Biomedical subjects

Gabriele Lohmann

Publications and source records attributed to Gabriele Lohmann.

16 recordsLinked to original sources

PubMed2005

Magnetic resonance imaging of the human frontal cortex reveals differential anterior-posterior variability of sulcal basins.

MRI data of 100 healthy human brains were analyzed to establish a neuroanatomical map of the most frequently occurring 'sulcal basins' of the human frontal cortex. Sulcal basins are defined to be concavities in the white matter surface constituting/representing components of entire sulci. We determined their volume, depth, and interindividual variability. The sulcal basins were found to fall into two groups, on average, eight anterior basins in the prefrontal and premotor region and four posterior ones in the motor region of the frontal lobe. Compared to posterior basins, anterior basins are characterized by lower volume and depth. Furthermore, they showed greater interindividual variability in volume, depth, and occurrence. Our results indicate the existence of a mechanism for cortical folding which shows a greater flexibility in the phylogenetically younger, anterior prefrontal areas.

Adult↗

PubMed2005

Morphometry demonstrates loss of cortical thickness in cerebral microangiopathy.

OBJECTIVE: To evaluate the role of MR morphometry in the characterization of cerebral microangiopathy (CMA) in relation to clinical and neuropsychological impairment. SUBJECTS AND METHODS: 3D MR images of 27 patients and 27 age-matched controls were morphometrically analysed for regional thickness. The normalized values were related to the patients' clinical and neuropsychological scores. The patients were categorised according to the amount of structural MR signal changes. A ventricle index reflecting internal atrophy was related to MR morphology and cortical thickness as an indicator for external atrophy. RESULTS: Cortical thickness was significantly reduced in the patients group (3.03 mm +/- 0.26 vs. 3.22 mm +/-0.13 in controls, p=0.001). The severest loss of cortical thickness occurred in severe CMA. Internal and external atrophy evolved in parallel and both showed a significant relationship with structural MR-abnormalities (p<0.05; r=-0.7; r=0.67; r=-0.74, respectively). Neuropsychological performance correlated strongly with the loss of cortical thickness. CONCLUSIONS: Cortical thickness was identified as the most sensitive parameter to characterize CMA. A strong correlation was found of morphometric parameters to the severity of CMA based on a score derived from T2-weighted MRI. The degree of cortical atrophy was directly related to the degree of neuropsychological impairment. Our findings suggest that the cortical thickness is a valid marker in the structural and clinical characterization of CMA.

Analysis of Variance↗

PubMed2005

Meta-analysis of functional imaging data using replicator dynamics.

Despite the rapidly growing number of meta-analyses in functional neuroimaging, the field lacks formal mathematical tools for the quantitative and qualitative evaluation of meta-analytic data. We propose to use replicator dynamics in the meta-analysis of functional imaging data to address an important aspect of neuroimaging research, the search for functional networks of cortical areas that underlie a specific cognitive task. The replicator process requires as input only a list of activation locations, and it results in a network of locations that jointly show significant activation in most studies included in the meta-analysis. These locations are likely to play a critical role in solving the investigated cognitive task. Our method was applied to a meta-analysis of the Stroop interference task using data provided by the publicly accessible database BrainMap DBJ.

Brain↗

PubMed2005

The correlation between blood oxygenation level-dependent signal strength and latency.

PURPOSE: To investigate the relationship between signal strength and latency of the blood oxygenation level-dependent (BOLD) signal. MATERIALS AND METHODS: Several correlation analyses were performed on data obtained in a functional magnetic resonance imaging (fMRI) experiment, where subjects were presented with a simple visual stimulus. The BOLD signal strength was correlated with both the phase shift of the spectral density matrix and time-to-peak calculated from trial-averaged time courses. Correlation coefficients were calculated for visual stimuli of 2, 6, and 15 seconds in duration. RESULTS: Analyzing all functional runs for the same subject separately, i.e., including for each run all significantly activated voxels, we observed that correlations between phase shift and signal strength, as well as between time-to-peak and signal strength, decreased with increasing stimulus length. However, when analyses were restricted to voxels found activated in all functional runs, we observed similar correlations between BOLD signal strength and latency in all runs, independent of the length of stimulation. This result was again obtained for both latency measures: the spectral density phase shift and time-to-peak. CONCLUSION: For both latency measures, phase shift and time-to-peak, a high correlation between BOLD signal strength and latency was observed. We have shown that this correlation is independent of the length of visual stimulation. Thus, the correlation between BOLD signal strength and latency seems to be an inherent property of the BOLD response that is independent of the length of stimulation and can be observed using different methods for determining signal latency.

