Spatially independent activity patterns in functional MRI data during the Stroop color-naming task

A method is given for determining the time course and spatial extent of consistently and transiently task-related activations from other physiological and artifactual components that contribute to functional MRI (fMRI) recordings. Independent component analysis (ICA) was used to analyze two fMRI data sets from a subject performing 6-min trials composed of alternating 40-sec Stroop color-naming and control task blocks. Each component consisted of a fixed three-dimensional spatial distribution of brain voxel values (a “map”) and an associated time course of activation. For each trial, the algorithm detected, without a priori knowledge of their spatial or temporal structure, one consistently task-related component activated during each Stroop task block, plus several transiently task-related components activated at the onset of one or two of the Stroop task blocks only. Activation patterns occurring during only part of the fMRI trial are not observed with other techniques, because their time courses cannot easily be known in advance. Other ICA components were related to physiological pulsations, head movements, or machine noise. By using higher-order statistics to specify stricter criteria for spatial independence between component maps, ICA produced improved estimates of the temporal and spatial extent of task-related activation in our data compared with principal component analysis (PCA). ICA appears to be a promising tool for exploratory analysis of fMRI data, particularly when the time courses of activation are not known in advance.

Prospective motion correction of 3D echo-planar imaging data for functional MRI using optical tracking.

We evaluated the performance of an optical camera based prospective motion correction (PMC) system in improving the quality of 3D echo-planar imaging functional MRI data. An optical camera and external marker were used to dynamically track the head movement of subjects during fMRI scanning. PMC was performed by using the motion information to dynamically update the sequence's RF excitation and gradient waveforms such that the field-of-view was realigned to match the subject's head movement. Task-free fMRI experiments on five healthy volunteers followed a 2×2×3 factorial design with the following factors: PMC on or off; 3.0mm or 1.5mm isotropic resolution; and no, slow, or fast head movements. Visual and motor fMRI experiments were additionally performed on one of the volunteers at 1.5mm resolution comparing PMC on vs PMC off for no and slow head movements. Metrics were developed to quantify the amount of motion as it occurred relative to k-space data acquisition. The motion quantification metric collapsed the very rich camera tracking data into one scalar value for each image volume that was strongly predictive of motion-induced artifacts. The PMC system did not introduce extraneous artifacts for the no motion conditions and improved the time series temporal signal-to-noise by 30% to 40% for all combinations of low/high resolution and slow/fast head movement relative to the standard acquisition with no prospective correction. The numbers of activated voxels (p<0.001, uncorrected) in both task-based experiments were comparable for the no motion cases and increased by 78% and 330%, respectively, for PMC on versus PMC off in the slow motion cases. The PMC system is a robust solution to decrease the motion sensitivity of multi-shot 3D EPI sequences and thereby overcome one of the main roadblocks to their widespread use in fMRI studies.

The Stability of the Blood Oxygenation Level-Dependent Functional MRI Response to Motor Tasks Is Altered in Patients With Chronic Ischemic Stroke

Background and Purpose-Functional MRI is a powerful tool to investigate recovery of brain function in patients with stroke. An inherent assumption in functional MRI data analysis is that the blood oxygenation level-dependent (BOLD) signal is stable over the course of the examination. In this study, we evaluated the validity of such assumption in patients with chronic stroke. Methods-Fifteen patients performed a simple motor task with repeated epochs using the paretic and the unaffected hand in separate runs. The corresponding BOLD signal time courses were extracted from the primary and supplementary motor areas of both hemispheres. Statistical maps were obtained by the conventional General Linear Model and by a parametric General Linear Model. Results-Stable BOLD amplitude was observed when the task was executed with the unaffected hand. Conversely, the BOLD signal amplitude in both primary and supplementary motor areas was progressively attenuated in every patient when the task was executed with the paretic hand. The conventional General Linear Model analysis failed to detect brain activation during movement of the paretic hand. However, the proposed parametric General Linear Model corrected the misdetection problem and showed robust activation in both primary and supplementary motor areas. Conclusions-The use of data analysis tools that are built on the premise of a stable BOLD signal may lead to misdetection of functional regions and underestimation of brain activity in patients with stroke. The present data urge the use of caution when relying on the BOLD response as a marker of brain reorganization in patients with stroke. (Stroke. 2010; 41:1921-1926.)

