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Torben E Lund

Publications and source records attributed to Torben E Lund.

8 recordsLinked to original sources
PubMed2006

An fMRI study of the neural correlates of graded visual perception.

The neural correlates of clearly perceived visual stimuli have been reported previously in contrast to unperceived stimuli, but it is uncertain whether intermediate or graded perceptual experiences correlate with different patterns of neural activity. In this study, the subjective appearance of briefly presented visual stimuli was rated individually by subjects with respect to perceptual clarity: clear, vague or no experience of a stimulus. Reports of clear experiences correlated with activation in a widespread network of brain areas, including parietal cortex, prefrontal cortex, premotor cortex, supplementary motor areas, insula and thalamus. The reports of graded perceptual clarity were reflected in graded neural activity in a network comprising the precentral gyrus, intraparietal sulcus, basal ganglia and the insula. In addition, the reports of vague experiences demonstrated unique patterns of activation. Different degrees of perceptual clarity were reflected both in the degree to which activation was found within parts of the network serving a clear conscious percept, and additional unique activation patterns for different degrees of perceptual clarity. Our findings support theories proposing the involvement of a widespread network of brain areas during conscious perception.

Adult↗
PubMed2006

Similar brain networks for detecting visuo-motor and visuo-proprioceptive synchrony.

The ability to recognize feedback from own movement as opposed to the movement of someone else is important for motor control and social interaction. The neural processes involved in feedback recognition are incompletely understood. Two competing hypotheses have been proposed: the stimulus is compared with either (a) the proprioceptive feedback or with (b) the motor command and if they match, then the external stimulus is identified as feedback. Hypothesis (a) predicts that the neural mechanisms or brain areas involved in distinguishing self from other during passive and active movement are similar, whereas hypothesis (b) predicts that they are different. In this fMRI study, healthy subjects saw visual cursor movement that was either synchronous or asynchronous with their active or passive finger movements. The aim was to identify the brain areas where the neural activity depended on whether the visual stimulus was feedback from own movement and to contrast the functional activation maps for active and passive movement. We found activity increases in the right temporoparietal cortex in the condition with asynchronous relative to synchronous visual feedback from both active and passive movements. However, no statistically significant difference was found between these sets of activated areas when the active and passive movement conditions were compared. With a posterior probability of 0.95, no brain voxel had a contrast effect above 0.11% of the whole-brain mean signal. These results do not support the hypothesis that recognition of visual feedback during active and passive movement relies on different brain areas.

Adult↗
PubMed2006

Images of illusory motion in primary visual cortex.

Illusory motion can be generated by successively flashing a stationary visual stimulus in two spatial locations separated by several degrees of visual angle. In appropriate conditions, the apparent motion is indistinguishable from real motion: The observer experiences a luminous object traversing a continuous path from one stimulus location to the other through intervening positions where no physical stimuli exist. The phenomenon has been extensively investigated for nearly a century but little is known about its neurophysiological foundation. Here we present images of activations in the primary visual cortex in response to real and apparent motion. The images show that during apparent motion, a path connecting the cortical representations of the stimulus locations is filled in by activation. The activation along the path of apparent motion is similar to the activation found when a stimulus is presented in real motion between the two locations.

Adult↗
PubMed2005

Non-white noise in fMRI: does modelling have an impact?

The sources of non-white noise in Blood Oxygenation Level Dependent (BOLD) functional magnetic resonance imaging (fMRI) are many. Familiar sources include low-frequency drift due to hardware imperfections, oscillatory noise due to respiration and cardiac pulsation and residual movement artefacts not accounted for by rigid body registration. These contributions give rise to temporal autocorrelation in the residuals of the fMRI signal and invalidate the statistical analysis as the errors are no longer independent. The low-frequency drift is often removed by high-pass filtering, and other effects are typically modelled as an autoregressive (AR) process. In this paper, we propose an alternative approach: Nuisance Variable Regression (NVR). By inclusion of confounding effects in a general linear model (GLM), we first confirm that the spatial distribution of the various fMRI noise sources is similar to what has already been described in the literature. Subsequently, we demonstrate, using diagnostic statistics, that removal of these contributions reduces first and higher order autocorrelation as well as non-normality in the residuals, thereby improving the validity of the drawn inferences. In addition, we also compare the performance of the NVR method to the whitening approach implemented in SPM2.

