Applications of magnetic resonance imaging to the study of human brain function

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

Effects of Pet Insects on Cognitive Function among the Elderly: An fMRI Study

Animal-assisted therapy has positive effects on cognitive function, depression, performance ability, and social functioning in elderly patients. The aim of this study was to evaluate the effects of rearing pet insects on the cognitive function of healthy elderly participants, with fMRI (functional magnetic resonance imaging) being used for this purpose. Community-dwelling right-handed elderly women (≥60 years) with normal cognitive function were enrolled and randomized at a 1:1 ratio into two groups: insect-rearing and control (n = 16) groups, with the insect-rearing group being further classified into two groups for analysis according to the subjects' scores in the Wisconsin Card Sorting Test, WCST) at the baseline fMRI: Insect-rearing group I with a relatively high score (n = 13), and insect-rearing group II with a relatively low score (n = 6). The insect-rearing groups received and reared crickets as pet insects for 8 weeks. The WCST consisted of two variations, a high level baseline (HLB) and semi-WCST version. There was a significant difference accuracy of the HLB-semi-WCST (p < 0.05) in insect-rearing group II after 8 weeks from the baseline test. In the fMRI analysis involving the WCST reaction test, increased activation was observed in the right dorsal lateral prefrontal cortex and parietal cortex in insect-rearing group II when the semi-WCST, rather than the HLB, was performed. Rearing pet insects showed positive effects on executive functions and performance improvement in elderly women. Further larger studies on the effects of pet insects on cognitive function are warranted.

Risonanza magnetica funzionale dell'encefalo in pazienti candidati ad intervento chirurgico con guida stereotassica

Negli ultimi anni la risonanza magnetica funzionale dell'encefalo (fMRI) si è proposta con successo nello studio dell'attività encefalica durante lo svolgimento di determinate funzioni. Si è inoltre delineato per essa un ruolo paricolarmente interessante nell'ambito della pianificazione preoperatoria di pazienti destinati ad un intervento chirurgico in sedi encefaliche critiche, ovvero ad alto rischio di invalidtà postoperatoria.

Il presente lavoro mira alla realizzazione di mappe di attivazione motoria in pazienti con lesioni occupanti spazio nell'encefalo mediante un tomografo a 1,5 T di non recentissima tecnologia; successivamente si è voluto valutare l'informatività di tali mappe, sia dal punto di vista neurochirurgico sia dal punto di vista neurofisiologico.

Sono stati presi in esame 5 pazienti, tutti portatori di una lesione occupante spazio in prossimità della corteccia perirolandica e tutti destinati ad un intervento chirurgico in stereotassi per l'asportazione di tale lesione. I pazienti venivano sottoposti in un'unica seduta allo studio funzionale e morfologico dell'encefalo con il casco stereotassico in sede.

Lo studio di fMRI si è svolto su uno o massimo due piani assiali orientati ortogonalmente al decorso della scissura di Rolando. Esso prevedeva come prova di attivazione il movimento della mano controlaterale all'emisfero con la lesione da asportare.

In un paziente la funzione motoria è stata studiata anche intraoperatoriamente mediante la stimolazione elettrica della corteccia cerebrale.

In 4 pazienti si è avuta una netta rappresentazione della corteccia sensitivo-motoria di tale emisfero. In 3 di essi si è vista inoltre un'attivazione dell'emisfero opposto, ipsilaterale alla mano in movimento. Altre zone di attivazione visualizzate nei nostri studi sono state messe in relazione con l'area motoria supplementare (3 pazienti) e l'area premotoria (2 pazienti).

Nel paziente in cui si è proceduto intraoperatoriamente con la stimolazione elettrica della corteccia cerebrale è stata riscontrata una buona concordanza tra lo studio elettrofisiologico e l'indagine di fMRI.

Analizzando i risultati da noi conseguiti possiamo concludere che anche un tomografo a 1,5 T di tipo convenzionale si presta ad un'indagine di fMRI in pazienti con lesioni encefaliche. Le mappe di attivazione realizzate su uno o due piani assiali sono in grado di fornire delle informazioni sui rapporti tra la lesione da asportare e la corteccia sensitivo-motoria, in particolare l'area della mano. Ciò ha notevoli implicazioni dal punto di vista neurochirurgico, considerando quanto sia importante la salvaguardia dell'integrità funzionale del soggetto che si sottopone ad un intervento neurochirurgico in una sede critica come la corteccia perirolandica.

