29 research outputs found
Identification and segmentation of the central sulcus from human brain MR image
Master'sMASTER OF SCIENC
Validating a new methodology for optical probe design and image registration in fNIRS studies
Functional near-infrared spectroscopy (fNIRS) is an imaging technique that relies on the principle of shining near-infrared light through tissue to detect changes in hemodynamic activation. An important methodological issue encountered is the creation of optimized probe geometry for fNIRS recordings. Here, across three experiments, we describe and validate a processing pipeline designed to create an optimized, yet scalable probe geometry based on selected regions of interest (ROIs) from the functional magnetic resonance imaging (fMRI) literature. In experiment 1, we created a probe geometry optimized to record changes in activation from target ROIs important for visual working memory. Positions of the sources and detectors of the probe geometry on an adult head were digitized using a motion sensor and projected onto a generic adult atlas and a segmented head obtained from the subject's MRI scan. In experiment 2, the same probe geometry was scaled down to fit a child's head and later digitized and projected onto the generic adult atlas and a segmented volume obtained from the child's MRI scan. Using visualization tools and by quantifying the amount of intersection between target ROIs and channels, we show that out of 21 ROIs, 17 and 19 ROIs intersected with fNIRS channels from the adult and child probe geometries, respectively. Further, both the adult atlas and adult subject-specific MRI approaches yielded similar results and can be used interchangeably. However, results suggest that segmented heads obtained from MRI scans be used for registering children's data. Finally, in experiment 3, we further validated our processing pipeline by creating a different probe geometry designed to record from target ROIs involved in language and motor processing
Neuroinformatics in Functional Neuroimaging
This Ph.D. thesis proposes methods for information retrieval in functional neuroimaging through automatic computerized authority identification, and searching and cleaning in a neuroscience database. Authorities are found through cocitation analysis of the citation pattern among scientific articles. Based on data from a single scientific journal it is shown that multivariate analyses are able to determine group structure that is interpretable as particular “known ” subgroups in functional neuroimaging. Methods for text analysis are suggested that use a combination of content and links, in the form of the terms in scientific documents and scientific citations, respectively. These included context sensitive author ranking and automatic labeling of axes and groups in connection with multivariate analyses of link data. Talairach foci from the BrainMap ™ database are modeled with conditional probability density models useful for exploratory functional volumes modeling. A further application is shown with conditional outlier detection where abnormal entries in the BrainMap ™ database are spotted using kernel density modeling and the redundancy between anatomical labels and spatial Talairach coordinates. This represents a combination of simple term and spatial modeling. The specific outliers that were found in the BrainMap ™ database constituted among others: Entry errors, errors in the article and unusual terminology
Multimodal (EEG-fMRI) functional connectivity study of levodopa effect in Parkinson’s disease
Aim: To assess if the intake of levodopa in patients with Parkinson’s Disease (PD) changes cerebral connectivity, as revealed by simultaneous recording of hemodynamic (functional MRI, or fMRI) and electric (electroencephalogram, EEG) signals. Particularly, we hypothesize that the strongest changes in FC will involve the motor network, which is the most impaired in PD.
Methods: Eight patients with diagnosis of PD “probable”, therapy with levodopa exclusively, normal cognitive and affective status, were included. Exclusion criteria were: moderate-severe rest tremor, levodopa induced dyskinesia, evidence of gray or white matter abnormalities on structural MRI. Scalp EEG (64 channels) were acquired inside the scanner (1.5 Tesla) before and after the intake of levodopa. fMRI functional connectivity was computed from four regions of interest: right and left supplementary motor area (SMA) and right and left precentral gyrus (primary motor cortex). Weighted partial directed coherence (w-PDC) was computed in the inverse space after the removal of EEG gradient and cardioballistic artifacts.
