Biomedical subjects

Christian Schwarzbauer

Publications and source records attributed to Christian Schwarzbauer.

7 recordsLinked to original sources

PubMed2006

Perirhinal cortex activity during visual object discrimination: an event-related fMRI study.

Previous fMRI studies have demonstrated preferential involvement of the perirhinal cortex and hippocampus in tasks of object and spatial memory, respectively. Here we investigated whether similar activity would also be present when object and spatial discrimination was assessed in the absence of explicit declarative memory demands. On each trial in the scanner, participants were presented simultaneously with two arrays of objects and were asked to indicate whether both arrays were identical, differed with respect to the identity of one object or differed with respect to the spatial arrangement of the objects. It was found that the detection of an object identity change was associated with significant right perirhinal cortex activity. We suggest that this perirhinal activity indicates a role of this structure in processes beyond declarative memory, for example, short-term visual working memory or higher order perception. Significantly greater hippocampal activity was not, however, observed during the spatial arrangement condition, perhaps due to the relatively low spatial processing demands of this task.

Adolescent↗

PubMed2005

Interleaved silent steady state (ISSS) imaging: a new sparse imaging method applied to auditory fMRI.

The acoustic scanner noise that is generated by rapid gradient switching in echo planar imaging (EPI) is an important confounding factor in auditory fMRI. "Sparse imaging" designs overcome the influence of scanner noise on stimulus presentation by acquiring single brain volumes following a silent stimulus presentation period. However, conventional sparse imaging requires assumptions about the time-to-peak of the evoked hemodynamic response and reduces the amount of EPI data which can be acquired and hence statistical power. In this article, we describe an "interleaved silent steady state" (ISSS) sampling scheme in which we rapidly acquire a set of EPI volumes following each silent stimulus presentation period. We avoid T1-related signal decay during the acquisition of the EPI volumes by maintaining the steady state longitudinal magnetization with a train of silent slice-selective excitation pulses during the silent period, ensuring that signal contrast is constant across successive scans. A validation study comparing ISSS to conventional sparse imaging demonstrates that ISSS imaging provides time course information that is absent in conventional sparse imaging data. The ISSS sequence has a temporal resolution like event-related (ER) imaging within a single trial (unlike conventional sparse imaging, where ER-like temporal resolution can only be achieved by compiling data across many jittered trials of the same stimulus type). This temporal resolution within trials makes ISSS particularly suitable for experiments in which a) scanner noise would interfere with the perception and processing of the stimulus; b) stimuli are several seconds in duration, and activation is expected to evolve and change as the stimulus unfolds; and c) it is impractical to present a single stimulus more than once (for example, repetition priming or familiarity effects would be expected).

Acoustic Stimulation↗

PubMed2005

Undirected graphs of frequency-dependent functional connectivity in whole brain networks.

We explored properties of whole brain networks based on multivariate spectral analysis of human functional magnetic resonance imaging (fMRI) time-series measured in 90 cortical and subcortical subregions in each of five healthy volunteers studied in the (no-task) resting state. We note that undirected graphs representing conditional independence between multivariate time-series can be more readily approached in the frequency domain than the time domain. Estimators of partial coherency and normalized partial mutual information phi, an integrated measure of partial coherence over an arbitrary frequency band, are applied. Using these tools, we replicate the prior observations that bilaterally homologous brain regions tend to be strongly connected and functional connectivity is generally greater at low frequencies [0.0004, 0.1518 Hz]. We also show that long-distance intrahemispheric connections between regions of prefrontal and parietal cortex were more salient at low frequencies than at frequencies greater than 0.3 Hz, whereas many local or short-distance connections, such as those comprising segregated dorsal and ventral paths in posterior cortex, were also represented in the graph of high-frequency connectivity. We conclude that the partial coherency spectrum between a pair of human brain regional fMRI time-series depends on the anatomical distance between regions: long-distance (greater than 7 cm) edges represent conditional dependence between bilaterally symmetric neocortical regions, and between regions of prefrontal and parietal association cortex in the same hemisphere, are predominantly subtended by low-frequency components.

Brain↗

PubMed2005

Positive or negative blips? The effect of phase encoding scheme on susceptibility-induced signal losses in EPI.

