Methods for Dichoptic Stimulus Presentation in Functional Magnetic Resonance Imaging - A Review

Dichoptic stimuli (different stimuli displayed to each eye) are increasingly being used in functional brain imaging experiments using visual stimulation. These studies include investigation into binocular rivalry, interocular information transfer, three-dimensional depth perception as well as impairments of the visual system like amblyopia and stereodeficiency. In this paper, we review various approaches of displaying dichoptic stimulus used in functional magnetic resonance imaging experiments. These include traditional approaches of using filters (red-green, red-blue, polarizing) with optical assemblies as well as newer approaches of using bi-screen goggles

Different patterns of white matter degeneration using multiple diffusion indices and volumetric data in mild cognitive impairment and Alzheimer patients

Alzheimeŕs disease (AD) represents the most prevalent neurodegenerative disorder that causes cognitive decline in old age. In its early stages, AD is associated with microstructural abnormalities in white matter (WM). In the current study, multiple indices of diffusion tensor imaging (DTI) and brain volumetric measurements were employed to comprehensively investigate the landscape of AD pathology. The sample comprised 58 individuals including cognitively normal subjects (controls), amnestic mild cognitive impairment (MCI) and AD patients. Relative to controls, both MCI and AD subjects showed widespread changes of anisotropic fraction (FA) in the corpus callosum, cingulate and uncinate fasciculus. Mean diffusivity and radial changes were also observed in AD patients in comparison with controls. After controlling for the gray matter atrophy the number of regions of significantly lower FA in AD patients relative to controls was decreased; nonetheless, unique areas of microstructural damage remained, e.g., the corpus callosum and uncinate fasciculus. Despite sample size limitations, the current results suggest that a combination of secondary and primary degeneration occurrs in MCI and AD, although the secondary degeneration appears to have a more critical role during the stages of disease involving dementia

Neural correlates of executive attention in adults born very preterm

Very preterm birth is associated with an increased prevalence of attention problems and may especially impair executive attention, i.e., top-down control of attentional selection in situations where distracting information interferes with the processing of task-relevant stimuli. While there are initial findings linking structural brain alterations in preterm-born individuals with attention problems, the functional basis of these problems are not well understood. The present study used an fMRI adaptation of the Attentional Network Test to examine the neural correlates of executive attention in a large sample of N = 86 adults born very preterm and/or with very low birth weight (VP/VLBW), and N = 100 term-born controls. Executive attention was measured by comparing task behavior and brain activations associated with the processing of incongruent vs. congruent arrow flanker stimuli. Consistent with subtle impairments of executive attention, the VP/VLBW group showed lower accuracy and a tendency for increased response times during the processing of incongruent stimuli. Both groups showed similar activation patters, especially within expected fronto-cingulo-parietal areas, but no significant between-group differences. Our results argue for a maintained attention-relevant network organization in high-functioning preterm born adults in spite of subtle deficits in executive attention. Gestational age and neonatal treatment variables showed associations with task behavior, and brain activation in the dorsal ACC and lateral occipital areas, suggesting that the degree of prematurity (and related neonatal complications) has subtle modulatory influences on executive attention processing

White matter alterations of the corticospinal tract in adults born very preterm and/or with very low birth weight

White matter (WM) injury, either visible on conventional magnetic resonance images (MRI) or measurable by diffusion tensor imaging (DTI), is frequent in preterm born individuals and often affects the corticospinal tract (CST). The relation between visible and invisible white mater alterations in the reconstructed CST of preterm subjects has so far been studied in infants, children and up to adolescence. Therefore, we probabilistically tracked the CST in 53 term-born and 56 very preterm and/or low birth weight (VP/VLBW, < 32 weeks of gestation and/or birth weight < 1,500 g) adults (mean age 26 years) and compared their DTI parameters (axial, radial, mean diffusivity—AD, RD, MD, fractional anisotropy—FA) in the whole CST and slice-wise along the CST. Additionally, we used the automatic, tract-based-spatial-statistics (TBSS) as an alternative to tractography. We compared control and VP/VLBW and subgroups with and without CST WM lesions visible on conventional MRI. Compared to controls, VP/VLBW subjects had significantly higher diffusivity (AD, RD, MD) in the whole CST, slice-wise along the CST, and in multiple regions along the TBSS skeleton. VP/VLBW subjects also had significantly lower (TBSS) and higher (tractography) FA in regions along the CST, but no different mean FA in the tracked CST as a whole. Diffusion changes were weaker, but remained significant for both, tractography and TBSS, when excluding subjects with visible CST lesions. Chronic CST injury persists in VP/VLBW adults even in the absence of visible WM lesions, indicating long-term structural WM changes induced by premature birth

