Advances in functional and structural MR image analysis and implementation as FSL.

A baseline or control state is fundamental to the understanding of most complex systems. Defining a baseline state in the human brain, arguably our most complex system, poses a particular challenge. Many suspect that left unconstrained, its activity will vary unpredictably. Despite this prediction we identify a baseline state of the normal adult human brain in terms of the brain oxygen extraction fraction or OEF. The OEF is defined as the ratio of oxygen used by the brain to oxygen delivered by flowing blood and is remarkably uniform in the awake but resting state (e.g., lying quietly with eyes closed). Local deviations in the OEF represent the physiological basis of signals of changes in neuronal activity obtained with functional MRI during a wide variety of human behaviors. We used quantitative metabolic and circulatory measurements from positron-emission tomography to obtain the OEF regionally throughout the brain. Areas of activation were conspicuous by their absence. All significant deviations from the mean hemisphere OEF were increases, signifying deactivations, and resided almost exclusively in the visual system. Defining the baseline state of an area in this manner attaches meaning to a group of areas that consistently exhibit decreases from this baseline, during a wide variety of goal-directed behaviors monitored with positron-emission tomography and functional MRI. These decreases suggest the existence of an organized, baseline default mode of brain function that is suspended during specific goal-directed behaviors.

/pdf/a-default-mode-of-brain-function-zwenkbmjw7.pdf

A default mode of brain function.

Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

Pattern Recognition and Machine Learning

An automated method for segmenting magnetic resonance head images into brain and non-brain has been developed. It is very robust and accurate and has been tested on thousands of data sets from a wide variety of scanners and taken with a wide variety of MR sequences. The method, Brain Extraction Tool (BET), uses a deformable model that evolves to fit the brain's surface by the application of a set of locally adaptive model forces. The method is very fast and requires no preregistration or other pre-processing before being applied. We describe the new method and give examples of results and the results of extensive quantitative testing against "gold-standard" hand segmentations, and two other popular automated methods.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/hbm.10062

Fast robust automated brain extraction

Co-Planar Stereotaxic Atlas of the Human Brain—3-Dimensional Proportional System: An Approach to Cerebral Imaging, J. Talairach, P. Tournoux. Georg Thieme Verlag, New York (1988), 122 pp., 130 figs. DM 268

Objective: To define the extent of neuronal injury and loss in thalamic gray matter in patients with relapsing-remitting (RR) MS and to characterize how these neuronal pathologic changes are related to disease duration. Methods: The authors studied 14 patients with RRMS (Expanded Disability Status Scale score, mean 3.25, range 2.0 to 6.0) and 14 (8 men, 6 women) age-matched healthy controls. Structural MR and MRS studies were performed in a single scanning session using a 3T MR system. Results: N -acetylaspartate (NAA) concentrations (a measure of the apparent neuronal density) were decreased approximately 11% in the thalami of the patients with RRMS relative to controls ( p p p p = 0.044) and normalized thalamic volume (r = −0.60, p = 0.01) were correlated inversely with disease duration. There was a trend for a correlation between the thalamic NAA/creatine (Cr) ratio and the NAA/Cr in the frontal normal-appearing white matter (r = 0.56, p Conclusions: The reduction of both NAA concentration and thalamic volume suggests that a neurodegenerative component may contribute to the pathology of MS even in the earlier RR stage. The trend toward a relationship between thalamic NAA/Cr and distant normal-appearing white matter changes implies that there may be a common mechanism for the white matter axonal loss and thalamic neuronal injury.

Thalamic neurodegeneration in relapsing-remitting multiple sclerosis.

Attention to sensory stimulation modulates behavioural responses and cortical activity. Attention to movement can also modulate motor responses. For example, directing attention away from cued movements can increase reaction times. This study used fMRI to determine where in the motor cortex attention to movement modulates activity. Attention to movement was reduced by asking subjects to perform a concurrent distractor task (counting backwards). Sensori-motor areas showing a negative interaction between counting and movement (i.e. reduced activation in the dual task condition relative to the sum of the single task conditions) included the supplementary motor area (SMA), cingulate cortex, insula and post-central gyrus. A separate volumes-of-interest analysis revealed significant reductions in mean percent signal change in the dual task compared to the single task in a portion of the pre-central gyrus, deep in the central sulcus (thought to correspond to area 4p) and SMA. We conclude that the brain network for motor control is modulated by attention at multiple sites, including the primary motor cortex. These results are also discussed with reference to theories concerning the neural correlates of dual task performance and mental calculation and have implications for the interpretation of functional imaging studies of normal and impaired motor performance.