Quantitative Assessment of Head Motion toward Functional Magnetic Resonance Imaging during Stepping

Purpose:

Stepping motions have been often used as gait-like patterns in functional magnetic resonance imaging (fMRI) to understand gait control. However, it is still very difficult to stabilize the task-related head motion. Our main purpose is to provide characteristics of the task-related head motion during stepping to develop robust restraints toward fMRI.

Methods:

Multidirectional head and knee position during stepping were acquired using a motion capture system outside MRI room in 13 healthy participants. Six phases in a stepping motion were defined by reference to the left knee angles and the mean of superior-inferior head velocity (V_mean) in each phase was investigated. Furthermore, the correlation between the standard deviation of the knee angle (θ_sd) and the maximum of the head velocity (V_max) was evaluated.

Results:

The standard deviation of each superior-inferior head position and pitch were significantly larger than the other measurements. V_mean showed a characteristic repeating pattern associated with the knee angle. Additionally, there were significant correlations between θ_sd and V_max.

Conclusions:

This is the first report to reveal the characteristics of the task-related head motion during stepping. Our findings are an essential step in the development of robust restraint toward fMRI during stepping task.

Attention deficit hyperactivity disorder

Over the last two decades, there have been numerous technical and methodological advances available to clinicians and researchers to better understand attention deficit hyperactivity disorder (ADHD) and its etiology. Despite the growing body of literature investigating the disorder's pathophysiology, ADHD remains a complex psychiatric disorder to characterize. This chapter will briefly review the literature on ADHD, with a focus on its history, the current genetic insights, neurophysiologic theories, and the use of neuroimaging to further understand the etiology. We address some of the major concerns that remain unclear about ADHD, including subtype instability, heterogeneity, and the underlying neural correlates that define the disorder. We highlight that the field of ADHD is rapidly evolving; the descriptions provided here will hopefully provide a sturdy foundation for which to build and improve our understanding of the disorder.

Primary sensory cortices contain distinguishable spatial patterns of activity for each sense

Whether primary sensory cortices are essentially multisensory or whether they respond to only one sense is an emerging debate in neuroscience. Here we use a multivariate pattern analysis of functional magnetic resonance imaging data in humans to demonstrate that simple and isolated stimuli of one sense elicit distinguishable spatial patterns of neuronal responses, not only in their corresponding primary sensory cortex, but in other primary sensory cortices. These results indicate that primary sensory cortices, traditionally regarded as unisensory, contain unique signatures of other senses and, thereby, prompt a reconsideration of how sensory information is coded in the human brain.

Human primary sensory cortices are traditionally regarded as being able to process only one sensory modality. Liang and colleagues use brain imaging to show that, as well as being processed in typically corresponding cortical areas, different sensory modalities are also processed in atypical cortical areas.

Functional and effective connectivity of visuomotor control systems demonstrated using generalized partial least squares and structural equation modeling

Tasks employing parametric variation in movement rate are associated with predictable modulations in neural activity and provide a convenient context for developing new techniques for system identification. Using a multistage approach, we explored the functional and effective connectivity of a visuomotor control system by combining generalized partial least squares (gPLS) with subsequent structural equation modeling (SEM) to reveal the relationships between neural activity and finger movement rate in an experiment involving visually paced left or right thumb flexion. The gPLS in the first analysis stage automatically identified spatially distributed sets of BOLD-contrast signal changes using linear combinations of sigmoidal basis functions parameterized by kinematic variables. The gPLS provided superior sensitivity in detecting task-related functional activity patterns via a step-wise comparison with both classical linear modeling and behavior correlation analysis. These activity patterns were used in the second analysis stage, which employed SEM to characterize the areal regional interactions. The hybrid gPLS/SEM procedure allowed modeling of complex regional interactions in a network including primary motor cortex, premotor areas, cerebellum, thalamus, and basal ganglia, with differential activity modulations with respect to rate observed in the corticocerebellar and corticostriate subsystems. This effective connectivity analysis of visuomotor control circuits showed that both the left and right corticocerebellar and corticostriate circuits exhibited movement rate-related modulation. The identification of the functional connectivity among regions participating particular classes of behavior using gPLS, followed by the estimation of the effective connectivity using SEM is an efficient means to characterize the neural interactions underlying variations in sensorimotor behavior.