Automated quantitative evaluation of brain MRI may be more accurate for discriminating preterm born adults

Objective To investigate the structural brain abnormalities and their diagnostic accuracy through qualitative and quantitative analysis in term born and very preterm birth or with very low birth weight (VP/VLBW) adults. Methods We analyzed 3-T MRIs acquired in 2011–2013 from 67 adults (27 term born controls, mean age 26.4 years, 8 females; 40 VP/VLBWs, mean age 26.6 years, 16 females). We compared automatic segmentations of the white matter, deep gray matter and cortical gray matter, manual corpus callosum measurements and visual ratings of the ventricles and white matter with t tests, logistic regression, and receiver operator characteristic (ROC) curves. Results Automatic segmentation correctly classified 84% of cases; visual ratings correctly classified 63%. Quantitative volumetry based on automatic segmentation revealed higher ventricular volume, lower posterior corpus callosum, and deep gray matter volumes in VP/VLBW subjects compared to controls (p < 0.01). Visual rating and manual measurement revealed a thinner corpus callosum in VP/VLBW adults (p = 0.04) and deformed lateral ventricles (p = 0.03) and tendency towards more “dirty” white matter (p = 0.06). Automatic/manual measures combined with visual ratings correctly classified 87% of cases. Stepwise logistic regression identified three independent features that correctly classify 81% of cases: ventricular volume, deep gray matter volume, and white matter aspect. Conclusion Enlarged and deformed lateral ventricles, thinner corpus callosum, and “dirty” white matter are prevalent in preterm born adults. Their visual evaluation has low diagnostic accuracy. Automatic volume quantification is more accurate but time consuming. It may be useful to ask for prematurity before initiating further diagnostics in subjects with these alterations

White matter macro- and microstructure alterations in preterm born adults

Introduction: White matter (WM) injury, both macroscopic (visible on conventional MRI) and microscopic (detectable by diffusion tensor imaging, DTI) is frequent among preterm born infants investigated in later life (Volpe, 2009). Degree and localization vary, but the corticospinal tract (CST) is often affected (Eikenes et al., 2011). The relation between the micro- and macroscopic CST alterations has only been studied to our knowledge up to adolescence (Bassi et al., 2011, Estep et al., 2014 and Groeschel et al., 2014) but not in adults. We aimed to assess group differences in micro- and macroscopic WM integrity of the CST between preterm and full-term born adults and to evaluate the impact of macroscopic lesions onto the DTI parameters in and along the CST. Methods: As part of a prospective study of preterm birth (Bavarian Longitudinal Study), we investigated adults at 26 years of age by means of MRI. Artifact-free DTI data were available from 51 full-term and 55 preterm born subjects (<32 weeks gestation or birth weight < 1500 g). In these subjects we probabilistically reconstructed the CST (Giorgio et al., 2010) and extracted DTI parameters (mean, axial, radial diffusivity - MD, AD, RD, and fractional anisotropy, FA) in the CST and slice-wise along the CST. On FLAIR-MRI images we identified macroscopic WM lesions as hyperintensities, noted their relation to the CST and correlated their presence with the DTI. Results and discussion: WM hyperintensities, periventricular or in the corona radiata, were present in 9 of 51 (18%) full-term and 26 of 55 (47%) preterm born subjects. In 3 of 51 (6%) full-term and 17 of 55 (30%) preterm born subjects these ”lesions” affected the CST. Compared to full-term individuals, the preterm born subjects had significantly higher diffusivity (MD, AD, RD) in and along the CST but no difference in FA. These changes were weaker, but nevertheless remained significant when excluding subjects with macroscopic lesions. This suggests chronic CST injury with increased tissue water content (Aung et al., 2013) even in the absence of visible WM lesions and points towards a high susceptibility of the CST after preterm birth