Chronometric features of processing unpleasant stimuli: a functional MRI-based transcranial magnetic stimulation study.

The quick identification of potentially threatening events is a crucial cognitive capacity to survive in a changing environment. Previous functional MRI data revealed the right dorsolateral prefrontal cortex and the region of the left intraparietal sulcus (IPS) to be involved in the perception of emotionally negative stimuli. For assessing chronometric aspects of emotion processing, we applied transcranial magnetic stimulation above these areas at different times after negative and neutral picture presentation. An interference with emotion processing was found with transcranial magnetic stimulation above the dorsolateral prefrontal cortex 200-300 ms and above the left intraparietal sulcus 240/260 ms after negative stimuli. The data suggest a parallel and conjoint involvement of prefrontal and parietal areas for the identification of emotionally negative stimuli.

Special issue on methods and learning in functional MRI: Introductory remarks

This special issue represents a further exploration of some issues raised at a symposium entitled “Functional magnetic resonance imaging: From methods to madness” presented during the 15th annual Theoretical and Experimental Neuropsychology (TENNET XV) meeting in Montreal, Canada in June, 2004. The special issue’s theme is methods and learning in functional magnetic resonance imaging (fMRI), and it comprises 6 articles (3 reviews and 3 empirical studies). The first (Amaro and Barker) provides a beginners guide to fMRI and the BOLD effect (perhaps an alternative title might have been “fMRI for dummies”). While fMRI is now commonplace, there are still researchers who have yet to employ it as an experimental method and need some basic questions answered before they venture into new territory. This article should serve them well. A key issue of interest at the symposium was how fMRI could be used to elucidate cerebral mechanisms responsible for new learning. The next 4 articles address this directly, with the first (Little and Thulborn) an overview of data from fMRI studies of category-learning, and the second from the same laboratory (Little, Shin, Siscol, and Thulborn) an empirical investigation of changes in brain activity occurring across different stages of learning. While a role for medial temporal lobe (MTL) structures in episodic memory encoding has been acknowledged for some time, the different experimental tasks and stimuli employed across neuroimaging studies have not surprisingly produced conflicting data in terms of the precise subregion(s) involved. The next paper (Parsons, Haut, Lemieux, Moran, and Leach) addresses this by examining effects of stimulus modality during verbal memory encoding. Typically, BOLD fMRI studies of learning are conducted over short time scales, however, the fourth paper in this series (Olson, Rao, Moore, Wang, Detre, and Aguirre) describes an empirical investigation of learning occurring over a longer than usual period, achieving this by employing a relatively novel technique called perfusion fMRI. This technique shows considerable promise for future studies. The final article in this special issue (de Zubicaray) represents a departure from the more familiar cognitive neuroscience applications of fMRI, instead describing how neuroimaging studies might be conducted to both inform and constrain information processing models of cognition.

