Biomedical subjects

Christoph Segebarth

Publications and source records attributed to Christoph Segebarth.

9 recordsLinked to original sources

PubMed2006

A low temperature embedding and section registration strategy for 3D image reconstruction of the rat brain from autoradiographic sections.

In studies on animal models of human brain pathologies, three-dimensional reconstruction from histological sections is particularly useful when assessing the morphologic, functional and biochemical changes induced by pathology. It allows assessing lesion heterogeneity in planes different from the cutting plane and allows correlating the histology with images obtained in vivo, such as by means of magnetic resonance imaging. To create a 3D volume from autoradiographic sections with minimal distortion, both cryosectioning as well as section registration need to be optimal. This paper describes a strategy whereby four external fiducial markers are positioned outside the rat brain with the use of a low temperature brain embedding procedure. The fiducial markers proposed here can be rapidly added to any frozen tissue block with no impact on the subsequent histological operations. Since embedding is performed at a low temperature, no tissue degradation occurs due to sample heating. The markers enable robust and almost error free registration, even in the presence of missing sections and poor image quality. Furthermore, the markers may be used to partially correct for global distortions.

Acrylamides↗

PubMed2006

When Broca experiences the Janus syndrome: an ER-fMRI study comparing sentence comprehension and cognitive sequence processing.

The determining of brain regions that exhibit specific activity during sentence comprehension compared to other non-linguistic cognitive tasks constitutes one of the important challenges in the domain of functional neuroimaging of the faculty of language. In the current paper we report an event-related functional magnetic resonance imaging (ER-fMRI) experiment, in which we directly compared the cerebral basis of sentence comprehension on the one hand, and of abstract sequence processing on the other hand. Previous experimental work done in our group, as well as different observations from recent behavioural, neurophysiological and functional neuroimaging experiments led us to propose the hypothesis that both of these tasks would share certain computational properties. Thus, this experiment was designed to show which brain regions would be implicated in both tasks and compare them to brain regions that would be specifically engaged in sentence comprehension. Results from this experiment suggest that distinct sub-regions in the left prefrontal cortex, potentially including Broca's area show distinct activation patterns during both of these tasks. Results are discussed in the context of a construction-based model of sentence processing (see Dominey and Hoen, 2006, this issue) that is based on a dual-path processing mechanism separating function and content information processing. We propose and discuss the hypothesis that subparts of Broca's area BA 44 and BA 45 would respectively be implicated in two different aspects of sentence comprehension: i) a general structure mapping capability and ii) the online integration of semantic representations onto structural constraints.

Adult↗

PubMed2005

Focal brain ischemia in rat: acute changes in brain tissue T1 reflect acute increase in brain tissue water content.

Several recent studies have reported changes of brain tissue T(1) in ischemic models during the first minutes after occlusion of the middle cerebral artery (MCA). In order to assess whether these tissue T(1) changes are related to an increase in tissue water content, we performed T(1) (7 T) and tissue water content measurements in a rat model (n = 10, Sprague-Dawley) of focal cerebral ischemia (intraluminal occlusion model). The tissue water content was determined using a gravimetric technique. The animals were divided into two groups: an ischemic group, with an effective MCA occlusion (n = 6) and a control group, with animals having undergone sham surgery but no MCA occlusion (n = 4). In the ipsilateral cortex, the tissue water content was 81.1 +/- 0.7% at 2 h 15 min following ischemic insult (contralateral value: 79.3 +/- 0.5%). Concomitantly, the tissue T(1) in the ipsilateral cortex was 2062 +/- 60 ms at ischemia onset + 1 h (contralateral 1811 +/- 28 ms) and 2100 +/- 38 ms at ischemia onset + 2 h (contralateral 1807 +/- 18 ms). The tissue T(1) and tissue water content values measured in the contralateral area do not differ from the values obtained in the control group. A significant T(1) increase is observed at ischemia onset + 1 h (+ 14%) and ischemia onset (+ 2 h) + 16%, together with a significant increase in tissue water content (+ 2.3%). This suggests that there is an increase in tissue water content concomitant with cell swelling during the first hours of ischemia.

Animals↗

PubMed2004

Involvement of right piriform cortex in olfactory familiarity judgments.

Previous studies have shown activation of right orbitofrontal cortex during judgments of odor familiarity. In the present study, we sought to extend our knowledge about the neural circuits involved in such a task by exploring the involvement of the right prefrontal areas and limbic/primary olfactory structures. Fourteen right-handed male subjects were tested using fMRI with a single functional run of two olfactory conditions (odor detection and familiarity judgments). Each condition included three epochs. During the familiarity condition, subjects rated whether odors were familiar or unfamiliar. During the detection condition, participants decided if odors were present. When contrasting the familiarity with the detection conditions, activated areas were found mainly in the right piriform cortex (PC) and hippocampus, the left inferior frontal gyrus and amygdala, and bilaterally in the mid-fusiform gyrus. Further analyses demonstrated that the right PC was more strongly activated than the left PC. This result supports the notion that the right PC is preferentially involved in judgments of odor familiarity.

Adolescent↗

PubMed2004

Cerebral regions and hemispheric specialization for processing spatial frequencies during natural scene recognition. An event-related fMRI study.

