Intracortical inhibition (SICI) and facilitation (ICF) in the human motor cortex can be measured using a paired pulse transcranial magnetic stimulation (ppTMS) protocol. Recently, a technical device ...has been introduced, which allows recording electroencephalographic (EEG) responses to TMS of a given scalp site. The latency, amplitude and scalp topography of such responses are considered a reflection of cortico-cortical connectivity and functional state. The aim of the present study is to better characterize the neuronal circuits underlying motor cortex connectivity as well as the mechanisms regulating its balance between inhibition and facilitation by means of EEG navigated-ppTMS coregistration.
Sub-threshold and supra-threshold single and ppTMS of the left primary motor cortex were carried out during a multi-channel EEG recording on 8 healthy volunteers; the between-pulse intervals used in the paired pulse trials were 3 (for SICI) and 11ms (for ICF). Motor evoked potentials (MEPs) from the opposite hand were simultaneously recorded.
Single and ppTMS induced EEG responses characterized by a sequence of negative deflections peaking at approximately 7, 18, 44, 100 and 280ms alternated with positive peaks at approximately 13, 30, 60 and 190ms post-TMS. Moreover, ppTMS modulated both EEG evoked activity and MEPs. Amplitude variability of EEG responses was correlated with – and therefore might partially explain – amplitude variability of MEPs.
EEG-ppTMS is a promising tool to better characterize the neuronal circuits underlying cortical effective connectivity as well as the mechanisms regulating the balance between inhibition and facilitation within the human cortices and the corticospinal pathway.
Highlights • We used TMS-EEG to study the effects of physiological aging on sensorimotor system. • We demonstrated that some TEPs of the left M1 are selectively modulated by aging. • Some ...sensorimotor system alterations might reflect secondary consequences of aging. • Some others alterations might be compensatory mechanisms to such changes.
When linking in time electrical stimulation of the peripheral nerve with transcranial magnetic stimulation (TMS), the excitability of the motor cortex can be modulated to evoke clear inhibition, as ...reflected by the amplitude decrement in the motor-evoked potentials (MEPs). This specific property, designated short-latency afferent inhibition (SAI), occurs when the nerve-TMS interstimulus interval (ISI) is approximately 25 ms and is considered to be a corticothalamic phenomenon. The aim of the present study was to use the electroencephalographic (EEG) responses to navigated-TMS coregistration to better characterize the neuronal circuits underlying SAI. The present experimental set included magnetic resonance imaging (MRI)-navigated TMS and 60-channel TMS-compatible EEG devices. TMS-evoked EEG responses and MEPs were analyzed in eight healthy volunteers; ISIs between median nerve and cortical stimulation were determined relative to the latency of the individual N20 component of the somatosensory-evoked potential (SEP) obtained after stimulation of the median nerve. ISIs from the latency of the N20 plus 3 ms and N20 plus 10 ms were investigated. In all experimental conditions, TMS-evoked EEG responses were characterized by a sequence of negative deflections peaking at approximately 7, 44, and 100 ms alternating with positive peaks at approximately 30, 60, and 180 ms post-TMS. Moreover, ISI N20+3 ms modulated both EEG-evoked activity and MEPs. In particular, it inhibited MEP amplitudes, attenuated cortical P60 and N100 responses, and induced motor cortex beta rhythm selective decrement of phase locking. The findings of the present experiment suggest the cortical origin of SAI that could result from the cortico-cortical activation of GABAergic-mediated inhibition onto the corticospinal neurons modulated by cholinergic activation able to reducing intralaminar inhibition and promoting intracolumnar inhibition.
Background:
Early and affordable identification of subjects with amnestic mild cognitive impairment (aMCI) who will convert to Alzheimer’s disease (AD) is a major scientific challenge.
Objective:
To ...investigate the neurophysiological hallmarks of sensorimotor cortex function in aMCI under the hypothesis that some may represent the plastic rearrangements induced by neurodegeneration, hence predictors of future conversion to AD. We sought to determine (1) whether the sensorimotor network shows peculiar alterations in patients with aMCI and (2) if sensorimotor network alterations predict long-term disease progression at the individual level.
Methods:
We studied several transcranial magnetic stimulation (TMS)-electroencephalogram (EEG) parameters of the sensorimotor cortex in a group of patients with aMCI and followed them for 6 years. We then identified aMCI who clinically converted to AD prodromal to AD-MCI (pAD-MCI) and those who remained cognitively stable non-prodromal to AD-MCI (npAD-MCI).