The observation of blood oxygenation level-dependent (BOLD) effect in functional magnetic resonance imaging (fMRI) studies is often hampered by the presence of magnetic field inhomogeneities. These are caused by abrupt changes in the magnetic susceptibility that typically occur near air/tissue interfaces and may result in substantial image distortions and signal losses. In this article, we investigate the effect of susceptibility-induced magnetic field inhomogeneities on the signal intensity (I) and the BOLD sensitivity (BS) for two different phase encoding schemes in blipped echo-planar imaging (EPI), which use either positive (EPIpos) or negative (EPIneg) phase gradient blips for stepping through k-space. Based on magnetic field maps, we generate computer simulations of I and BS for both phase encoding schemes and demonstrate good agreement with the experimental image intensities. We show that regions compromised by susceptibility effects are affected very differently by EPIpos and EPIneg. Further simulations are performed in two representative regions of interest (orbitofrontal cortex and lower temporal lobe) to investigate the dependence of I and BS on the slice angle (alpha), the magnitude of a moderate compensation gradient applied in the slice direction (GScomp), and the phase encoding scheme. We find that I and BS can be considerably increased if the appropriate phase encoding scheme is applied in addition to optimizing alpha and GScomp. Our results suggest that this optimization method would be useful in future fMRI studies to improve the sensitivity in regions compromised by susceptibility effects.

Artifacts↗

PubMed2005

An evaluation of the use of passive shimming to improve frontal sensitivity in fMRI.

The presence of the head in an MRI scanner leads to inhomogeneities in the magnetic field. These cause the 'susceptibility artifacts' of image distortion and signal dropout. In this paper, we evaluate a technique called passive shimming, which has the potential to reduce field inhomogeneities and the resultant artifacts. A piece of a magnetically active material (pyrolytic graphite) is held on the roof of the participant's mouth by a plastic mouth mould. We evaluate the effects in several different ways. We show that the presence of a shim reduces field inhomogeneity across much of the brain. From field maps, we generate simulations of EPI image intensity and BOLD sensitivity. Both of these are mainly improved by the presence of shim, although there were small reductions in some regions. Measured EPI image intensity also mostly increased. Finally, we ran a reward-punishment task in our subjects, and found that the presence of a shim increased functional sensitivity in the orbitofrontal cortex. Using the BOLD sensitivity measure, we provide estimates of the improvement to be expected in functional studies for a range of neural structures. Passive shims are quick to make and reasonably comfortable to wear, and have substantial potential for researchers investigating inferior frontal brain regions using MRI.

Artifacts↗

PubMed2005

A comparison of signal instability in 2D and 3D EPI resting-state fMRI.

Spatiotemporally structured noise, such as physiological noise, is a potential source of artifacts in functional magnetic resonance imaging (fMRI) and is the main limiting factor for the detection of small blood oxygen level-dependent (BOLD) signal variations. fMRI was employed to detect low-frequency BOLD signal fluctuations, which are thought to be related to spontaneous neuronal activity in the resting human brain. The sensitivity to noise, that is, signal variations of non-BOLD origin, was investigated for two- (2D) and three-dimensional (3D) imaging techniques. Incomplete relaxation between subsequent scans increases the level of temporally and spatially correlated signal variations originating from physiological and/or systemic noise. Although inflow effects are suspected to be reduced in 3D echo-planar imaging (EPI) compared with multi-slice 2D EPI, the noise level was higher in the 3D technique. The noise level in 3D fMRI experiments was significantly increased by instabilities of the transverse steady-state magnetization as the repetition time was of the order of T(2). By implementing radiofrequency spoiling, temporal signal fluctuations and erroneous inter-regional correlation in connectivity maps were diminished to a level present in data sets acquired with 2D EPI.

Adult↗

PubMed2004

The role of the lateral frontal cortex in causal associative learning: exploring preventative and super-learning.

Prediction error--a mismatch between expected and actual outcome--is critical to associative accounts of inferential learning. However, it has proven difficult to explore the effects of prediction error using functional magnetic resonance imaging (fMRI) while excluding the confounding effects of stimulus novelty and incorrect responses. In this event-related fMRI study we used a three-stage experiment generating preventative- and super-learning conditions. In both cases, it was possible to generate prediction error within a causal associative learning experiment while subtracting the effects of novelty and error. We show that right lateral prefrontal cortex (PFC) activation is sensitive to the magnitude of prediction error. Furthermore, super-learning activation in this region of PFC correlates, across subjects, with the amount learned. We thus provide direct evidence for a brain correlate of the surprise-dependent mechanisms proposed by associative accounts of causal learning. We show that activity in right lateral PFC is sensitive to the magnitude, though not the direction, of the prediction error. Furthermore, its activity is not directly explicable in terms of novelty or response errors and appears directly related to the learning that arises out of prediction error.

Adult↗