Biomedical subjects

Rainer Goebel

Publications and source records attributed to Rainer Goebel.

51 records · Page 3Linked to original sources

PubMed2003

In vivo mapping of functional domains and axonal connectivity in cat visual cortex using magnetic resonance imaging.

Noninvasive cognitive neuroimaging studies based on functional magnetic resonance imaging (fMRI) are of ever-increasing importance for basic and clinical neurosciences. The explanatory power of fMRI could be greatly expanded, however, if the pattern of the neuronal circuitry underlying functional activation could be made visible in an equally noninvasive manner. In this study, blood oxygenation level-dependent (BOLD)-based fMRI and diffusion tensor imaging (DTI) were performed in the same cat visual cortex, and the foci of fMRI activation utilized as seeding points for 3D DTI fiber reconstruction algorithms, thus providing the map of the axonal circuitry underlying visual information processing. The methods developed in this study will lay the foundation for in vivo neuroanatomy and the ability for noninvasive longitudinal studies of brain development.

Algorithms↗

PubMed2003

Investigating directed cortical interactions in time-resolved fMRI data using vector autoregressive modeling and Granger causality mapping.

We present a framework aimed to reveal directed interactions of activated brain areas using time-resolved fMRI and vector autoregressive (VAR) modeling in the context of Granger causality. After describing the underlying mathematical concepts, we present simulations helping to characterize the conditions under which VAR modeling and Granger causality can reveal directed interactions from fluctuations in BOLD-like signal time courses. We apply the proposed approach to a dynamic sensorimotor mapping paradigm. In an event-related fMRI experiment, subjects performed a visuomotor mapping task for which the mapping of two stimuli ("faces" vs "houses") to two responses ("left" or "right") alternated periodically between the two possible mappings. Besides expected activity in sensory and motor areas, a fronto-parietal network was found to be active during presentation of a cue indicating a change in the stimulus-response (S-R) mapping. The observed network includes the superior parietal lobule and premotor areas. These areas might be involved in setting up and maintaining stimulus-response associations. The Granger causality analysis revealed a directed influence exerted by the left lateral prefrontal cortex and premotor areas on the left posterior parietal cortex.

Brain Mapping↗

PubMed2003

Cortical capacity constraints for visual working memory: dissociation of fMRI load effects in a fronto-parietal network.

Working memory (WM) capacity limitations and their neurophysiological correlates are of special relevance for the understanding of higher cognitive functions. Evidence from behavioral studies suggests that restricted attentional resources contribute to these capacity limitations. In an event-related functional magnetic resonance imaging (fMRI) study, we probed the capacity of the human visual WM system for up to four complex nonnatural objects using a delayed discrimination task. A number of prefrontal and parietal areas bilaterally showed increased blood oxygen level-dependent activity, relative to baseline, throughout the task when more than one object had to be held in memory. Monotonic increases in response to memory load were observed bilaterally in the dorsolateral prefrontal cortex (DLPFC) and the presupplementary motor area (pre-SMA). Conversely, activity in the frontal eye fields (FEFs) and in areas along the intraparietal sulcus (IPS) peaked when subjects had to maintain only two or three objects and decreased in the highest load condition. This dissociation of memory load effects on cortical activity suggests that the cognitive operations subserved by the IPS and FEF, which are most likely related to attention, fail to support visual WM when the capacity limit is approached. The correlation of brain activity with performance implies that only the operations performed by the DLPFC and pre-SMA, which support an integrated representation of visual information, helped subjects to maintain a reasonable level of performance in the highest load condition. These results indicate that at least two distinct cortical subsystems are recruited for visual WM, and that their interplay changes when the capacity limit is reached.

Adult↗

PubMed2003

Real-time independent component analysis of fMRI time-series.