Algorithms↗
PubMed2005

Motion or activity: their role in intra- and inter-subject variation in fMRI.

Functional MRI (fMRI) carries the potential for non-invasive measurements of brain activity. Typically, what are referred to as activation images are actually thresholded statistical parametric maps. These maps possess large inter-session variability. This is especially problematic when applying fMRI to pre-surgical planning because of a higher requirement for intra-subject precision. The purpose of this study was to investigate the impact of residual movement artefacts on intra-subject and inter-subject variability in the observed fMRI activation. Ten subjects were examined using three different word-generation tasks. Two of the subjects were examined 10 times on 10 different days using the same paradigms. We systematically investigated one approach of correcting for residual movement effects: the inclusion of regressors describing movement-related effects in the design matrix of a General Linear Model (GLM). The data were analysed with and without modeling the residual movement artefacts and the impact on inter-session variance was assessed using F-contrasts. Inclusion of motion parameters in the analysis significantly reduced both the intra-subject as well as the inter-subject-variance.

Adult↗
PubMed2004

Hypercapnic normalization of BOLD fMRI: comparison across field strengths and pulse sequences.

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal response to neural stimulation is influenced by many factors that are unrelated to the stimulus. These factors are physiological, such as the resting venous cerebral blood volume (CBV(v)) and vessel size, as well as experimental, such as pulse sequence and static magnetic field strength (B(0)). Thus, it is difficult to compare task-induced fMRI signals across subjects, field strengths, and pulse sequences. This problem can be overcome by normalizing the neural activity-induced BOLD fMRI response by a global hypercapnia-induced BOLD signal. To demonstrate the effectiveness of the BOLD normalization approach, gradient-echo BOLD fMRI at 1.5, 4, and 7 T and spin-echo BOLD fMRI at 4 T were performed in human subjects. For neural stimulation, subjects performed sequential finger movements at 2 Hz, while for global stimulation, subjects breathed a 5% CO(2) gas mixture. Under all conditions, voxels containing primarily large veins and those containing primarily active tissue (i.e., capillaries and small veins) showed distinguishable behavior after hypercapnic normalization. This allowed functional activity to be more accurately localized and quantified based on changes in venous blood oxygenation alone. The normalized BOLD signal induced by the motor task was consistent across different magnetic fields and pulse sequences, and corresponded well with cerebral blood flow measurements. Our data suggest that the hypercapnic normalization approach can improve the spatial specificity and interpretation of BOLD signals, allowing comparison of BOLD signals across subjects, field strengths, and pulse sequences. A theoretical framework for this method is provided.

Adult↗
PubMed2002

Cortical deactivation induced by visual stimulation in human slow-wave sleep.

It has previously been demonstrated that sleeping and sedated young children respond with a paradoxical decrease in the blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal in the rostro-medial occipital visual cortex during visual stimulation. It is unresolved whether this negative BOLD response pattern is of developmental neurobiological origin particular to a given age or to a general effect of sleep or sedative drugs. To further elucidate this issue, we used fMRI and positron emission tomography (PET) to study the brain activation pattern during visual stimulation in spontaneously sleeping adult volunteers. In five sleeping volunteers fMRI studies confirmed a robust signal decrease during stimulation in the rostro-medial occipital cortex. A similar relative decrease at the same location was found during visual stimulation and polysomnographically verified slow-wave sleep in a separate group of six subjects using H(2)(15)O PET measures of the regional cerebral blood flow (rCBF). This decrease was more rostro-dorsal compared to the relative rCBF increase along the calcarine sulcus found during visual stimulation in the awake state. This study reconfirms the previously described paradoxical stimulation-correlated negative BOLD signal change in the rostro-medial occipital cortex, expanding this response mode to an age spectrum ranging from the newborn to the adult. Further, the use of complementary brain mapping techniques suggests that this decrease was secondary to a relative rCBF decrease. Possible mechanisms for the paradoxical response pattern during sleep include an active inhibition of the visual cortex or a disruption of an energy-consuming process.

Adult↗