Functional MRI Mapping of the Human Auditory Cortex

Mapping the human auditory cortex with standard functional imaging techniques is difficult because of its small size and angular position along the Sylvian fissure. As a result, the exact number and location of auditory cortex areas in the human remains unknown. In a first experiment, we measured the two largest tonotopic areas of primary auditory cortex (PAC, Al and R) using high-resolution functional MRI at 7 Tesla relative to the underlying anatomy of Heschl's gyrus (HG). The data reveals a clear anatomical- functional relationship that indicates the location of PAC across the range of common morphological variants of HG (single gyri, partial duplication and complete duplication). Human PAC tonotopic areas are oriented along an oblique posterior-to-anterior axis with mirror-symmetric frequency gradients perpendicular to HG, as in the macaque. In a second experiment, we tested whether these primary frequency-tuned units were modulated by selective attention to preferred vs. non-preferred sound frequencies in the dynamic manner needed to account for human listening abilities in noisy environments, such as cocktail parties or busy streets. We used a dual-stream selective attention experiment where subjects attended to one of two competing tonal streams presented simultaneously to different ears. Attention to low-frequency tones (250 Hz) enhanced neural responses within low-frequency-tuned voxels relative to high (4000 Hz), and vice versa when at-tention switched from high to low. Human PAC is able to tune into attended frequency channels and can switch frequencies on demand, like a radio. In a third experiment, we investigated repetition suppression effects to environmental sounds within primary and non-primary early-stage auditory areas, identified with the tonotopic mapping design. Repeated presentations of sounds from the same sources, as compared to different sources, gave repetition suppression effects within posterior and medial non-primary areas of the right hemisphere, reflecting their potential involvement in semantic representations. These three studies were conducted at 7 Tesla with high-resolution imaging. However, 7 Tesla scanners are, for the moment, not yet used for clinical diagnosis and mostly reside in institutions external to hospitals. Thus, hospital-based clinical functional and structural studies are mainly performed using lower field systems (1.5 or 3 Tesla). In a fourth experiment, we acquired tonotopic maps at 3 and 7 Tesla and evaluated the consistency of a tonotopic mapping paradigm between scanners. Mirror-symmetric gradients within PAC were highly similar at 7 and 3 Tesla across renderings at different spatial resolutions. We concluded that the tonotopic mapping paradigm is robust and suitable for definition of primary tonotopic areas, also at 3 Tesla. Finally, in a fifth study, we considered whether focal brain lesions alter tonotopic representations in the intact ipsi- and contralesional primary auditory cortex in three patients with hemispheric or cerebellar lesions, without and with auditory complaints. We found evidence for tonotopic reorganisation at the level of the primary auditory cortex in cases of brain lesions independently of auditory complaints. Overall, these results reflect a certain degree of plasticity within primary auditory cortex in different populations of subjects, assessed at different field strengths. - La cartographie du cortex auditif chez l'humain est difficile à réaliser avec des techniques d'imagerie fonctionnelle standard, étant donné sa petite taille et position angulaire le long de la fissure sylvienne. En conséquence, le nombre et l'emplacement exacts des différentes aires du cortex auditif restent inconnus chez l'homme. Lors d'une première expérience, nous avons mesuré, avec de l'imagerie par résonance magnétique à haute intensité (IRMf à 7 Tesla) chez des sujets humains sains, deux larges aires au sein du cortex auditif primaire (PAC; Al et R) avec une représentation spécifique des fréquences pures préférées - ou tonotopie. Nos résultats ont démontré une relation anatomico- fonctionnelle qui définit clairement la position du PAC à travers toutes les variantes du gyrus d'Heschl's (HG). Les aires tonotopiques du PAC humain sont orientées le long d'un axe postéro-antérieur oblique avec des gradients de fréquences spécifiques perpendiculaires à HG, d'une manière similaire à celles mesurées chez le singe. Dans une deuxième expérience, nous avons testé si ces aires primaires pouvaient être modulées, de façon dynamique, par une attention sélective pour des fréquences préférées par rapport à celles non-préférées. Cette modulation est primordiale lors d'interactions sociales chez l'humain en présence de bruits distracteurs tels que d'autres discussions ou un environnement sonore nuisible (comme par exemple, dans la circulation routière). Dans cette étude, nous avons utilisé une expérience d'attention sélective où le sujet devait être attentif à une des deux voies sonores présentées simultanément à chaque oreille. Lorsque le sujet portait était attentif aux sons de basses fréquences (250 Hz), la réponse neuronale relative à ces fréquences augmentait par rapport à celle des hautes fréquences (4000 Hz), et vice versa lorsque l'attention passait des hautes aux basses fréquences. De ce fait, nous pouvons dire que PAC est capable de focaliser sur la fréquence attendue et de changer de canal selon la demande, comme une radio. Lors d'une troisième expérience, nous avons étudié les effets de suppression due à la répétition de sons environnementaux dans les aires auditives primaires et non-primaires, d'abord identifiées via le protocole de la première étude. La présentation répétée de sons provenant de la même source sonore, par rapport à de sons de différentes sources sonores, a induit un effet de suppression dans les aires postérieures et médiales auditives non-primaires de l'hémisphère droite, reflétant une implication de ces aires dans la représentation de la catégorie sémantique. Ces trois études ont été réalisées avec de l'imagerie à haute résolution à 7 Tesla. Cependant, les scanners 7 Tesla ne sont pour le moment utilisés que pour de la recherche fondamentale, principalement dans des institutions externes, parfois proches du patient mais pas directement à son chevet. L'imagerie fonctionnelle et structurelle clinique se fait actuellement principalement avec des infrastructures cliniques à 1.5 ou 3 Tesla. Dans le cadre dune quatrième expérience, nous avons avons évalués la cohérence du paradigme de cartographie tonotopique à travers différents scanners (3 et 7 Tesla) chez les mêmes sujets. Nos résultats démontrent des gradients de fréquences définissant PAC très similaires à 3 et 7 Tesla. De ce fait, notre paradigme de définition des aires primaires auditives est robuste et applicable cliniquement. Finalement, nous avons évalués l'impact de lésions focales sur les représentations tonotopiques des aires auditives primaires des hémisphères intactes contralésionales et ipsilésionales chez trois patients avec des lésions hémisphériques ou cérébélleuses avec ou sans plaintes auditives. Nous avons trouvé l'évidence d'une certaine réorganisation des représentations topographiques au niveau de PAC dans le cas de lésions cérébrales indépendamment des plaintes auditives. En conclusion, nos résultats démontrent une certaine plasticité du cortex auditif primaire avec différentes populations de sujets et différents champs magnétiques.

