A Review of Transcranial Magnetic Stimulation and Multimodal Neuroimaging to Characterize Post-Stroke Neuroplasticity

Abstract

Following stroke, the brain undergoes various stages of recovery where the central nervous system can reorganize neural circuitry (neuroplasticity) both spontaneously and with the aid of behavioral rehabilitation and non-invasive brain stimulation. Multiple neuroimaging techniques can characterize common structural and functional stroke-related deficits, and importantly, help predict recovery of function. Diffusion tensor imaging (DTI) typically reveals increased overall diffusivity throughout the brain following stroke, and is capable of indexing the extent of white matter damage. Magnetic resonance spectroscopy (MRS) provides an index of metabolic changes in surviving neural tissue after stroke, serving as a marker of brain function. The neural correlates of altered brain activity after stroke have been demonstrated by abnormal activation of sensorimotor cortices during task performance, and at rest, using functional magnetic resonance imaging (fMRI). Electroencephalography (EEG) has been used to characterize motor dysfunction in terms of increased cortical amplitude in the sensorimotor regions when performing upper limb movement, indicating abnormally increased cognitive effort and planning in individuals with stroke. Transcranial magnetic stimulation (TMS) work reveals changes in ipsilesional and contralesional cortical excitability in the sensorimotor cortices. The severity of motor deficits indexed using TMS has been linked to the magnitude of activity imbalance between the sensorimotor cortices. In this paper, we will provide a narrative review of data from studies utilizing DTI, MRS, fMRI, EEG, and brain stimulation techniques focusing on TMS and its combination with uni- and multimodal neuroimaging methods to assess recovery after stroke. Approaches that delineate the best measures with which to predict or positively alter outcomes will be highlighted.

Action Observation and Cortical Connectivity: Evidence from EEG Analysis

“…30 Interhemispheric imbalance is a key factor in functional impairment after a stroke. 31 In relation to the poor motor recovery of stroke patients, when interhemispheric inhibition increases from the contralesional motor cortex (the unaffected hemisphere) to the ipsilesional motor cortex (the affected hemisphere), the excitability of the ipsilesional motor cortex decreases with functional loss of the paretic extremity.…”

Promoting recovery from ischemic stroke

“…Network analysis may help to evaluate the efficacy of neurorestorative treatments which aim to reinforce compensatory neural circuits. [48] In future, fMRI might also be used to identify patterns of early post-stroke neural activity that are associated with better outcomes. [48] 3 Neurorestorative treatments…”

“…[47] Functional MRI determines the blood oxygen level dependent (BOLD) signal, which reflects deoxygemoglobin levels in a tissue. [48] As deoxyhemoglobin levels are influenced by Downloaded by [University of California, San Diego] at 14:06 30 December 2015 regional oxygen metabolism and regional blood volume, which increase with increased neural activity, the BOLD signal thus represents an indirect measure of neural activity. [48] Functional MRI enables the analysis of widespread alterations in neural acitivity after a focal lesion and the identification of "compensatory" network patterns.…”

“…[48] As deoxyhemoglobin levels are influenced by Downloaded by [University of California, San Diego] at 14:06 30 December 2015 regional oxygen metabolism and regional blood volume, which increase with increased neural activity, the BOLD signal thus represents an indirect measure of neural activity. [48] Functional MRI enables the analysis of widespread alterations in neural acitivity after a focal lesion and the identification of "compensatory" network patterns. Network analysis may help to evaluate the efficacy of neurorestorative treatments which aim to reinforce compensatory neural circuits.…”

“…[47,48] Diffusion tensor imaging (DTI) enables the evaluation of white matter tract integrity by water diffusion characteristics. [49] It is based on the principle that water molecules move parallel to nerve fibers under physiological conditions, whereas diffusion across fiber tracts is limited, causing anisotropy.…”