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

An MRI time course of 512 echo-planar images (EPI) in resting human brain obtained every 250 ms reveals fluctuations in signal intensity in each pixel that have a physiologic origin. Regions of the sensorimotor cortex that were activated secondary to hand movement were identified using functional MRI methodology (FMRI). Time courses of low frequency (< 0.1 Hz) fluctuations in resting brain were observed to have a high degree of temporal correlation (P < 10(-3)) within these regions and also with time courses in several other regions that can be associated with motor function. It is concluded that correlation of low frequency fluctuations, which may arise from fluctuations in blood oxygenation or flow, is a manifestation of functional connectivity of the brain.

Functional connectivity in the motor cortex of resting human brain using echo-planar MRI.

Thirty years of brain imaging research has converged to define the brain’s default network—a novel and only recently appreciated brain system that participates in internal modes of cognition Here we synthesize past observations to provide strong evidence that the default network is a specific, anatomically defined brain system preferentially active when individuals are not focused on the external environment Analysis of connectional anatomy in the monkey supports the presence of an interconnected brain system Providing insight into function, the default network is active when individuals are engaged in internally focused tasks including autobiographical memory retrieval, envisioning the future, and conceiving the perspectives of others Probing the functional anatomy of the network in detail reveals that it is best understood as multiple interacting subsystems The medial temporal lobe subsystem provides information from prior experiences in the form of memories and associations that are the building blocks of mental simulation The medial prefrontal subsystem facilitates the flexible use of this information during the construction of self-relevant mental simulations These two subsystems converge on important nodes of integration including the posterior cingulate cortex The implications of these functional and anatomical observations are discussed in relation to possible adaptive roles of the default network for using past experiences to plan for the future, navigate social interactions, and maximize the utility of moments when we are not otherwise engaged by the external world We conclude by discussing the relevance of the default network for understanding mental disorders including autism, schizophrenia, and Alzheimer’s disease

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The Brain's Default Network Anatomy, Function, and Relevance to Disease

During performance of attention-demanding cognitive tasks, certain regions of the brain routinely increase activity, whereas others routinely decrease activity. In this study, we investigate the extent to which this task-related dichotomy is represented intrinsically in the resting human brain through examination of spontaneous fluctuations in the functional MRI blood oxygen level-dependent signal. We identify two diametrically opposed, widely distributed brain networks on the basis of both spontaneous correlations within each network and anticorrelations between networks. One network consists of regions routinely exhibiting task-related activations and the other of regions routinely exhibiting task-related deactivations. This intrinsic organization, featuring the presence of anticorrelated networks in the absence of overt task performance, provides a critical context in which to understand brain function. We suggest that both task-driven neuronal responses and behavior are reflections of this dynamic, ongoing, functional organization of the brain.

https://www.pnas.org/content/pnas/102/27/9673.full.pdf

The human brain is intrinsically organized into dynamic, anticorrelated functional networks

A neglected question regarding cognitive control is how control processes might detect situations calling for their involvement. The authors propose here that the demand for control may be evaluated in part by monitoring for conflicts in information processing. This hypothesis is supported by data concerning the anterior cingulate cortex, a brain area involved in cognitive control, which also appears to respond to the occurrence of conflict. The present article reports two computational modeling studies, serving to articulate the conflict monitoring hypothesis and examine its implications. The first study tests the sufficiency of the hypothesis to account for brain activation data, applying a measure of conflict to existing models of tasks shown to engage the anterior cingulate. The second study implements a feedback loop connecting conflict monitoring to cognitive control, using this to simulate a number of important behavioral phenomena.

Conflict monitoring and cognitive control.

Functional magnetic resonance imaging (FMRI) is a new, noninvasive imaging tool thought to measure changes related to regional cerebral blood flow (rCBF). Previous FMRI studies have demonstrated functional changes within the primary cerebral cortex in response to simple activation tasks, but it is unknown whether FMRI can also detect changes within the nonprimary cortex in response to complex mental activities. We therefore scanned six right-handed healthy subjects while they performed self-paced simple and complex finger movements with the right and left hands. Some subjects also performed the tasks at a fixed rate (2 Hz) or imagined performing the complex task. Functional changes occurred (1) in the contralateral primary motor cortex during simple, self-paced movements; (2) in the contralateral (and occasionally ipsilateral) primary motor cortex, the supplementary motor area (SMA), the premotor cortex of both hemispheres, and the contralateral somatosensory cortex during complex, self-paced movements; (3) with less intensity during paced movements, presumably due to the slower movement rates associated with the paced (relative to self-paced) condition; and (4) in the SMA and, to a lesser degree, the premotor cortex during imagined complex movements. These preliminary results are consistent with hierarchical models of voluntary motor control.