Neurobiological correlates of orientation-specific interocular transfer in humans

Visual information is processed hierarchically in the human visual system. Early during processing basic features are analysed separately while at later stages of processing, they are integrated into a unified percept. By investigating a basic visual feature and following its integration at different levels of processing one can identify specific patterns. In certain visual impairments, these patterns can function defectively and their detailed study can clarify the cause of the visual deficit. Here we investigate orientation as a basic feature and use a property of the visual system called adaptation. Adaptation occurs as a decrease in the level of neural activity during repetitive presentation of the same stimulus. Psychophysical studies have shown that adaptation transfers interocularly, meaning that if only one eye is adapted the other eye shows also adaptation effects. Our aim was to investigate interocular transfer by means of functional magnetic resonance imaging (fMRI). Even though adaptation was demonstrated in the fMRI environment, the interocular transfer was never investigated in such a setup. First, we developed a method to measure interocular transfer of adaptation to gratings with fMRI. We then went further to test it in various groups of subjects. In normally sighted humans interocular transfer was present both in early (striate) as well as later visual areas (extrastriate). In subjects with impaired stereovision (with or without normal visual acuity) interocular transfer was absent in the investigated regions. Detailed analysis of the recorded differences between subjects with and subjects without stereovision was performed. The results of this analysis are presented in detail in this book. These results suggest that the neuronal mechanisms involved in the interocular transfer of pattern adaptation share, at least in part, the neural circuitry underlying binocular functions and stereopsis. We conclude that fMRI adaptation can be used for the assessment of cortical binocularity in humans with normal and impaired stereopsis. Further investigations are needed to address more subtle aspects of the lack of interocular transfer. Towards this purpose, through a fourth experiment we propose further directions that might shed more light on the issue of stereovision and its clinical implications. We show that carefully tuned variations in our experimental procedure might reveal other aspects of binocularity in the human visual system. We believe that the method we developed, apart from the interesting results shown here, has a high potential to be further used for other research questions. Following the above summarized ideas, the thesis comprises of three parts (chapters). The first chapter provides the main theoretical backgrounds of the visual system and of the MRI imaging technique, chapter two describes the experimental procedures while the results and their detailed discussion are detailed in chapter three.Im visuellen System des Menschen werden Informationen hierarchisch verarbeitet. Zu Beginn der Verarbeitung werden die grundlegendsten Eigenschaften der visuellen Reize getrennt analysiert und in späteren Phasen werden diese Eigenschaften zu einer einheitlichen Wahrnehmung wieder zusammen geführt. Durch die Beobachtung der Integration einer grundlegenden visuellen Funktion auf verschiedenen Gehirnebenen ist es möglich, spezifische Muster in der visuellen Verarbeitung zu identifizieren. Bei bestimmten Sehstörungen, wie beispielsweise bei Störungen des binokularen Sehens, sind diese beeinträchtigt und eine detaillierte Untersuchung der Muster kann möglicherweise Aufschluss über die Ursachen der visuellen Defizite geben. In diesem Kontext, untersuchen wir die Verarbeitung von visuellen Reizen in Form von Gittermustern mit unterschiedlichen Orientierungen mittels funktioneller Magnetresonanztomographie (fMRT). Beobachtet werden primäre visuelle Areale (V1) sowie höhere, extrastriäre visuelle Areale im ventralen und dorsalen Verarbeitungspfad. Nach der Entwicklung einer geeigneten Methode, haben wir für die Studie Probanden mit normalen und Probanden mit gestörten binokularem Sehen untersucht und die Ergebnisse miteinander verglichen. Basierend auf dem oben genannten Grundgedanken, besteht die Dissertation aus drei Teilen (Kapitel). Der erste Teil beschreibt die wichtigsten theoretischen Grundlagen des visuellen Systems und der MRT-Bildgebung. Die experimentellen Verfahren sind in Kapitel zwei detailliert dargestellt, während die Ergebnisse der Experimente im dritten Kapitel zusammen mit einer ausführlichen Diskussion präsentiert werden. Im Folgenden werden diese drei Teile zusammenfassend dargesestellt