Attention to movement modulates activity in sensori-motor areas, including primary motor cortex

Blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) is a powerful approach to defining activity in the healthy and diseased human brain. BOLD fMRI detects local increases in relative blood oxygenation that are most probably a direct consequence of neurotransmitter action and thus reflect local neuronal signalling. The method allows localisation to volumes of the order of a few to several cubic millimetres and can be used in serial studies of individual subjects. Basic approaches to experimental design and analysis are reviewed briefly, as well as potential clinical applications. The latter include three broad areas: anatomical characterisation of normal or pathological patterns of brain functioning; distinguishing pathological traits; and monitoring treatment responses. New research is emphasising the integration of fMRI with other techniques, particularly electrophysiological. In conjunction with MRI methods for characterising pathological load, fMRI promises a refined understanding of when disease processes begin and how they can be modified by new treatments.

https://jnnp.bmj.com/content/jnnp/75/1/6.full.pdf

Functional magnetic resonance imaging

The left hemisphere specialization for speech perception might arise from asymmetries at more basic levels of auditory processing. In particular, it has been suggested that differences in "temporal" and "spectral" processing exist between the hemispheres. Here we used functional magnetic resonance imaging to test this hypothesis further. Fourteen healthy volunteers listened to sequences of alternating pure tones that varied in the temporal and spectral domains. Increased temporal variation was associated with activation in Heschl's gyrus (HG) bilaterally, whereas increased spectral variation activated the superior temporal gyrus (STG) bilaterally and right posterior superior temporal sulcus (STS). Responses to increased temporal variation were lateralized to the left hemisphere; this left lateralization was greater in posteromedial HG, which is presumed to correspond to the primary auditory cortex. Responses to increased spectral variation were lateralized to the right hemisphere specifically in the anterior STG and posterior STS. These findings are consistent with the notion that the hemispheres are differentially specialized for processing auditory stimuli even in the absence of linguistic information.

/pdf/hemispheric-specialization-for-processing-auditory-nonspeech-3emgb6w9gq.pdf

Hemispheric Specialization for Processing Auditory Nonspeech Stimuli

Morphology is the aspect of language concerned with the internal structure of words, and languages vary in the extent to which they rely on morphological structure. Consequently, it is not clear whether morphology is a basic element of a linguistic structure or whether it emerges from systematic regularities between the form and meaning of words. Here, we looked for evidence of morphological structure at a neural systems level by using a visual masked priming paradigm and functional MRI. Form and meaning relations were manipulated in a 2 × 2 design to identify reductions in blood oxygenation level-dependent signal related to shared form (e.g., corner-corn), shared meaning (e.g., idea-notion), and shared morphemes (e.g., boldly-bold, which overlapped in both form and meaning). Relative to unrelated pairs (e.g., ozone-hero), morphologically related items reduced blood oxygenation level-dependent signal in the posterior angular gyrus bilaterally, left occipitotemporal cortex, and left middle temporal gyrus. In the posterior angular gyrus, a neural priming effect was observed for all three priming conditions, possibly reflecting reduced attentional demands rather than overlapping linguistic representations per se. In contrast, the reductions seen in the left occipitotemporal cortex and left middle temporal gyrus corresponded, respectively, to main effects of orthographic and semantic overlap. As neural regions sensitive to morphological structure overlapped almost entirely with regions sensitive to orthographic and semantic relatedness, our results suggest that morphology emerges from the convergence of form and meaning.

https://www.pnas.org/content/pnas/101/41/14984.full.pdf

Paul M. Matthews

Papers

Thalamic neurodegeneration in relapsing-remitting multiple sclerosis.

Attention to movement modulates activity in sensori-motor areas, including primary motor cortex

Functional magnetic resonance imaging

Hemispheric Specialization for Processing Auditory Nonspeech Stimuli

Morphology and the internal structure of words.