Adult↗

PubMed2004

Investigating the wavelet coherence phase of the BOLD signal.

PURPOSE: To investigate the temporal behavior of the blood oxygenation-level dependent (BOLD) signal applying the wavelet coherence phase to functional magnetic resonance imaging (fMRI) data. MATERIALS AND METHODS: The wavelet coherence phase was computed for a group of four subjects using three functional runs of different visual stimulation lengths. In order to consider the variability of the wavelet coherence phase, a correlation analysis was performed between the variance of the phase shift and the associated activation strength. In activated brain regions, correlation was performed between the mean wavelet coherence phase and the spectral density phase shift. RESULTS: The scalograms of the wavelet coherence phase show the temporal variability of the phase shift between fMRI time series. For brain regions with weakly stationary behavior, the mean wavelet coherence phase supports the results obtained by spectral analyses. CONCLUSION: The wavelet coherence phase provides a description of the temporal behavior of the BOLD signal even for the nonstationary case. In particular, temporal changes of the phase shift can be investigated. This makes the wavelet coherence phase more suitable for the investigation of BOLD dynamics than an average phase lag obtained by correlation or spectral methods.

Adult↗

PubMed2004

Towards a standard analysis for functional near-infrared imaging.

Functional near-infrared spectroscopy (fNIRS) allows the ability to monitor brain activation by measuring changes in the concentration of oxy- and deoxy-hemoglobin. Until now no standardized approach for fNIRS data analysis has been established, although this has to be regarded as a precondition for future application. Hence, we applied the well-established general linear model to optical imaging data. Further, fNIRS data were analyzed in the frequency domain. Two visual tasks were investigated with optical imaging: a checkerboard paradigm supposed to activate the primary and secondary visual cortex, and a paradigm consisting of moving colored stimuli (rotating 'L's) additionally involving the motion area V5. Analysis with the general linear model detected the activation focus in the primary and secondary visual cortex during the first paradigm. For the second paradigm, a second laterally localized activated brain region was found, most likely representing V5. Spatially resolved spectral analysis confirmed the results by showing maxima of power spectral density and coherence in the same respective brain regions. Moreover, it demonstrated a delay of the hemodynamic response in the motion area. In summary, the present study suggests that the general linear model and spatially resolved spectral analysis can be used as standard statistical approaches for optical imaging data, particularly because they are almost independent of the assumed differential path length factors.

Adult↗

PubMed2003

Wavelet statistics of functional MRI data and the general linear model.

PURPOSE: To improve the signal-to-noise ratio (SNR) of functional magnetic resonance imaging (fMRI) data, an approach is developed that combines wavelet-based methods with the general linear model. MATERIALS AND METHODS: Ruttimann et al. (1) developed a wavelet-based statistical procedure to test wavelet-space partitions for significant wavelet coefficients. Their method is applicable for the detection of differences between images acquired under two experimental conditions using long blocks of stimulation. However, many neuropsychological questions require more complicated event-related paradigms and more experimental conditions. Therefore, in order to apply wavelet-based methods to a wide range of experiments, we present a new approach that is based on the general linear model and wavelet thresholding. RESULTS: In contrast to a monoresolution filter, the application of the wavelet method increased the SNR and showed a set of clearly dissociable activations. Furthermore, no relevant decrease of the local maxima was observed. CONCLUSION: Wavelet-based methods can increase the SNR without diminishing the signal amplitude, while preserving the spatial resolution of the image. The anatomical localization is strongly improved.

Adult↗

PubMed2003

Investigating the stimulus-dependent temporal dynamics of the BOLD signal using spectral methods.