Results and discussion: fMRI group analysis shows that the intake of levodopa increases hemodynamic functional connectivity among the SMAs / primary motor cortex and: sensory-motor network itself, attention network and default mode network. w-PDC analysis shows that EEG connectivity among regions of the motor network has the tendency to decrease after the intake the levodopa; furthermore, regions belonging to the DMN have the tendency to increase their outflow toward the rest of the brain. These findings, even if in a small sample of patients, suggest that other resting state physiological functional networks, beyond the motor one, are affected in patients with PD. The behavioral and cognitive tasks corresponding to the affected networks could benefit from the intake of levodopa
The neurocognitive processing of plausibility and real-world knowledge:A cross-linguistic investigation
Our knowledge about concepts and meanings is at the very heart of human cognition.
In everyday life, we have to interact with our environment in a variety of different
ways. Our actions are guided by what we know and believe about the world and this
knowledge derives primarily from previous sensory and perceptual experiences. The
fact that we are capable of engaging with our environment in an appropriate and efficient
way means that we have learnt (how) to make sense of the events and entities we
are faced with in day-to-day life. We are thus able to recognise and name both physical
objects and abstract concepts, to categorise and associate them based on their specific
properties, to interpret other people’s intentions, and to judge cause and effect of
their actions as well as our own. Moreover, the ability to represent this wealth of
knowledge about the real world in the conceptualised and symbolic form of language
is believed to be exclusive to humans. Our language capacity allows us to communicate
with others about past and future events or to describe fictitious scenarios by
combining previously acquired concepts in a novel way without the need for external
stimulation. Thus language forms a primary means of interacting with those around us
by allowing us to express our own thoughts and comprehend those of others. As long
as language processing proceeds in an undisturbed manner, we are largely unaware of
the underlying mechanisms that support the seemingly effortless interpretation of linguistic
input. The importance of these processes for successful communication, however,
becomes all the more apparent when language processing is disrupted, for example,
by brain lesions that render semantic analysis difficult or impossible.
Scientific research that aims to uncover and define cognitive or neural mechanisms
underlying semantic processing is inevitably faced with the complexity and
wealth of semantic relationships that need to be taken into account. In absence of noninvasive
neurocognitive methods and insights gleaned from modern neurobiology,
early research had a limited impact on our understanding of how semantic processing
is implemented in the human brain. Traditional neurological models of language have
been based primarily on lesion-deficit data, and thus supported the view that certain
areas of the brain were exclusively dedicated to the processing of language-specific
functions (Geschwind, 1970; Lichtheim, 1885; Wernicke, 1874). Furthermore, classical
theories of sensory processing viewed the brain as a purely stimulus-driven system that retrieves and combines individual low-level aspects or features in an automated,
passive and context-independent manner (Biederman, 1987; Burton & Sinclair, 1996;
Hubel & Wiesel, 1965; Massaro, 1998).
After a recent paradigm shift in the cognitive neurosciences, current theories
of sensory processing are now based on the concept of the brain as a highly active,
adaptive and dynamic device. In this sense, language comprehension, like many other
higher-cognitive functions, is shaped by a flexible interaction of a number of different
processes and information sources that include so-called bottom-up signals, i.e., the
actual sensory input and processes related to their forward propagation, and top-down
processes that generate predictions and expectations based on prior experience and
perceived probabilities. Therefore, accounts that view semantic processing as a dynamic
and active construction of meaning that is highly sensitive to contextual influences
seem most probable from a neurobiological perspective. Results from electrophysiological
and neuroimaging research on semantic analysis in sentence and discourse
context have provided evidence for top-down influences from the very beginning.
In addition, recent ERP results have suggested that the interaction between topdown
and bottom-up information is more flexible and dynamic than previously assumed.
Yet, the importance of predictions and expectations has long been neglected in
models of semantic processing and language comprehension in general.
Neuroimaging data have provided us with a long list of brain regions that have
been implicated in different aspects of semantic analysis. We are only beginning to
understand the role(s) that these regions play and how they interact to support the
flexible and efficient construction of meaning.