Results:
Patients with aMCI showed reduced motor cortex (M1) excitability and disrupted EEG synchronization decreased intertrial coherence (ITC) in alpha, beta and gamma frequency bands compared to the control subjects. The degree of alteration in M1 excitability and alpha ITC was comparable between pAD-MCI and npAD-MCI. Importantly, beta and gamma ITC impairment in the stimulated M1 was greater in pAD-MCI than npAD-MCI. Furthermore, an additional parameter related to the waveform shape of scalp signals, reflecting time-specific alterations in global TMS-induced activity stability of the dipolar activity (sDA), discriminated npAD-MCI from MCI who will convert to AD.
Discussion:
The above mentioned specific cortical changes, reflecting deficit of synchronization within the cortico-basal ganglia-thalamo-cortical loop in aMCI, may reflect the pathological processes underlying AD. These changes could be tested in larger cohorts as neurophysiological biomarkers of AD.
•M1-prefrontal coupling in the beta-2 band led to larger MEPs both in the young and the elderly.•M1-parietoccipital coupling in the delta band effectively led to significantly larger MEPs in the ...young but not in the elderly.•In elderly delta coherence is high both for high and for low MEPs. M1 excitability is led by the coupling of specific EEG rhythms depending on age.
It was recently demonstrated that the characteristics of EEG rhythms preceding a transcranial magnetic stimulation (TMS) of the motor cortex (M1) influence the motor-evoked potential (MEP) amplitude with a peculiar pattern, thus reflecting the M1 functional state. As physiological aging is related to a decrease in motor performance and changes in excitability and connectivity strength within cerebral sensorimotor circuits, we aimed to explore whether aging affects EEG-MEP interactions. Using MRI-navigated TMS and multichannel EEG, we compared the EEG-MEP interactions observed in healthy aged subjects with those observed in young volunteers. We divided the MEPs amplitude into two different subgroups consisting of “high” and “low” MEPs, based on the 50th percentile of their amplitude distribution. Then we analysed the characteristics of the pre-stimulus EEG from M1 and correlated areas separately for the “high” and “low” MEPs, comparing the two conditions. In both young and old subjects, significantly larger MEPs were evoked when the stimulated M1 was coupled in the beta-2 band with the homolateral prefrontal cortex. Conversely, only in young participants was the MEP size modulated when the M1 and homolateral parieto-occipital cortices were coupled in the delta band. The elderly didn’t show this kind of pattern. Importantly, this coupling was significantly higher in elderly brains than in young brains, both for high and low MEPs. Our results suggest an age-related significant influence of time-varying coupling of spatially patterned EEG rhythms on motor cortex excitability in response to TMS.
Highlights • Motor cortex has enhanced excitability in AD and SIVD patients with respect to control subjects and it is plastically rearranged, without significant differences between the two demented ...groups. • In the whole sample of subjects examined a significant direct correlation between parameters associated to cortical excitability and those associated to cortical plasticity is evident. • Common compensatory mechanisms may act in different kind of dementing diseases.
The last years have witnessed a significant increase in our understanding of brain functions in survivors of severe brain injuries with disorders of consciousness (DOC). Despite there is currently no ...effective standardized treatment for DOC patients, in the past decade many potential pharmacological as well as non-pharmacological therapies have been proposed. A promising and increasingly growing field of non-pharmacological therapeutic trials has been supported by the application of electrophysiological techniques. This article reviews the most relevant studies in the literature in order to provide the reader with a clear picture of the current available neurophysiological instruments that could be used to treat DOC patients. We will hereinafter briefly discuss the basic principles of deep brain stimulation (DBS), repetitive transcranial magnetic stimulation (rTMS), spinal cord stimulation (SCS) and peripheral nerve stimulation (MNS) that are the main techniques now used by researchers as a treatment and we will explain the rationale of these therapies. Then, we will outline the more relevant studies regarding their application in DOC patients. Finally, due to the fact that only a moderate amount of individual or clinically-dependent approaches are available, we conclude that more standardized studies are necessary to address the role of electrophysiological treatment strategies in DOC as well as to further elucidate their therapeutic effects and define optimal stimulation parameters. Undoubtedly, at present the multidimensional approach is the most interesting.
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