Real-time functional magnetic resonance imaging (fMRI) enables one to monitor a subject's brain activity during an ongoing session. The availability of online information about brain activity is essential for developing and refining interactive fMRI paradigms in research and clinical trials and for neurofeedback applications. Data analysis for real-time fMRI has traditionally been based on hypothesis-driven processing methods. Off-line data analysis, conversely, may be usefully complemented by data-driven approaches, such as independent component analysis (ICA), which can identify brain activity without a priori temporal assumptions on brain activity. However, ICA is commonly considered a time-consuming procedure and thus unsuitable to process the high flux of fMRI data while they are acquired. Here, by specific choices regarding the implementation, we exported the ICA framework and implemented it into real-time fMRI data analysis. We show that, reducing the ICA input to a few points within a time-series in a sliding-window approach, computational times become compatible with real-time settings. Our technique produced accurate dynamic readouts of brain activity as well as a precise spatiotemporal history of quasistationary patterns in the form of cumulative activation maps and time courses. Results from real and simulated motor activation data show comparable performances for the proposed ICA implementation and standard linear regression analysis applied either in a sliding-window or in a cumulative mode. Furthermore, we demonstrate the possibility of monitoring transient or unexpected neural activities and suggest that real-time ICA may provide the fMRI researcher with a better understanding and control of subjects' behaviors and performances.

Algorithms↗

PubMed2003

Tracking cognitive processes with functional MRI mental chronometry.

Functional magnetic resonance imaging (fMRI) is used widely to determine the spatial layout of brain activation associated with specific cognitive tasks at a spatial scale of millimeters. Recent methodological improvements have made it possible to determine the latency and temporal structure of the activation at a temporal scale of few hundreds of milliseconds. Despite the sluggishness of the hemodynamic response, fMRI can detect a cascade of neural activations - the signature of a sequence of cognitive processes. Decomposing the processing into stages is greatly aided by measuring intermediate responses. By combining event-related fMRI and behavioral measurement in experiment and analysis, trial-by-trial temporal links can be established between cognition and its neural substrate.

Brain↗

PubMed2003

Physiological self-regulation of regional brain activity using real-time functional magnetic resonance imaging (fMRI): methodology and exemplary data.

A brain-computer interface (BCI) based on real-time functional magnetic resonance imaging (fMRI) is presented which allows human subjects to observe and control changes of their own blood oxygen level-dependent (BOLD) response. This BCI performs data preprocessing (including linear trend removal, 3D motion correction) and statistical analysis on-line. Local BOLD signals are continuously fed back to the subject in the magnetic resonance scanner with a delay of less than 2 s from image acquisition. The mean signal of a region of interest is plotted as a time-series superimposed on color-coded stripes which indicate the task, i.e., to increase or decrease the BOLD signal. We exemplify the presented BCI with one volunteer intending to control the signal of the rostral-ventral and dorsal part of the anterior cingulate cortex (ACC). The subject achieved significant changes of local BOLD responses as revealed by region of interest analysis and statistical parametric maps. The percent signal change increased across fMRI-feedback sessions suggesting a learning effect with training. This methodology of fMRI-feedback can assess voluntary control of circumscribed brain areas. As a further extension, behavioral effects of local self-regulation become accessible as a new field of research.

Adult↗

PubMed2002

The role of feedback in shaping neural representations in cat visual cortex.

In the primary visual cortex, neurons with similar response preferences are grouped into domains forming continuous maps of stimulus orientation and direction of movement. These properties are widely believed to result from the combination of ascending and lateral interactions in the visual system. We have tested this view by examining the influence of deactivating feedback signals descending from the visuoparietal cortex on the emergence of these response properties and representations in cat area 18. We thermally deactivated the dominant motion-processing region of the visuoparietal cortex and used optical and electrophysiological methods to assay neural activity evoked in area 18 by stimulation with moving gratings and fields of coherently moving randomly distributed dots. Feedback deactivation decreased signal strength in both orientation and direction maps and virtually abolished the global layout of direction maps, whereas the basic structure of the orientation maps was preserved. These findings could be accounted for by a selective silencing of highly direction-selective neurons and by the redirection of preferences of less selective neurons. Our data suggest that signals fed back from the visuoparietal cortex strongly contribute to the emergence of direction selectivity in early visual areas. Thus we propose that higher cortical areas have significant influence over fundamental neuronal properties as they emerge in lower areas.

