Biomedical subjects

Herbert Bauer

Publications and source records attributed to Herbert Bauer.

8 recordsLinked to original sources

PubMed2006

Neural correlates of monocular and binocular depth cues based on natural images: a LORETA analysis.

Functional imaging studies investigating perception of depth rely solely on one type of depth cue based on non-natural stimulus material. To overcome these limitations and to provide a more realistic and complete set of depth cues natural stereoscopic images were used in this study. Using slow cortical potentials and source localization we aimed to identify the neural correlates of monocular and binocular depth cues. This study confirms and extends functional imaging studies, showing that natural images provide a good, reliable, and more realistic alternative to artificial stimuli, and demonstrates the possibility to separate the processing of different depth cues.

Adolescent↗

PubMed2006

EEG reveals the effect of fMRI scanner noise on noise-sensitive subjects.

One drawback of fMRI is that subjects must endure intense noise during testing. This may be annoying to some people and acceptable to others. The aim of this study was to examine, by means of event-related potentials (ERPs), the possible influence of this noise on brain activity while performing a mental reasoning task. Subjects carrying out tasks in a silent environment were compared with two groups executing the same tasks in an "fMRI-like" noisy environment, one of which consisted of subjects who were annoyed by the noise and the other of subjects who tolerated it easily. Subjects who were annoyed performed less well (i.e., produced more errors compared to the "no noise" group) and "not annoyed" subjects showed a speed-accuracy trade-off (i.e., reacted faster but made more errors compared to "no noise" subjects). Noise led to more pronounced N1 and P2 peaks but attenuated N2. As early ERP components are influenced by attention, this observation most likely reflects different attentional requirements. The slow cortical negative shift during task processing was significantly attenuated with "annoyed" subjects compared to "not annoyed" subjects. Emotion-related subcortical structures may be responsible for the observed difference. These findings suggest that individual reactions to fMRI scanner noise should be taken into account when designing fMRI studies and interpreting results.

Adolescent↗

PubMed2005

Individual differences in brain activity during visuo-spatial processing assessed by slow cortical potentials and LORETA.

Using slow-cortical potentials (SCPs), Vitouch et al. demonstrated that subjects with low ability to solve a complex visuo-spatial imagery task show higher activity in occipital, parietal and frontal cortex during task processing than subjects with high ability. This finding has been interpreted in the sense of the so-called "neural efficiency" hypothesis, which assumes that the central nervous system of individuals with higher intellectual abilities is functioning in a more efficient way than the one of individuals with lower abilities. Using a higher spatial resolution of SCP recordings, and by employing the source localization method of LORETA (low-resolution electromagnetic tomography), we investigated this hypothesis by performing an extended replication of Vitouch et al.'s study. SCPs during processing of a visuo-spatial imagery task were recorded in pre-selected subjects with either high or low abilities in solving the imagery task. Topographic and LORETA analyses of SCPs revealed that a distributed network of extrastriate occipital, superior parietal, temporal, medial frontal and prefrontal areas was active during task solving. This network is well in line with former studies of the functional neuroanatomy of visuo-spatial imagery. Contrary to our expectations, however, the results of Vitouch et al. as well as of other studies supporting the neural efficiency hypothesis could not be confirmed since no difference in brain activity between groups was observed. This inconsistency between studies might be due to differing task processing strategies. While subjects with high abilities in the Vitouch et al. study seemed to use a visuo-perceptual task solving approach, all other subjects relied upon a visuo-motor task processing strategy.

Artifacts↗

PubMed2005

Using ICA for removal of ocular artifacts in EEG recorded from blind subjects.

One of the standard applications of Independent Component Analysis (ICA) to EEG is removal of artifacts due to movements of the eye bulbs. Short blinks as well as slower saccadic movements are removed by subtracting respective independent components (ICs). EEG recorded from blind subjects poses special problems, since it shows a higher quantity of eye movements, which are also more irregular and very different across subjects. It is demonstrated that ICA can still be of use by comparing results from four blind subjects with results from one subject without eye bulbs who therefore does not show eye movement artifacts at all.

Algorithms↗

PubMed2003

Fuzzy cluster analysis of high-field functional MRI data.

