to affect behavioral outcome, particularly in online rTMS paradigms like the one we used [ 74 , 75 ]. Consequently, the impact of an rTMS-induced perturbation might change with varying task conditions and complexity [ 76 ].…”

How Can Transcranial Magnetic Stimulation Be Used to Modulate Episodic Memory?: A Systematic Review and Meta-Analysis

“…Generally, researchers will adjust stimulation intensity based upon an individual's resting or active motor threshold (RMT, AMT; Ngomo et al, 2012), which is defined as the minimal stimulator intensity needed to induce a muscle evoked potential in 5 out of 10 occurrences in either a relaxed (RMT) or actively engaged muscle (AMT). Prior work has found evidence that low (below) and high (above) stimulation intensities mirror facilitatory vs. inhibitory neural activity that is also associated with enhancements vs. decrements in behavioral outcomes (Silvanto et al, 2018). In addition, recent findings have shown that the effects of stimulation intensity on cortical excitability also may interact with specific frequencies, which in turn can lead to divergent effects on cognitive performance (Chung et al, 2018).…”

Combining HF rTMS over the Left DLPFC with Concurrent Cognitive Activity for the Offline Modulation of Working Memory in Healthy Volunteers: A Proof-of-Concept Study

“…Another possible explanation for the non-significant effects of combined protocols can be the high intensity (80%-100% of rMT) of HF rTMS protocols used in our study, which might contribute to the state-dependency of rTMS effects, as proposed by Silvanto et al (2017) for online-TMS-approaches [44]. According to their model, high-intensity stimulation might lead to inhibition of both task-relevant and irrelevant (noisy) neural activity, diminishing the resulting rTMS effects compared to mid-and low-intensity rTMS [45]. Chung et al (2018) have also shown that only mid-intensity, but not low-or high-intensity stimulation over the left DLPFC, demonstrated a significant effect on TMS-evoked potentials measured by TMS-EEG [18].…”