The aim of the present thesis is to gain a more comprehensive view on the
computational mechanisms underlying language processing by investigating how bottom-
up and top-down information and processes interactively contribute to the semantic
analysis in sentences and discourse. To this end, we conducted a total of five studies
that used either event-related potentials or functional neuroimaging to shed light
on this matter from different perspectives.
The thesis is divided into two main parts: Part I (chapters 1-5) provides an
overview on previous results from electrophysiology and neuroimaging on semantic
processing as well as a description and discussion of the studies conducted in the present thesis. Part II (chapters 6-9) consists of three research articles that describe
and discuss the results of five experimental studies.
In Part I, Chapter 2 gives a brief introduction to the event-related potential and
functional neuroimaging techniques and reviews the most relevant results and theories
that have emerged from studies on sentence and discourse processing. Chapter 3 highlights
the research questions targeted in each of the experimental studies and describes
and discusses the most relevant findings against the background established by Chapter
2. Chapters 4 and 5 conclude Part I by placing the presented results in a broader
context and by briefly outlining future directions.
Part II begins with a survey of the three studies reported in the subsequent
chapters. Chapter 7 highlights the results of the first study, a German ERP experiment
that investigated the impact of capitalisation, i.e., a purely form-based and contextually
independent bottom-up manipulation, on the processing of semantic anomalies
in single sentences. Chapter 8 comprises three ERP experiments that used both easy
and hard to detect semantic anomalies in German and English to corroborate the assumption
that the weighting of top-down and bottom-up information cues might be
determined in a language-specific way. Chapter 9, the final chapter of the thesis, describes
and discusses the results of the third study, in which the impact of embedding
context on the required depth of semantic processing was examined using functional
neuroimaging
Early and Late Stage Mechanisms for Vocalization Processing in the Human Auditory System
The human auditory system is able to rapidly process incoming acoustic information, actively filtering, categorizing, or suppressing different elements of the incoming acoustic stream. Vocalizations produced by other humans (conspecifics) likely represent the most ethologically-relevant sounds encountered by hearing individuals. Subtle acoustic characteristics of these vocalizations aid in determining the identity, emotional state, health, intent, etc. of the producer. The ability to assess vocalizations is likely subserved by a specialized network of structures and functional connections that are optimized for this stimulus class. Early elements of this network would show sensitivity to the most basic acoustic features of these sounds; later elements may show categorically-selective response patterns that represent high-level semantic organization of different classes of vocalizations. A combination of functional magnetic resonance imaging and electrophysiological studies were performed to investigate and describe some of the earlier and later stage mechanisms of conspecific vocalization processing in human auditory cortices. Using fMRI, cortical representations of harmonic signal content were found along the middle superior temporal gyri between primary auditory cortices along Heschl\u27s gyri and the superior temporal sulci, higher-order auditory regions. Additionally, electrophysiological findings also demonstrated a parametric response profile to harmonic signal content. Utilizing a novel class of vocalizations, human-mimicked versions of animal vocalizations, we demonstrated the presence of a left-lateralized cortical vocalization processing hierarchy to conspecific vocalizations, contrary to previous findings describing similar bilateral networks. This hierarchy originated near primary auditory cortices and was further supported by auditory evoked potential data that suggests differential temporal processing dynamics of conspecific human vocalizations versus those produced by other species. Taken together, these results suggest that there are auditory cortical networks that are highly optimized for processing utterances produced by the human vocal tract. Understanding the function and structure of these networks will be critical for advancing the development of novel communicative therapies and the design of future assistive hearing devices
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Impacts of systemic hemodynamic factors on cerebral and peripheral perfusion