Arousal dissociates amygdala and hippocampal fear responses: evidence from simultaneous fMRI and skin conductance recording

The experience and appraisal of threat is essential to human and animal survival. Lesion evidence suggests that the subjective experience of fear relies upon amygdala-medial frontal activity (as well as autonomic arousal), whereas the factual context of threat stimuli depends upon hippocampal-lateral frontal activity. This amygdala-hippocampus dissociation has not previously been demonstrated in vivo. To explore this differentiation, we employed functional magnetic resonance imaging (fMRI) and simultaneous skin conductance response (SCR) measures of phasic arousal, while subjects viewed fearful versus neutral faces. fMRI activity was subaveraged according to whether or not the subject evoked an arousal SCR to each discrete face stimulus. The fMRI-with arousal and fMRI-without arousal data provided a distinct differentiation of amygdala and hippocampal networks. Amygdala-medial frontal activity was observed only with SCRs, whereas hippocampus-lateral frontal activity occurred only in the absence of SCRs. The findings provide direct evidence for a dissociation between human amygdala and hippocampus networks in the visceral experience versus declarative fact processing of fear.

Quantifying head motion associated with motor tasks used in fMRI

In functional magnetic resonance imaging (fMRI) studies, long experiment times and small intensity changes associated with brain activation frequently lead to image artifacts due to head motion. Methods to minimize and correct for head motion by restraint, fast imaging, and retrospective image registration are typically combined but do not completely solve the problem, particularly for specific patient populations. As an initial step toward optimizing future designs of head restraints and improving motion correction techniques, the head motion characteristics of groups of stroke subjects, age-matched controls, and young adults were investigated with the aid of an MR simulator and a highly accurate position tracking system. Position measurements were recorded during motor tasks involving either the hand or the foot. Head motion was strongly dependent on the subject group and less upon the task conditions based on ANOVA calculations (P < 0.05). The stroke subjects exhibited approximately twice the head motion compared to that of age-matched controls, and the latter's head motion was about twice that of young adults. Moreover, the range of head motion in stroke subjects over all tasks was approximately 2 +/- 1 mm, with the motion occurring predominantly as translation in the superior-inferior direction and pitch rotation (nodding). These results lead to several recommendations on the design of fMRI motor experiments and suggest that improved motion correction strategies are required to examine such patient populations comprehensively.

Hierarchical organization of the human auditory cortex revealed by functional magnetic resonance imaging

The concept of hierarchical processing--that the sensory world is broken down into basic features later integrated into more complex stimulus preferences--originated from investigations of the visual cortex. Recent studies of the auditory cortex in nonhuman primates revealed a comparable architecture, in which core areas, receiving direct input from the thalamus, in turn, provide input to a surrounding belt. Here functional magnetic resonance imaging (fMRI) shows that the human auditory cortex displays a similar hierarchical organization: pure tones (PTs) activate primarily the core, whereas belt areas prefer complex sounds, such as narrow-band noise bursts.

Neuroimaging in hyperkinetic children and adults: an overview

The application of brain imaging techniques to children with Attention Deficit/Hyperactivity Disorders is reviewed, stressing methodological aspects. Findings are still provisional, but suggest minor structural changes in frontal and candate areas, especially on the right side. Functional studies suggest reduced activation in these and other areas. The techniques do not yet contribute to individual diagnosis.

Correspondence between functional magnetic resonance imaging somatotopy and individual brain anatomy of the central region: comparison with intraoperative stimulation in patients with brain tumors

OBJECT

The goal of this study was to determine the somatotopical structure-function relationships of the primary motor cortex in individual patients by using functional magnetic resonance (fMR) imaging. This was done to assess whether there is a displacement of functional areas compared with anatomical landmarks in patients harboring brain tumors close to the central region, and to validate these findings with intraoperative cortical stimulation.

METHODS

One hundred twenty hemispheres in 60 patients were studied by obtaining blood oxygen level-dependent fMR images in patients while they performed movements of the foot, hand, and face on both sides. There was a good correspondence between anatomical landmarks in the deep portion of the central sulcus on axial slices and the somatotopical organization of primary motor areas. Pixels activated during hand movements were centered on a small characteristic digitation; those activated during movements in the face and foot areas were located in the lower portion of the central sulcus (lateral to the hand area) and around the termination of the central sulcus, respectively. In diseased hemispheres, signal-intensity changes were still observed in the projection of the expected anatomical area. The fMR imaging data mapped intraoperative electrical stimulation in 92% of positive sites.

CONCLUSIONS

There was a high correspondence between the somatotopical anatomy and function in the central sulcus, which was similar in normal and diseased hemispheres. The fMR imaging and electrical stimulation data were highly concordant. These findings may enable the neurosurgeon to locate primary motor areas more easily during surgery.