PURPOSE: To compare several spectral parameters using different durations of visual hemifield stimulation in order to explore the different temporal behavior of the blood oxygenation-level dependent (BOLD) signal in various brain regions. MATERIALS AND METHODS: Spectral methods were applied to three different groups of subjects with visual stimulation lasting 6, 12, and 30 seconds. Furthermore, diffusion weighting was applied in an interleaved way. The core of the data processing was the computation of the spectral density matrix using the multidimensional weighted covariance estimate. Spectral parameters of coherence and phase shift were computed. RESULTS: The correlation between signal changes and phase shifts was dependent on the duration of the visual stimulation. The shorter the duration of visual stimulation, the stronger the correlation between percentage signal change and phase shift. CONCLUSION: The experiments with short and long stimuli differed mainly in the distribution of the activated voxels in the plane of percentage signal change and phase shift. It was revealed that the height of the signal change depends on the phase shift, whereas the diffusion weighting has no influence.

Adult↗

PubMed2003

Event-related analysis for event types of fixed order and restricted spacing by temporal quantification of trial-averaged fMRI time courses.

PURPOSE: To develop a method for event-related fMRI that allows rapidly presented event sequences to be analyzed, without requiring transitions of different event-types to be counterbalanced. MATERIALS AND METHODS: A cued task switching procedure was investigated with an experimental trial comprising a visual task cue that indicated how to process a subsequent visual target stimulus. Cue and target were either presented quasi-simultaneously, separated by a 100 msec cue-target-interval (CTI100), or the target presentation was delayed by 2000 msec (CTI2000). To characterize the trial-related BOLD-response in terms of its temporal relation to the underlying event structure, the pattern of onset latency differences and peak latency differences for CTI2000 minus CTI100 was evaluated. Independent estimates of onset latencies and peak latencies were determined for preprocessed trial-averaged time courses by jackknife resampling. RESULTS: Validating results were obtained for two brain areas with known characteristics: the visual cortex (cue-locked plus target-locked activation) and the motor cortex (response-locked activation). Extending the analysis to prefrontal areas with a priori unknown characteristics differentiated between several meaningful temporal activation patterns. CONCLUSION: The method yielded a fine-grained temporal description of trial-related BOLD-responses that could be successfully used for the event-related analysis of an experimental design that was highly restricted with respect to event order and event spacing.

Adult↗

PubMed2003

Morphology-based cortical thickness estimation.

We describe a new approach to estimating the cortical thickness of human brains using magnetic resonance imaging data. Our algorithm is part of a processing chain consisting of a brain segmentation (skull stripping), as well as white and grey matter segmentation procedures. In this paper, only the grey matter segmentation together with the cortical thickness estimation is described. In contrast to many existing methods, our estimation method is voxel-based and does not use any surface meshes. While this fact poses a principal limit on the accuracy that can be achieved by our method, it offers tremendous advantages with respect to practical applicability. In particular, it is applicable to data sets showing severe cortical atrophies that involve areas of high curvature and extremely thin gyral stalks. In contrast to many other methods, it is entirely automatic and very fast with computation times of a few minutes. Our method has been used in two clinical studies involving a total of 27 patients and 23 healthy subjects.

Algorithms↗

PubMed2003

Within-subject variability of BOLD response dynamics.

In this paper we investigate the within-subject variability of dynamical aspects of the BOLD response obtained in a series of fMRI sessions several days apart. Five different parameters describing the temporal behavior of trial-averaged time courses, such as time-to-peak and time-to-onset, were estimated and analyzed with respect to their variability across nine sessions. Results show that small variances of the estimated parameters can be found, provided that the analysis is restricted to voxels activated in all individual sessions. Among the investigated parameters, time-to-peak shows the most stable behavior. These results were obtained using two different analysis methods, the estimation of the parameters directly from trial-averaged time courses and fitting trial-averaged time courses to an assumed hemodynamic response function. Both methods yield comparable results.

Adult↗

PubMed2003

Auditory what, where, and when: a sensory somatotopy in lateral premotor cortex.

