Cognitive Enhancement in Neurological and Psychiatric Disorders Using Transcranial Magnetic Stimulation (TMS): A Review of Modalities, Potential Mechanisms and Future Implications

Abstract

Cognitive enhancement refers to the improvement of cognitive function related to deficits that occurred as part of a certain illness. However, the term cognitive enhancement does not yet have a definitive meaning, and its connotations often vary depending on the research of interest. Recently, research interests are growing towards enhancing human cognition beyond what has traditionally been considered necessary using various brain devices. The phenomenon of exceeding the cognitive abilities of individuals who are already functional has also introduced new terminologies as means to classify between cognitive enhancing procedures that are part of treatment versus simply supplementary. Of the many devices used to attain cognitive enhancement, transcranial magnetic stimulation (TMS) is a unique neurostimulatory device that has demonstrated significant improvements in various cognitive domains including memory and cognitive processing skills. While many studies have supported the safety and efficacy of TMS in treatment, there has yet to be an optimization in parameter for TMS that is catered to a certain target group. The current paper aims to review with perspective the many studies that have used TMS for the purpose of cognitive enhancement and provide further insight on the development of an optimal stimulation parameter. The paper reviews 41 peer-reviewed articles that used TMS for cognitive enhancement, summarizes the findings that were apparent for each distinct parameter, and discusses future directions regarding TMS as an elective tool for healthy individuals while considering some of the ethical perspectives that may be warranted.

Keywords: Cognition; Memory; Neuropsychological tests; Noninvasive; Transcranial magnetic stimulation.

Fig. 1. Schematic diagram of the underlying mechanism of rTMS. (A) Three-dimensional images of the brain that highlight the frontoparietal network of the brain. The top image shows the axial view of the brain, alongside with the area of stimulation for the rTMS treatment summarized in the current review colored in light green. The bottom image shows the sagittal view of the brain. Each circle is a node that represents the region of the brain that is part of the frontoparietal network, and each grey line is an edge that represents the inter-regional connectivity between the nodes. (B) The two potential neural pathways that underlie rTMS treatment to the DLPFC. A T1-weighted image of the brain is shown in sagittal, coronal, and axial view to show the DLPFC as highlighted in yellow. R, right; L, left; DLPFC, dorsolateral prefrontal cortex; MFG, middle frontal gyrus; IPL, inferior parietal lobe; IPS, inferior parietal sulcus; CC, corpus callosum; rTMS, repetitive transcranial magnetic stimulation; FPN, frontoparietal network; DMN, default mode network.