Functional magnetic resonance imaging of complex human movements

Magnetic resonance imaging methods recently demonstrated regional cerebral signal changes in response to limb movement and visual stimulation, attributed to blood flow enhancement We studied 5 normal subjects scanned while listening to auditory stimuli including nonspeech noise, meaningless speech sounds, single words, and narrative text Imaged regions included the lateral aspects of both hemispheres Signal changes in the superior temporal gyrus and superior temporal sulcus were observed bilaterally in all subjects Speech stimuli were associated with significantly more widespread signal changes than was the noise stimulus, while no consistent differences were observed between responses to different speech stimuli Considerable intersubject variability in the topography of signal changes was observed These observations confirm the utility of magnetic resonance imaging in the study of human brain structure-function relationships and emphasize the role of the superior temporal gyrus in perception of acoustic-phonetic features of speech, rather than processing of semantic features

Functional magnetic resonance imaging of human auditory cortex

Conceptual reasoning deficits are common in patients with multiple sclerosis (MS) and are typically associated with focal lesions involving the frontal lobes. In this study, we predicted that MS patients with frontal white matter lesions (MS-F) would be more impaired on a standard conceptual reasoning task (Wisconsin Card Sorting Test; WCST) than patients with minimal frontal lesions (MS-NF), even if the total cerebral lesion area (TLA), measured from MRI, was equivalent across groups. We subdivided 43 definite MS patients into three groups based on MRI findings: seven in the MS-F group (mean TLA = 41.4 cm2) and seven in the MS-NF group (mean TLA = 50.0 cm2); 29 MS patients served as a low lesion burden control group (MS-C; mean TLA = 6.4 cm2). The groups did not differ with regard to demographic and illness characteristics. Although the three subgroups obtained comparable scores on a measure of global cognitive functioning (verbal intelligence), the MS-F group achieved significantly fewer categories and made more total errors on the WCST than did the MS-NF and MS-C groups. The MS-F group made significantly more perseverative responses than the MS-C group and nonsignificantly more than the MS-NF group. These results suggest that the pattern of cognitive decline in MS is a function of the location of demyelinating lesions within the cerebral hemispheric white matter. Finally, we supplement the group study results with a case report of an MS patient who was studied serially with MRI and cognitive testing.

Relationship between frontal lobe lesions and Wisconsin Card Sorting Test performance in patients with multiple sclerosis

PURPOSE To evaluate the spatial specificity of functional MR imaging by comparing it with intraoperative electrocortical mapping. METHODS Functional MR imaging was performed in 28 patients before awake craniotomy and intraoperative electrocortical mapping. Activation was mapped for finger movement, lip movement, tongue movement, word generation, and counting paradigms. During surgery, finger movement, lip movement, tongue movement, counting, and/or speaking were mapped. The functional images and the photographic recordings of the brain functions mapped during surgery were converted to bit maps and coregistered by a computer program. The distance between the intraoperatively mapped function site and the MR activation site for a comparable function was measured. RESULTS Forty-six functions were recorded on MR images and intraoperative maps. In 100% of correlations, the intraoperative site and the MR activation site were within 20 mm; in 87% of correlations they were within 10 mm. For each paradigm, 67% or more of the intraoperative stimulation maps correlated within 10 mm of the MR activation site. CONCLUSIONS For the tasks used in this study, the activation site on functional MR images correlated well with the site at which intraoperative stimulation identified function.

http://www.ajnr.org/content/ajnr/18/7/1311.full.pdf

F Z Yetkin

Papers

Functional connectivity in the motor cortex of resting human brain using echo-planar MRI.

Functional magnetic resonance imaging of complex human movements

Functional magnetic resonance imaging of human auditory cortex

Relationship between frontal lobe lesions and Wisconsin Card Sorting Test performance in patients with multiple sclerosis

Functional MR activation correlated with intraoperative cortical mapping.