Recent findings have demonstrated that attention to visual events engages the lateral premotor cortex even in the absence of motor planning. Here, we used functional magnetic resonance imaging to explore acoustically triggered activations within the lateral premotor cortex. Temporal (when), object-related (what), and spatial (where) auditory patterns were to be monitored for violations in a serial prediction task. As a result, we found a modality-dependent modulation for auditory events within the inferior ventrolateral premotor cortex, an area engaged in vocal plans. In addition, however, auditory activations were distributed within the entire premotor cortex depending on which stimulus property was attended to. Attention to where patterns was found to engage fields for gaze and reaching (dorsolateral premotor cortex), what patterns to engage fields for hand movements (superior ventrolateral premotor cortex), and when patterns to engage fields for vocal plans. Together, the findings confirm the idea of a sensory somatotopy in lateral premotor cortex, according to which a perceptual pattern triggers representations within that motor effector which would be most appropriate to generate it as an action effect.

Acoustic Stimulation↗

PubMed2003

Bayesian second-level analysis of functional magnetic resonance images.

We propose a new method for the second-level analysis of functional MRI data based on Bayesian statistics. Our method does not require a computationally costly Bayesian model on the first level of analysis. Rather, modeling for single subjects is realized by means of the commonly applied General Linear Model. On the basis of the resulting parameter estimates for single subjects we calculate posterior probability maps and maps of the effect size for effects of interest in groups of subjects. A comparison of this method with the conventional analysis based on t statistics shows that the new approach is more robust against outliers. Moreover, our method overcomes some of the severe problems of null hypothesis significance tests such as the need to correct for multiple comparisons and facilitates inferences which are hard to formulate in terms of classical inferences.

Algorithms↗

PubMed2002

FMRI reveals brain regions mediating slow prosodic modulations in spoken sentences.

By means of fMRI measurements, the present study identifies brain regions in left and right peri-sylvian areas that subserve grammatical or prosodic processing. Normal volunteers heard 1) normal sentences; 2) so-called syntactic sentences comprising syntactic, but no lexical-semantic information; and 3) manipulated speech signals comprising only prosodic information, i.e., speech melody. For all conditions, significant blood oxygenation signals were recorded from the supratemporal plane bilaterally. Left hemisphere areas that surround Heschl gyrus responded more strongly during the two sentence conditions than to speech melody. This finding suggests that the anterior and posterior portions of the superior temporal region (STR) support lexical-semantic and syntactic aspects of sentence processing. In contrast, the right superior temporal region, in especially the planum temporale, responded more strongly to speech melody. Significant brain activation in the fronto-opercular cortices was observed when participants heard pseudo sentences and was strongest during the speech melody condition. In contrast, the fronto-opercular area is not prominently involved in listening to normal sentences. Thus, the functional activation in fronto-opercular regions increases as the grammatical information available in the sentence decreases. Generally, brain responses to speech melody were stronger in right than left hemisphere sites, suggesting a particular role of right cortical areas in the processing of slow prosodic modulations.

Adult↗

PubMed2002

Using replicator dynamics for analyzing fMRI data of the human brain.

The understanding of brain networks becomes increasingly the focus of current research. In the context of functional magnetic resonance imagery (fMRI) data of the human brain, networks have been mostly detected using standard clustering approaches. In this work, we present a new method of detecting functional networks using fMRI data. The novelty of this method is that these networks have the property that every network member is closely connected with every other member. This definition might to be better suited to model important aspects of brain activity than standard cluster definitions. The algorithm that we present here is based on a concept from theoretical biology called "replicator dynamics."

Algorithms↗

PubMed2002

Bach speaks: a cortical "language-network" serves the processing of music.

The aim of the present study was the investigation of neural correlates of music processing with fMRI. Chord sequences were presented to the participants, infrequently containing unexpected musical events. These events activated the areas of Broca and Wernicke, the superior temporal sulcus, Heschl's gyrus, both planum polare and planum temporale, as well as the anterior superior insular cortices. Some of these brain structures have previously been shown to be involved in music processing, but the cortical network comprising all these structures has up to now been thought to be domain-specific for language processing. To what extent this network might also be activated by the processing of non-linguistic information has remained unknown. The present fMRI-data reveal that the human brain employs this neuronal network also for the processing of musical information, suggesting that the cortical network known to support language processing is less domain-specific than previously believed.

Acoustic Stimulation↗