Assessment and discrimination of odor stimuli in rat olfactory bulb by dynamic functional MRI

Dynamic blood oxygenation level-dependent functional MRI was applied at 7 T in the rat olfactory bulb (OB) with pulsed delivery of iso-amyl acetate (IAA) and limonene. Acquisition times for single-slice and whole OB data were 8 and 32 s, respectively, with spatial resolution of 220 × 220 × 250 μm. On an intrasubject basis, short IAA exposures of 0.6 min separated by 3.5-min intervals induced reproducible spatial activity patterns (SAPs) in the olfactory nerve layer, glomerular layer, and external plexiform layer. During long exposures (≈10 min), the initially dominant dorsal SAPs declined in intensity and area, whereas in some OB regions, the initially weak ventral/lateral SAPs increased first and then decreased. The SAPs of different concentrations were topologically similar, which implies that whereas an odor at various concentrations activates the same subsets of receptor cells, different concentrations are assessed and discriminated by variable magnitudes of laminarspecific activations. IAA and limonene reproducibly activated different subsets of receptor cells with some overlaps. Whereas qualitative topographical agreement was observed with results from other methods, the current dynamic blood oxygenation level-dependent functional MRI results can provide quantitative SAPs of the entire OB.

Mechanisms of migraine aura revealed by functional MRI in human visual cortex

Cortical spreading depression (CSD) has been suggested to underlie migraine visual aura. However, it has been challenging to test this hypothesis in human cerebral cortex. Using high-field functional MRI with near-continuous recording during visual aura in three subjects, we observed blood oxygenation level-dependent (BOLD) signal changes that demonstrated at least eight characteristics of CSD, time-locked to percept/onset of the aura. Initially, a focal increase in BOLD signal (possibly reflecting vasodilation), developed within extrastriate cortex (area V3A). This BOLD change progressed contiguously and slowly (3.5 ± 1.1 mm/min) over occipital cortex, congruent with the retinotopy of the visual percept. Following the same retinotopic progression, the BOLD signal then diminished (possibly reflecting vasoconstriction after the initial vasodilation), as did the BOLD response to visual activation. During periods with no visual stimulation, but while the subject was experiencing scintillations, BOLD signal followed the retinotopic progression of the visual percept. These data strongly suggest that an electrophysiological event such as CSD generates the aura in human visual cortex.

Event-related functional MRI: Past, present, and future

The past two decades have seen an enormous growth in the field of human brain mapping. Investigators have extensively exploited techniques such as positron emission tomography and MRI to map patterns of brain activity based on changes in cerebral hemodynamics. However, until recently, most studies have investigated equilibrium changes in blood flow measured over time periods upward of 1 min. The advent of high-speed MRI methods, capable of imaging the entire brain with a temporal resolution of a few seconds, allows for brain mapping based on more transient aspects of the hemodynamic response. Today it is now possible to map changes in cerebrovascular parameters essentially in real time, conferring the ability to observe changes in brain state that occur over time periods of seconds. Furthermore, because robust hemodynamic alterations are detectable after neuronal stimuli lasting only a few tens of milliseconds, a new class of task paradigms designed to measure regional responses to single sensory or cognitive events can now be studied. Such “event related” functional MRI should provide for fundamentally new ways to interrogate brain function, and allow for the direct comparison and ultimately integration of data acquired by using more traditional behavioral and electrophysiological methods.

