**3.5 TMS as a tool to measure mirror neuron system (MNS) activity**

Among the various networks involved in the pathology of social cognition, the mirror neuron system is most extensively studied. Typically, there is a quantifiable motor cortical reactivity facilitation (increased motor evoked potentials or reduced intracortical inhibition) or motor resonance in the same muscle group that is observed to be in action. This index of motor resonance in the primary motor cortex is likely to be driven by premotor MNS-activity and is used as a putative or indirect marker of the premotor mirror neuron system activity. Studies using TMS have demonstrated diminished modulation of motor cortical reactivity during both neutral action observation [30] and context-based action in schizophrenia [31]. Also, reduced MNS activity is related to poorer social cognition performance. In contrast, patients with mania demonstrate an elevated MNS response, perhaps reflecting disinhibition of the regulatory prefrontal brain regions. Higher MNS activity in mania was associated with higher manic symptom severity [32].

## **3.6 TMS as a tool for cortical mapping**

TMS methods allow for the identification of a direct association between the studied site and the behavioral outcome in a temporary and non-invasive fashion, allowing for mapping of areas of cortex less accessible by previous techniques, hence providing a powerful tool to identify the brain-behavior relationship. A single TMS pulse or a short sequence of pulses has the potential to transiently disrupt ongoing cortical activity in the region being stimulated. This phenomenon has been termed a virtual lesion. TMS is an important tool in cognitive neuroscience and has changed the way we understand cognitive function [33]. TMS can create virtual lesions, thereby allowing us to obtain information about the contribution of a given cortical region to a specific behavior. For example, subjects asked to memorize and repeat a list of words would likely show increased activity in the prefrontal cortex using fMRI. This increased activity would provide an indirect association between the prefrontal cortex and the task. However, if stimuli from TMS over the prefrontal cortex were found to obstruct the ability to learn and recall the list, then researchers would have more convincing evidence to support the involvement of the prefrontal cortex in short-term memory. Pascual-Leone investigated the role of the dorsolateral prefrontal cortex (DLPFC) in implicit procedural learning. In this study, low-intensity rTMS was applied to the DLPFC, to the supplementary motor area, or directly to the ipsilateral hand used in testing. It was demonstrated that DLPFC stimulation markedly impaired implicit procedural learning, whereas stimulation of the other areas did not impair learning [34].

#### **3.7 CombinatoryTMS approaches**

TMS may also be paired with other investigative modalities to investigate connectivity between brain regions and can be used as a brain-mapping tool to complement information gained from functional magnetic resonance imaging (fMRI) and electroencephalography (EEG), thus improving both spatial and temporal accuracy of the biological signals derived.

*Understanding the Neuropathophysiology of Psychiatry Disorder Using Transcranial Magnetic… DOI: http://dx.doi.org/10.5772/intechopen.103748*