Short communication: mapping of somatosensory cortices with functional magnetic resonance imaging in anaesthetized macaque monkeys

Functional magnetic resonance imaging (fMRI) in macaque monkeys is emerging as a potent candidate to bridge the gap between data from human fMRI studies and data from anatomy, electrophysiology and lesion studies in monkeys. The primary (SI) and secondary (SII) somatosensory cortices are the principal regions for somatosensory information processing and contain systematic representations of the body surface map (somatotopy). To examine the functional organization of the somatosensory cortices in anaesthetized macaque monkeys with fMRI, we asked whether focal and differential activation could be observed in SI and SII in response to tactile stimulation with two parameters: body sides (right and left) and body regions (hand and face). We found that changes in stimulus parameters elicited differential focal activation in both SI and SII in two ways. First, the hand and face stimulation activated SI and SII in the contralateral, but not in the ipsilateral, hemisphere. Second, the hand and face stimulation differentially activated two adjacent regions in both SI and SII. These fMRI results appear to correlate with previous mapping studies by other methods in the macaque somatosensory cortices. This study shows the feasibility of fMRI studies in mapping multiple sensory areas in monkeys by which we can distinguish between adjacent functionally distinct regions.

Safe electrical stimulation of the cochlear nerve at the promontory during functional magnetic resonance imaging

The purpose of this study was to evaluate possibilities and technical risks for combining intended electrical stimulation of the cochlear nerve and functional magnetic resonance imaging (fMRI). Theoretical considerations and experiments indicate that fMRI can be performed safely during electrical stimulation. A nerve stimulator was developed with minimized length of electrical conductors, current limiting resistance, high inner impedance of a current source, radio frequency (RF)-shielding, and avoidance of ferromagnetic materials. This nerve stimulator transfers the optically encoded stimulating current signal via a fiber optic cable located near the area of stimulation. There, the optical signal drives an MRI-compatible current source. This set-up was tested with transtympanic electrical stimulation of the cochlear nerve at the promontory during an fMRI examination. No hazardous effects could be detected. The stimulation resulted in activation of the Heschl's gyrus. Compared to the conventional promontory testing this method may allow a more objective examination of cochlear implant candidates. Magn Reson Med 42:371-378, 1999.

Functional imaging of the visual cortex with bold-contrast MRI: hyperventilation decreases signal response

Hypocapnia due to hyperventilation reduces cerebral blood flow and volume. To investigate the effects of hyperventilation on the regional signal response to visual activation using blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI), six volunteers were investigated during visual stimulation under normocapnia and hypocapnia conditions. Hyperventilation significantly decreased in visual cortex the BOLD MRI response to visual stimulation (3.97+/-0.5% [mean ( SD) in normocapnia vs. 0.77+/-0.7% in hypocapnia, P < 0.01]. In three of six subjects, functional signal changes were reduced to noise level. The reduced stimulus response during hyperventilation is probably due to a decreased overshoot in the blood oxygenation response. These results indicate that BOLD-contrast functional MRI is highly sensitive to pCO2 changes.

Proton magnetic resonance spectroscopy: an emerging technology in pediatric neurology research

Proton magnetic resonance spectroscopy (MRS) is an emerging technology that allows for the quantitative noninvasive assessment of regional brain biochemistry. The capacity to carry out MRS studies requires existing magnetic resonance imaging (MRI) technology platforms and the purchase of commercially available software modifications. In this review, the physical basis for MRS will be presented leading to an understanding of its potential applications and limitations within the clinical research milieu. Thus far, within pediatric neurology, proton MRS studies have been used to assist in the prediction of outcome in a variety of settings of acquired brain injuries (perinatal asphyxia, near drowning). In addition, proton MRS has been used to document disturbances in oxidative metabolism in neurometabolic disorders, assisting in defining phenotype and the response to therapeutic interventions. In epilepsy, spectroscopic studies have been useful in localizing the epileptogenic zone in intractable focal epilepsies. Future applications of proton MRS will also be highlighted. These include its use as a means of observing the transport and metabolism of various compounds in the brain, its concurrent application with other nuclear magnetic resonance techniques such as MRI and functional MRI, and finally its potential as a means of assessing the short-term effects of any CNS targeted pharmacologic interventions.