Premovement activity of the pre-supplementary motor area and the readiness for action: Studies of time-resolved event-related functional MRI

The supplementary motor area (SMA) is thought to play in important role in the preparation and organisation of voluntary movement. It has long been known that cortical activity begins to increase up to 2 s prior to voluntary self-initiated movement. This increasing premovement activity measured in EEG is known as the Bereitschaftspotential or readiness potential. Modern functional brain imaging methods, using event-related and time-resolved functional MRI techniques, are beginning to reveal the role of the SMA, and in particular the more anterior pre-SMA, in premovement activity associated with the readiness for action. In this paper we review recent studies using event-related time-resolved fMRI methods to examine the time-course of activation changes within the SMA throughout the preparation, readiness and execution of action. These studies suggest that the preSMA plays a common role in encoding or representing actions prior to our own voluntary self-initiated movements, during motor imagery, and from the observation of others' actions. We suggest that the pre-SMA generates and encodes motor representations which are then maintained in readiness for action. (c) 2005 Elsevier B.V. All rights reserved.

Visuospatial processing and the function of prefrontal-parietal networks in autism spectrum disorders: a functional MRI study

Objective: Individuals with autism spectrum disorders typically have normal visuospatial abilities but impaired executive functioning, particularly in abilities related to working memory and attention. The aim of this study was to elucidate the functioning of frontoparietal networks underlying spatial working memory processes during mental rotation in persons with autism spectrum disorders. Method: Seven adolescent males with normal IQ with an autism spectrum disorder and nine age- and IQ-matched male comparison subjects underwent functional magnetic resonance imaging scans while performing a mental rotation task. Results: The autism spectrum disorders group showed less activation in lateral and medial premotor cortex, dorsolateral prefrontal cortex, anterior cingulate gyrus, and caudate nucleus. Conclusions: The finding of less activation in prefrontal regions but not in parietal regions supports a model of dysfunction of frontostriatal networks in autism spectrum disorders.

Three-dimensional modeling of tongue during speech using MRI data

The tongue is the most important and dynamic articulator for speech formation, because of its anatomic aspects (particularly, the large volume of this muscular organ comparatively to the surrounding organs of the vocal tract) and also due to the wide range of movements and flexibility that are involved. In speech communication research, a variety of techniques have been used for measuring the three-dimensional vocal tract shapes. More recently, magnetic resonance imaging (MRI) becomes common; mainly, because this technique allows the collection of a set of static and dynamic images that can represent the entire vocal tract along any orientation. Over the years, different anatomical organs of the vocal tract have been modelled; namely, 2D and 3D tongue models, using parametric or statistical modelling procedures. Our aims are to present and describe some 3D reconstructed models from MRI data, for one subject uttering sustained articulations of some typical Portuguese sounds. Thus, we present a 3D database of the tongue obtained by stack combinations with the subject articulating Portuguese vowels. This 3D knowledge of the speech organs could be very important; especially, for clinical purposes (for example, for the assessment of articulatory impairments followed by tongue surgery in speech rehabilitation), and also for a better understanding of acoustic theory in speech formation.

Functional MRI of stimulus interference on auditory processing in normal listeners and tinnitus patients

We are constantly immersed in stimuli. Upon reaching our senses, stimuli are processed within various brain systems along various pathways into the brain, and eventually turned into a percept. However, there are percepts that do not result from responses to external source stimuli. A particular case of this situation is the auditory percept known as tinnitus. Tinnitus can be seen as a task-irrelevant auditory percept, commonly reported to interfere with normal daily tasks. This is known from reports made by tinnitus sufferers that refer to their phantom percept as distracting, and that it diverts their focus from the task-relevant stimuli.(...)

Long memory model of resting state functional MRI

Magdeburg, Univ., Fak. für Elektrotechnik und Informationstechnik, Diss., 2013

Visual analysis, clustering, and classification of contrast-enhanced tumor perfusion MRI data

Magdeburg, Univ., Fak. für Inf., Diss., 2014