Animals↗

PubMed2002

Activity patterns in human motion-sensitive areas depend on the interpretation of global motion.

Numerous imaging studies have contributed to the localization of motion-sensitive areas in the human brain. It is, however, still unclear how these areas contribute to global motion perception. Here, we investigate with functional MRI whether the motion-sensitive area hMT+/V5 is involved in perceptual segmentation and integration of motion signals. Stimuli were overlapping moving gratings that can be perceived either as two independently moving, transparent surfaces or as a single surface moving in an intermediate direction. We examined whether motion-sensitive area hMT+/V5 is involved in mediating the switches between the two percepts. The data show differential activation of hMT+/V5 with perceptual switches, suggesting that these are associated with a reconfiguration of cell assemblies in this area.

Brain↗

PubMed2002

Tracking the mind's image in the brain II: transcranial magnetic stimulation reveals parietal asymmetry in visuospatial imagery.

The functional relevance of brain activity during visuospatial tasks was investigated by combining functional magnetic resonance imaging with unilateral repetitive transcranial magnetic stimulation (rTMS). The cognitive tasks involved visuospatial operations on visually presented and mentally imagined material ("mental clock task"). While visuospatial operations were associated with activation of the intraparietal sulcus region bilaterally, only the group which received rTMS to the right parietal lobe showed an impairment of performance during and immediately after rTMS. This functional parietal asymmetry might indicate a capacity of the right parietal lobe to compensate for a temporary suppression of the left. This is compatible with current theories of spatial hemineglect and constitutes a constraint for models of distributed information processing in the parietal lobes.

Acoustic Stimulation↗

PubMed2002

Tracking the mind's image in the brain I: time-resolved fMRI during visuospatial mental imagery.

Mental imagery, the generation and manipulation of mental representations in the absence of sensory stimulation, is a core element of numerous cognitive processes. We investigate the cortical mechanisms underlying imagery and spatial analysis in the visual domain using event-related functional magnetic resonance imaging during the mental clock task. The time-resolved analysis of cortical activation from auditory perception to motor response reveals a sequential activation of the left and right posterior parietal cortex, suggesting that these regions perform distinct functions in this imagery task. This is confirmed by a trial-by-trial analysis of correlations between reaction time and onset, width, and amplitude of the hemodynamic response. These findings pose neurophysiological constraints on cognitive models of mental imagery.

Acoustic Stimulation↗

PubMed2002

Apparent motion: event-related functional magnetic resonance imaging of perceptual switches and States.

When spatially segregated visual stimuli are presented in alternation, subjects may perceive a single stimulus moving between the two positions (apparent motion). By adjusting spatial and temporal parameters, an ambiguous condition can be created in which perception of back-and-forth motion alternates with the perception of two stationary blinking stimuli. We presented subjects with such ambiguous stimuli, asked them to signal periods of perceived motion and blinking, and measured brain activity with functional magnetic resonance imaging. Multiple regression analysis revealed that early visual areas responded with equal strength during both perceptual conditions, whereas hMT+(V5) (the human motion complex that includes the human homolog of MT and its satellites) was more active during the perception of apparent motion. These results indicate that neurons in hMT+ participate in the constructive process that creates a continuous motion percept from a discontinuous visual input.

Brain Mapping↗

PubMed2002

Spatial independent component analysis of functional MRI time-series: to what extent do results depend on the algorithm used?