The acquisition of skilled motor performance: fast and slow experience-driven changes in primary motor cortex

Behavioral and neurophysiological studies suggest that skill learning can be mediated by discrete, experience-driven changes within specific neural representations subserving the performance of the trained task. We have shown that a few minutes of daily practice on a sequential finger opposition task induced large, incremental performance gains over a few weeks of training. These gains did not generalize to the contralateral hand nor to a matched sequence of identical component movements, suggesting that a lateralized representation of the learned sequence of movements evolved through practice. This interpretation was supported by functional MRI data showing that a more extensive representation of the trained sequence emerged in primary motor cortex after 3 weeks of training. The imaging data, however, also indicated important changes occurring in primary motor cortex during the initial scanning sessions, which we proposed may reflect the setting up of a task-specific motor processing routine. Here we provide behavioral and functional MRI data on experience-dependent changes induced by a limited amount of repetitions within the first imaging session. We show that this limited training experience can be sufficient to trigger performance gains that require time to become evident. We propose that skilled motor performance is acquired in several stages: "fast" learning, an initial, within-session improvement phase, followed by a period of consolidation of several hours duration, and then "slow" learning, consisting of delayed, incremental gains in performance emerging after continued practice. This time course may reflect basic mechanisms of neuronal plasticity in the adult brain that subserve the acquisition and retention of many different skills.

Latencies in fMRI time-series: effect of slice acquisition order and perception

In BOLD fMRI a detailed analysis of the MRI signal time course sometimes shows time differences between different activated regions. Some researchers have suggested that these latencies could be used to infer the temporal order of activation of these cortical regions. Several effects must be considered, however, before interpreting these latencies. The effect of a slice-dependent time shift (SDTS) with multi-slice acquisitions, for instance, may be important for regions located on different slices. After correction for this SDTS effect the time dispersion between activated regions is significantly decreased and the correlation between the MRI signal time course and the stimulation paradigm is improved. Another effect to consider is the latency which may exist between perception and stimulus presentation. It is shown that the control of perception can be achieved using a finger-spanning technique during the fMRI acquisition. The use of this perception profile rather than an arbitrary waveform derived from the paradigm proves to be a powerful alternative to fMRI data processing, especially with chemical senses studies, when return to baseline is not always correlated to stimulus suppression. This approach should also be relevant to other kinds of stimulation tasks, as a realistic way of monitoring the actual task performance, which may depend on attention, adaptation, fatigue or even variability of stimulus presentation.

A Bayesian approach to introducing anatomo-functional priors in the EEG/MEG inverse problem

In this paper, we present a new approach to the recovering of dipole magnitudes in a distributed source model for magnetoencephalographic (MEG) and electroencephalographic (EEG) imaging. This method consists in introducing spatial and temporal a priori information as a cure to this ill-posed inverse problem. A nonlinear spatial regularization scheme allows the preservation of dipole moment discontinuities between some a priori noncorrelated sources, for instance, when considering dipoles located on both sides of a sulcus. Moreover, we introduce temporal smoothness constraints on dipole magnitude evolution, at time scales smaller than those of cognitive processes. These priors are easily integrated into a Bayesian formalism, yielding a maximum a posteriori (MAP) estimator of brain electrical activity. Results from EEG simulations of our method are presented and compared with those of classical quadratic regularization and a now popular generalized minimum-norm technique called low-resolution electromagnetic tomography (LORETA).

Primary motor and sensory cortex activation during motor performance and motor imagery: a functional magnetic resonance imaging study

The intensity and spatial distribution of functional activation in the left precentral and postcentral gyri during actual motor performance (MP) and mental representation [motor imagery (MI)] of self-paced finger-to-thumb opposition movements of the dominant hand were investigated in fourteen right-handed volunteers by functional magnetic resonance imaging (fMRI) techniques. Significant increases in mean normalized fMRI signal intensities over values obtained during the control (visual imagery) tasks were found in a region including the anterior bank and crown of the central sulcus, the presumed site of the primary motor cortex, during both MP (mean percentage increase, 2.1%) and MI (0.8%). In the anterior portion of the precentral gyrus and the postcentral gyrus, mean functional activity levels were also increased during both conditions (MP, 1.7 and 1.2%; MI, 0.6 and 0.4%, respectively). To locate activated foci during MI, MP, or both conditions, the time course of the signal intensities of pixels lying in the precentral or postcentral gyrus was plotted against single-step or double-step waveforms, where the steps of the waveform corresponded to different tasks. Pixels significantly (r > 0.7) activated during both MP and MI were identified in each region in the majority of subjects; percentage increases in signal intensity during MI were on average 30% as great as increases during MP. The pixels activated during both MP and MI appear to represent a large fraction of the whole population activated during MP. These results support the hypothesis that MI and MP involve overlapping neural networks in perirolandic cortical areas.