Functional magnetic resonance imaging (fMRI) based on blood-oxygen level dependent (BOLD) contrast today is an established brain research method and quickly gains acceptance for complementary clinical diagnosis. However, neither the basic mechanisms like coupling between neuronal activation and haemodynamic response are known exactly, nor can the various artifacts be predicted or controlled. Thus, modeling functional signal changes is non-trivial and exploratory data analysis (EDA) may be rather useful. In particular, identification and separation of artifacts as well as quantification of expected, i.e. stimulus correlated, and novel information on brain activity is important for both, new insights in neuroscience and future developments in functional MRI of the human brain. After an introduction on fuzzy clustering and very high-field fMRI we present several examples where fuzzy cluster analysis (FCA) of fMRI time series helps to identify and locally separate various artifacts. We also present and discuss applications and limitations of fuzzy cluster analysis in very high-field functional MRI: differentiate temporal patterns in MRI using (a) a test object with static and dynamic parts, (b) artifacts due to gross head motion artifacts. Using a synthetic fMRI data set we quantitatively examine the influences of relevant FCA parameters on clustering results in terms of receiver-operator characteristics (ROC) and compare them with a commonly used model-based correlation analysis (CA) approach. The application of FCA in analyzing in vivo fMRI data is shown for (a) a motor paradigm, (b) data from multi-echo imaging, and (c) a fMRI study using mental rotation of three-dimensional cubes. We found that differentiation of true "neural" from false "vascular" activation is possible based on echo time dependence and specific activation levels, as well as based on their signal time-course. Exploratory data analysis methods in general and fuzzy cluster analysis in particular may help to identify artifacts and add novel and unexpected information valuable for interpretation, classification and characterization of functional MRI data which can be used to design new data acquisition schemes, stimulus presentations, neuro(physio)logical paradigms, as well as to improve quantitative biophysical models.

Artifacts↗

PubMed2003

Human motor cortex activity during mental rotation.

The functional role of human premotor and primary motor cortex during mental rotation has been studied using functional MRI at 3 T. Fourteen young, male subjects performed a mental rotation task in which they had to decide whether two visually presented cubes could be identical. Exploratory Fuzzy Cluster Analysis was applied to identify brain regions with stimulus-related time courses. This revealed one dominant cluster which included the parietal cortex, premotor cortex, and dorsolateral prefrontal cortex that showed signal enhancement during the whole stimulus presentation period, reflecting cognitive processing. A second cluster, encompassing the contralateral primary motor cortex, showed activation exclusively after the button press response. This clear separation was possible in 3 subjects only, however. Based on these exploratory results, the hypothesis that primary motor cortex activity was related to button pressing only was tested using a parametric approach via a random-effects group analysis over all 14 subjects in SPM99. The results confirmed that the stimulus response via button pressing causes activation in the primary motor cortex and supplementary motor area while parietal cortex and mesial regions rostral to the supplementary motor area are recruited for the actual mental rotation process.

Adult↗

PubMed2003

Functional neuroanatomy of learned helplessness.

In the experiments reported here, female subjects were presented with reasoning tasks that changed from solvable to unsolvable, evoking "learned helplessness" or "loss of control" reactions in some subjects. Significant differences in slow cortical potential (SCP) changes were found between emotionally highly and lowly reactive subjects (grouped according to responses in postexperimental questionnaires) when processing unsolvable tasks. Cortical LORETA of SCP topographies and subsequent statistical nonparametric mapping (SnPM; analysis indicate clear reduction of anterior cingulate activity only with emotionally highly reactive subjects. In these subjects a region of the brain that is indispensable for goal-directed handling of tasks was switched off, whereas regions that are primarily engaged in processing the task stimuli were even more active during loss of control, although not at a statistically significant level. According to the anterior cingulate monitors the conflicts among brain regions and issues calls for further processing to the PFC that then guides behavior toward a goal. Learned helplessness might then be seen a state in which the function of the anterior cingulate is no longer maintained, perhaps due to the inhibitory influence of the amygdala possibly mediated via the brainstem dopaminergic ventral tegmental area.

Adult↗

PubMed2002

Consistency of inter-trial activation using single-trial fMRI: assessment of regional differences.

Recently, the technique of single-trial fMRI was introduced, which allows the assessment of hemodynamic responses to single task executions (e.g. sensory, motor, or cognitive). In this study, single-trial fMRI was used to examine regional differences in the inter-trial consistency (ITC) of brain activity related to the processing of a dynamic visuo-spatial imagery task. For every single trial, a t-statistic assessing task-related activation was calculated and thresholded at a p-value of P < or =l0.05 (uncorrected). The percentage of trials with t-values above the threshold was used to assess differences in the consistency of brain activity in occipital, parietal, premotor and prefrontal regions of interest. While most of these regions showed activation which was highly consistent across trials, the consistency of activation was significantly reduced in the dorsolateral prefrontal cortex. We hypothesize that the consistency of activation across trials may be interpreted as an indicator of the functional relevance of a brain region for processing and solving a cognitive task. Thus, the analysis concept presented here has the potential to provide new insights into the neuro-cognitive mechanisms of human information processing. In addition, the results of this study confirm that averaging across trials might result in a significant loss of information about functional neuroanatomy. Regions which are active in some trials only, which show only weak activity increases, or whose activity is not constant and time-locked, might not show up in averaged activation maps and could thus erroneously be interpreted as irrelevant for task processing.

Adult↗