Independent component analysis (ICA) has been successfully employed to decompose functional MRI (fMRI) time-series into sets of activation maps and associated time-courses. Several ICA algorithms have been proposed in the neural network literature. Applied to fMRI, these algorithms might lead to different spatial or temporal readouts of brain activation. We compared the two ICA algorithms that have been used so far for spatial ICA (sICA) of fMRI time-series: the Infomax (Bell and Sejnowski [1995]: Neural Comput 7:1004-1034) and the Fixed-Point (Hyvärinen [1999]: Adv Neural Inf Proc Syst 10:273-279) algorithms. We evaluated the Infomax- and Fixed Point-based sICA decompositions of simulated motor, and real motor and visual activation fMRI time-series using an ensemble of measures. Log-likelihood (McKeown et al. [1998]: Hum Brain Mapp 6:160-188) was used as a measure of how significantly the estimated independent sources fit the statistical structure of the data; receiver operating characteristics (ROC) and linear correlation analyses were used to evaluate the algorithms' accuracy of estimating the spatial layout and the temporal dynamics of simulated and real activations; cluster sizing calculations and an estimation of a residual gaussian noise term within the components were used to examine the anatomic structure of ICA components and for the assessment of noise reduction capabilities. Whereas both algorithms produced highly accurate results, the Fixed-Point outperformed the Infomax in terms of spatial and temporal accuracy as long as inferential statistics were employed as benchmarks. Conversely, the Infomax sICA was superior in terms of global estimation of the ICA model and noise reduction capabilities. Because of its adaptive nature, the Infomax approach appears to be better suited to investigate activation phenomena that are not predictable or adequately modelled by inferential techniques.

Adult↗

PubMed2002

Integration of multiple motion vectors over space: an fMRI study of transparent motion perception.

Visual scenes are frequently composed of objects that move in different directions. To segment such scenes into distinct objects or image planes, local motion cues have to be evaluated and integrated according to criteria of global coherence. When several populations of coherently moving random dots penetrate each other, the visual system tends to assign them to different planes-perceived as transparent motion. This process of integration was studied by changing the angle of motion trajectories with which groups of dots penetrate each other or by varying the spatial constellation of dots moving in opponent directions. Psychophysical testing revealed that stimuli providing almost identical local motion cues could be perceived in three very different ways: (1) as a matrix of stationary flickering dots, (2) as a single surface of coherently moving dots, and (3) as two transparent dot matrices moving in different directions. Behaviorally controlled functional magnetic resonance imaging (fMRI) was used to identify brain regions that contribute to the integration of local motion cues into coherently moving surfaces. Activation of the human motion complex (hMT+/V5) and of areas in the fusiform gyrus (FG) as well as in the intraparietal sulcus (IPS-occ) was correlated with the perception of coherent motion and especially hMT+/V5 took a central role in differentiating transparent motion from single-surface coherent motion.

Adult↗

PubMed2002

Coordinate and categorical judgements in spatial imagery. An fMRI study.

We aimed at verifying whether the hemispheric specialisation for categorical/coordinate spatial judgements also applies to the spatial imagery domain by the use of whole-brain fMRI. In a block-design experiment we used the "coordinate" mental clock test, contrasting it with a "categorical" task applied to the same clock stimuli; as a control task we used a syllable counting task requiring a verbal-phonological judgement on the same material of the two imagery tasks. Our results showed that categorical and coordinate spatial judgements on imagined stimuli rely on the activation of a set of cortical areas, centred upon the superior parietal lobule (SPL) bilaterally. These regions, together with other parietal and prefrontal areas, showed a pattern of relative lateralization, with the left hemisphere being mainly activated during the categorical task and the right in the coordinate task. These data confirm the strong involvement of the SPL in spatial processing. Moreover, our findings suggest that different interconnected neural networks are activated to comply with specific test requirements, giving rise to functional imaging patterns compatible with psychological theories on hemispheric specialization.

Adult↗

PubMed2002

Distributed cortical systems in visual short-term memory revealed by event-related functional magnetic resonance imaging.

The spatio-temporal distribution of brain activity as revealed by non-invasive functional imaging helps to elucidate the neuronal encoding and processing strategies required by complex cognitive tasks. We investigated visual short-term memory for objects, places and conjunctions in humans using event-related time-resolved functional magnetic resonance imaging that permitted segregation of encoding, retention and retrieval phases. All conditions were accompanied by the activation of a widespread network of parietal and prefrontal areas during the retention phase, but this retention-related activity showed additional modulations depending on task instructions. These modulations confirmed a posterior-anterior and right-left dissociation for spatial versus non-spatial memory and revealed that conjunction memory does not rely on a linear addition of the component processes.

Adult↗