Changes in physiological factors involving modulation of hemodynamics such as baroreflex sensitivity (BRS), heart rate, and/or blood pressure influence blood flow to downstream tissues leading to changes in response and/or function of tissues. For example, a sufficient increase in heart rate elicits greater accumulative shear stimuli on a per minute basis leading to a greater vasodilatory response of endothelial cells and providing greater perfusion to skeletal muscle. The high-flow and low-impedance nature of the cerebral circulation leads to increased susceptibility to damage from considerable blood pressure fluctuations. For this reason, the cerebrovasculature holds a very narrow range of operation of cerebral autoregulation in response to changes in perfusion pressure. In a nondemented elderly population and patients with Alzheimer’s disease, impaired BRS has been linked with poor cognitive function and a link between high pulsatile components of blood pressure (i.e., pulse pressure) and impaired cognitive function has also been reported. Three research investigations were included in this dissertation study. The first study was to determine the association between heart rate at rest and endothelium-dependent vasodilation as assessed by flow-mediated dilation (FMD) of the brachial artery. The primary findings from study 1 revealed an indirect association between heart rate and FMD through shear stimuli. The studies 2 and 3 sought to determine the association of regional cerebral perfusion with cardiovagal BRS and blood pressure components. A link between cardiovagal BRS and regional cerebral perfusion of the hippocampus was demonstrated in the study 2. This finding may add mechanistic insight to the relationship between impaired BRS and cognitive dysfunction. The primary finding from the study 3 revealed a significant relationship between peripheral pulsatile blood pressure components and regional cerebral perfusion of the hippocampus as well as anterior white matter. This finding highlights the importance of pulsatile pressure on cerebral vascular beds. Taken together, the overall findings from this dissertation study indicate the potential impacts of systemic hemodynamic factors on cerebral and peripheral perfusion. Future longitudinal studies in nondemented elderly and individuals with Alzheimer’s disease are warranted to reveal the causality of these associations.Kinesiology and Health Educatio
Properties of Visual Field Maps in Health and Disease
The visual world that surrounds us is represented in and processed by multiple topographically organised maps in the human brain. The organising principle underlying these retinotopic maps is also apparent across other sensory modalities and appears highly conserved across species. Moreover, the template for these visual maps is laid down during development, without the need for visual experience. This thesis binds and summarises seven publications describing work to characterise the functional properties of visual maps in the human brain. Initially, we describe TMS and fMRI measurements designed to probe the functional specificity of two spatially distinct but spatially adjacent maps, LO-1 and LO-2. Concurrently I developed software (visualisation tools) for precise dissection of these areas and to more broadly facilitate the visualisation of neuroimaging data. Our experiment revealed a double dissociation in the functional specificity of these areas, with preferential processing of orientation and shape information by LO-1 and LO-2, respectively. We then used fMRI to examine the effect of spatial attention on the responses measured from visual field maps. We showed that attention modulated visual responses by both enhancing attended locations and suppressing unattended locations; these effects were evident in the maps of early visual cortex and subcortical structures including the lateral geniculate and pulvinar nuclei. Finally, we examined the properties of visual field maps in patients with retinal lesions. Although maps can be abnormally organised with certain congenital visual deficits, we asked whether normally developed maps were able to reorganise when input to them is lost later in life, specifically due to central retinal lesions. Our measurements showed no evidence of reorganisation in the maps of patients with macular degeneration: the extent of activity measured in these maps was both highly predictable based on individual retinal lesions and could be reliably simulated in normally sighted individuals
Functional Magnetic Resonance Imaging
"Functional Magnetic Resonance Imaging - Advanced Neuroimaging Applications" is a concise book on applied methods of fMRI used in assessment of cognitive functions in brain and neuropsychological evaluation using motor-sensory activities, language, orthographic disabilities in children. The book will serve the purpose of applied neuropsychological evaluation methods in neuropsychological research projects, as well as relatively experienced psychologists and neuroscientists. Chapters are arranged in the order of basic concepts of fMRI and physiological basis of fMRI after event-related stimulus in first two chapters followed by new concepts of fMRI applied in constraint-induced movement therapy; reliability analysis; refractory SMA epilepsy; consciousness states; rule-guided behavioral analysis; orthographic frequency neighbor analysis for phonological activation; and quantitative multimodal spectroscopic fMRI to evaluate different neuropsychological states