It has been suggested that visual scene recognition is mainly based on spatial frequency (Fourier) analysis of the image. This analysis starts with processing low spatial frequencies (LSF), followed by processing high spatial frequencies (HSF). Within the framework of the spatial frequency analysis, the right/left hemisphere would be predominantly involved in LSF/HSF analysis, respectively. The aim of this event-related fMRI study was to evaluate neural correlates and hemispheric specialization of spatial frequency analysis during recognition of nonfiltered (NF) and filtered, either in LSF or HSF, natural scenes. Comparing LSF or NF to HSF scene recognition, significant activation was obtained within right anterior temporal cortex and right parahippocampal gyrus. As these regions are known to be involved in scene processing, we interpret this result as suggesting that scene recognition is mainly based on LSF extraction and analysis. When LSF scene was compared to HSF scene recognition, supplementary activation was obtained within the right inferior parietal lobule that likely reflects attentional modulation on spatial frequency processing. A direct interhemispheric comparison for each particular band of spatial frequencies highlighted predominance within the early visual areas (such as the middle occipital gyrus) to the right for LSF processing and to the left for HSF processing. This result provides supplementary evidence for hemispheric specialization at early levels of visual analysis when spatial frequencies are processed.

Adult↗

PubMed2004

Multistable representation of speech forms: a functional MRI study of verbal transformations.

We used functional magnetic resonance imaging (fMRI) to localize the brain areas involved in the imagery analogue of the verbal transformation effect, that is, the perceptual changes that occur when a speech form is cycled in rapid and continuous mental repetition. Two conditions were contrasted: a baseline condition involving the simple mental repetition of speech sequences, and a verbal transformation condition involving the mental repetition of the same items with an active search for verbal transformation. Our results reveal a predominantly left-lateralized network of cerebral regions activated by the verbal transformation task, similar to the neural network involved in verbal working memory: the left inferior frontal gyrus, the left supramarginal gyrus, the left superior temporal gyrus, the anterior part of the right cingulate cortex, and the cerebellar cortex, bilaterally. Our results strongly suggest that the imagery analogue of the verbal transformation effect, which requires percept analysis, form interpretation, and attentional maintenance of verbal material, relies on a working memory module sharing common components of speech perception and speech production systems.

Adult↗

PubMed2003

fMRI of emotional responses to odors: influence of hedonic valence and judgment, handedness, and gender.

Previous positron emission tomography studies of right-handed individuals show that the left orbitofrontal cortex is dominant during emotional processing of odors. We collected functional magnetic resonance imaging data from 28 subjects to study this network as a function of odor hedonic valence (pleasant vs. unpleasant), active hedonic judgments versus passive sensation of hedonically charged odors, handedness, and gender. Two functional runs were performed, with pleasant and unpleasant odors presented in different epochs. In the first run, subjects passively smelled odorants, whereas in the second run they rated degree of odor pleasantness or unpleasantness by using a "finger-span" technique that simulated a visual rating scale. Electrodermal and plethysmography responses were simultaneously recorded to control for covert, physiological manifestations of the emotional response. The piriform-amygdala area and ventral insula were activated more for unpleasant than pleasant odors. More extreme ratings were also associated with higher electrodermal amplitude, suggesting that activation stemmed more from emotional or hedonic intensity than valence, and that unpleasant odors induced more arousal than pleasant odors. Unpleasant odors activated the left ventral insula in right-handers and the right ventral insula in left-handers, suggesting lateralized processing of emotional odors as a function of handedness. Active decisions about odor pleasantness induced specific left orbitofrontal cortex activation, implicating the role of this area in the conscious assessment of the emotional quality of odors. Finally, left orbitofrontal cortex was more active in women than men, potentially in relation to women's well-documented advantage in odor identification.

Adult↗

PubMed2003

Retrospective intra-scan motion correction.

This paper analyzes the effects of intra-scan motion and demonstrates the possibility of correcting them directly in k-space with a new automatic retrospective method. The method is presented for series of 2D acquisitions with Cartesian sampling. Using a reference k-space acquisition (corrected for translations) within the series, intra-scan motion parameters are accurately estimated for each trajectory in k-space of each data set in the series resulting in pseudo-random sample positions. The images are reconstructed with a Bayesian estimator that can handle sparse arbitrary sampling in k-space and reduces intra-scan rotation artefacts to the noise level. The method has been assessed by means of a Monte Carlo study on axial brain images for different signal-to-noise ratios. The accuracy of motion estimates is better than 0.1 degrees for rotation, and 0.1 and 0.05 pixel, respectively, for translation along the read and phase directions for signal-to-noise ratios higher than 6 of the signals on each trajectory. An example of reconstruction from experimental data corrupted by head motion is also given.

Algorithms↗

PubMedDate not supplied

Global integration of local color differences in transparency perception: An fMRI study.

In normal viewing, the visual system effortlessly assigns approximately constant attributes of color and shape to perceived objects. A fundamental component of this process is the compensation for illuminant variations and intervening media to recover reflectance properties of natural surfaces. We exploited the phenomenon of transparency perception to explore the cortical regions implicated in such processes, using fMRI. By manipulating the coherence of local color differences around a region in an image, we interfered with their global perceptual integration and thereby modified whether the region appeared transparent or not. We found the major cortical activation due to global integration of local color differences to be in the anterior part of the parahippocampal gyrus. Regions differentially activated by chromatic versus achromatic geometric patterns showed no significant differential response related to the coherence/incoherence of local color differences. The results link the integration of local color differences in the extraction of a transparent layer with sites activated by object-related properties of an image.

Adult↗