The association between gray matter volume and reading proficiency. A longitudinal study of beginning readers

Neural systems involved in the processing of written language have been identified by a number of functional imaging studies. Structural changes in cortical anatomy that occur in the course of literacy acquisition, however, remain largely unknown. Here, [the authors] follow elementary school children over their first 2 years of formal reading instruction and use tensor-based morphometry to relate reading proficiency to cortical volume at baseline and follow-up measurement as well as to intraindividual longitudinal volume development between the two measurement time points. A positive relationship was found between baseline gray matter volume in the left superior temporal gyrus and subsequent changes in reading proficiency. Furthermore, a negative relationship was found between reading proficiency at the second measurement time point and intraindividual cortical volume development in the inferior parietal lobule and the precentral and postcentral gyri of the left hemisphere. These results are interpreted as evidence that reading acquisition is associated with preexisting structural differences as well as with experience-dependent structural changes involving dendritic and synaptic pruning. (DIPF/Orig.

Central nervous system imaging in childhood Langerhans cell histiocytosis – a reference center analysis

Background. The aim of our study was (1) to describe central nervous system (CNS) manifestations in children with Langerhans cell histiocytosis (LCH) based on images sent to a reference center and meeting minimum requirements and (2) to assess the inter-rater agreement of CNS-MRI results, which represents the overall reproducibility of this investigation

Aphasia associated with lacunar infarctions

Hintergrund: Aphasien gehören nicht zu den typischen klinischen Manifestationen lakunärer Hirninfarkte, sind jedoch im Rahmen seltener atypischer lakunärer Syndrome beschrieben. Ziel der Arbeit: Beschreibung von Aphasiemustern und betroffener Fasertrakte bei lakunären Infarkten. Material und Methoden: Fallserie von drei Patienten mit in der Magnetresonanztomographie nachgewiesenen lakunären Hirninfarkten und Aphasie. Identifikation betroffener Faserbahnen mittels Fasertraktographie der koregistrierten Schädigungsorte in Gehirnen zweier gesunder Probanden. Ergebnisse: Radiologisch waren die Lakunen, die Aphasien hervorriefen, weit lateral im Marklager der linken Hemisphäre gelegen und befanden sich im Vergleich zu der Lakune eines nichtaphasischen Kontrollpatienten weiter rostrodorsal. Klinisch fand sich trotz Aussparung des Kortex, Thalamus und weiter Teile der Basalganglien eine leichte bis moderate nichtflüssige Aphasie mit syntaktischen Defiziten. In der Fasertraktographie zeigten die aphasischen im Vergleich zum nichtaphasischen Patienten eine stärkere Affektion der Fasern des linken Fasciculus arcuatus sowie eine Beteiligung des frontostriatalen und frontalen Aslant-Trakts. Diskussion: Links lateral gelegene lakunäre Infarkte können durch Beteiligung sprachrelevanter Fasertrakte eine klinisch relevante Aphasie hervorrufen.Background: Typical lacunar syndromes do not include aphasia but aphasia has been reported in rare atypical lacunar syndromes. Objective; Description of the phenomenology and of affected fiber tracts. Material and methods: Case series of three patients with lacunar stroke as evidenced by magnetic resonance imaging. Identification of affected fiber tracts via fiber tracking from coregistered lesion sites in brains of two healthy participants. Results: The lacunar strokes that produced aphasia were located in the very lateral territory of perforating branches of the middle cerebral artery and extended along the external capsule into its most rostrodorsal aspect. Even though the cortex, thalamus and most parts of the basal ganglia were unaffected, patients exhibited a mild to moderate nonfluent aphasia with syntactic deficits. Fiber tracking revealed that in contrast to the nonaphasic control patient with a neighboring lacunar stroke, the aphasic patient strokes involved particularly fibers of the left arcuate fascicle as well as fibers of the frontostriatal and frontal aslant tracts. Conclusion: Left lateral lacunar stroke can cause clinically relevant aphasia through disruption of speech-relevant fiber tracts