NeuroImage
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Repetitive transcranial magnetic stimulation (rTMS) modulates brain activity in different ways according to the stimulation parameters. Although the after-effects of rTMS over motor cortex are well documented in healthy individuals, less is known about the stimulation of dorso-lateral prefrontal cortex (DLPFC). Here, we studied in 20 healthy subjects how cortical oscillations are modulated by four different active rTMS protocols (1Hz, 10Hz, continuous and intermittent theta bursts - cTBS and iTBS) of the left DLPFC, and by a sham protocol used as a control condition, by comparing the spectral power of pre- and post-rTMS electroencephalographic (EEG) recordings of 15min duration. ⋯ Because large delta and theta activity is usually associated with cortical inhibition, observed rTMS-induced EEG changes in low frequencies suggest that rTMS of DLPFC transiently decreases local cortical inhibition. Importantly, local responses take place in association with other unknown mechanisms that modulate inter-hemispheric connectivity between homologous regions, resulting in the increase or decrease of fast activity in each prefrontal lobe, depending on the stimulation protocol. Only decreases of fast activity following active rTMS could be detected as significant when compared to Sham stimulation.
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There is evidence that the timing of developmental changes in cortical volume and thickness varies across the brain, although the processes behind these differences are not well understood. In contrast to volume and thickness, the regional developmental trajectories of cortical surface area have not yet been described. The present study used a combined cross-sectional and longitudinal design with 201 MRI-scans (acquired at 1.5-T) from 135 typically developing children and adolescents. ⋯ Global gender differences were more pronounced in cortical volume and surface area than in average thickness. Our findings suggest that developmental trajectories of surface area and thickness differ across the brain, both in their pattern and their timing, and that they also differ from the developmental trajectory of global cortical volume. Taken together, these findings indicate that the development of surface area and thickness is driven by different processes, at least in part.
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White matter of the brain contains a majority of long T2 components as well as a minority of short T2 components. These are not detectable using clinical magnetic resonance imaging (MRI) sequences with conventional echo times (TEs). In this study we used ultrashort echo time (UTE) sequences to investigate the ultrashort T2 components in white matter of the brain and quantify their T2*s and relative proton densities (RPDs) (relative to water with a proton density of 100%) using a clinical whole body 3T scanner. ⋯ Nine healthy volunteers were studied. The IR-dUTE sequence provided excellent image contrast for the ultrashort T2 components in white matter of the brain with a mean signal to noise ratio of 18.7 ± 3.7 and a contrast to noise ratio of 14.6 ± 2.4 between the ultrashort T2 white matter and gray matter in a 4.4 min scan time with a nominal voxel size of 1.25 × 1.25 × 5.0mm(3). On average a T2* value of 0.42 ± 0.08 ms and a RPD of 4.05 ± 0.88% were demonstrated for the ultrashort T2 components in white matter of the brain of healthy volunteers at 3T.
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Laboratory mouse models represent a powerful tool to elucidate the biological foundations of disease, but translation to and from human studies rely upon valid cross-species measures. Resting-state functional connectivity (rsFC) represents a promising translational probe of brain function; however, no convincing demonstration of the presence of distributed, bilateral rsFC networks in the mouse brain has yet been reported. Here we used blood oxygen level dependent (BOLD) and cerebral blood volume (CBV) weighted fMRI to demonstrate the presence of robust and reproducible resting-state networks in the mouse brain. ⋯ Seed-based analysis confirmed the inter-hemispheric specificity of the correlations observed with ICA and highlighted the presence of distributed antero-posterior networks anatomically homologous to the human salience network (SN) and default-mode network (DMN). Consistent with rsFC investigations in humans, BOLD and CBV-weighted fMRI signals in the DMN-like network exhibited spontaneous anti-correlation with neighbouring fronto-parietal areas. These findings demonstrate the presence of robust distributed intrinsic functional connectivity networks in the mouse brain, and pave the way for the application of rsFC readouts in transgenic models to investigate the biological underpinnings of spontaneous BOLD fMRI fluctuations and their derangement in pathological states.
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Review
Magnetic Resonance Spectroscopy as a tool to study the role of GABA in motor-cortical plasticity.
Quantification of a number of neurochemicals within localised regions of tissue has long been possible using Magnetic Resonance Spectroscopy (MRS). In recent years, MRS has increasingly been utilised as a method to indirectly assess neuronal activity in vivo, primarily via measurement of the major neurotransmitters glutamate and γ-aminobutyric acid (GABA). To date a number of studies have highlighted relationships between local GABA levels and behaviour, and have demonstrated the modulation of GABA by protocols designed to induce synaptic plasticity. This review aims to examine the literature on MRS-assessed GABA changes in synaptic plasticity, focussing on the primary motor cortex (M1), to relate these to animal studies on the role of GABA in synaptic plasticity, and to highlight some of the important outstanding questions in interpreting MRS findings.