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Biomedical subjects

Hans-Joachim Freund

Publications and source records attributed to Hans-Joachim Freund.

At least 19 recordsLinked to original sources
PubMed2004

Neural circuits underlying imitation learning of hand actions: an event-related fMRI study.

The neural bases of imitation learning are virtually unknown. In the present study, we addressed this issue using an event-related fMRI paradigm. Musically naive participants were scanned during four events: (1) observation of guitar chords played by a guitarist, (2) a pause following model observation, (3) execution of the observed chords, and (4) rest. The results showed that the basic circuit underlying imitation learning consists of the inferior parietal lobule and the posterior part of the inferior frontal gyrus plus the adjacent premotor cortex (mirror neuron circuit). This circuit, known to be involved in action understanding, starts to be active during the observation of the guitar chords. During pause, the middle frontal gyrus (area 46) plus structures involved in motor preparation (dorsal premotor cortex, superior parietal lobule, rostral mesial areas) also become active. Given the functional properties of area 46, a model of imitation learning is proposed based on interactions between this area and the mirror neuron system.

Adult↗
PubMed2004

Alumina-supported vanadium nanoparticles: structural characterization and CO adsorption properties.

Alumina-supported vanadium particles were prepared under ultrahigh vacuum (UHV) conditions and characterized with respect to their structural and CO adsorption properties. As supporting oxide, we used a thin, well-ordered alumina film grown on NiAl(110). This allows the application of scanning tunneling microscopy (STM), infrared reflection-absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS) without charging effects. Vanadium evaporation under UHV conditions leads to the growth of nanometer-sized particles which strongly interact with the alumina support. At very low vanadium coverages, these particles are partially incorporated into the alumina film and get oxidized through the contact to alumina. Low-temperature CO adsorption in this coverage regime permits the preparation of isolated vanadium carbonyls, of which we have identified mono-, di-, and tricarbonyls of the V(CO)(y)() type. A charge-frequency relationship was set up which allows one to quantify the extent of charge transfer from vanadium to alumina. It turns out that this charge transfer depends on the V nucleation site.

Journal Article↗
PubMed2004

Perilesional pathological oscillatory activity in the magnetoencephalogram of patients with cortical brain lesions.

In the surrounding of focal ischemic brain lesions dysfunctional neuronal zones emerge often resulting in pathological oscillatory activity. Using whole-head magnetoencephalography we recorded brain activity during rest in 23 patients with ischemic cortical lesions to find out whether we can localise and characterise low-frequency oscillatory activity. We measured patients at different times after stroke and partly in a follow-up approach to determine the time course of slow-wave activity. Using the analysis tool Dynamic Imaging of Coherent Sources we computed tomographic maps of oscillatory power in the delta-band (0.5-3 Hz). Fifteen of 23 patients with cortical strokes showed delta-activity, which was localised in an area not more than 2 cm away from the lesion. We found this perilesional low-frequency activity in the acute as well as in the chronic stage of stroke. Follow-up measurements of individual patients revealed persistence of perilesional low-frequency activity for months and even years. No consistent relation between perilesional activity and clinical symptoms was observed. Our results indicate that perilesional delta activity is common after ischemic cortical stroke. However, the functional significance remains to be elucidated.

Action Potentials↗
PubMed2004

Long-term results of bilateral pallidal stimulation in Parkinson's disease.

We followed up 11 patients for up to 5 years after bilateral pallidal deep brain stimulation for advanced Parkinson's disease. Dyskinesias remained significantly reduced until the last assessment. The initial improvement of off-period motor symptoms and fluctuations, however, was not sustained and gradually declined. Beneficial effects of pallidal deep brain stimulation on activities of daily living in the on- and off-period were lost after the first year. Replacement of pallidal electrodes into the subthalamic nucleus in four patients could restore the initial benefit of deep brain stimulation and allowed a significant reduction of dopaminergic drug therapy.

Activities of Daily Living↗
PubMed2004

Neural mechanisms underlying reaching for remembered targets cued kinesthetically or visually in left or right hemispace.

Reaching for a target involves integrative coordinate transformation processes between the representation of the target location, the sensorimotor information of limb of reach, and body space. Although right hemisphere dominance for visuospatial information processing is well established, corresponding right hemisphere dominance for kinesthetic spatial information processing remains to be demonstrated. We explored neural mechanisms of encoding target locations using 15O-butanol positron emission tomography (PET) in normal volunteers in a factorial experiment, where modality (visual/kinesthetic) and hemispace of target presentation (left/right of midsagittal plane) were varied systematically. After target presentation, subjects reached to the encoded target location. PET data analysis using SPM99 showed increased neural activity (P < 0.05, corrected) associated with left hemispace target presentation in right hemisphere areas (sensorimotor, anterior cingulate, insular, and temporo-occipital cortex) only. By contrast, right hemispace target presentation activated bilateral temporo-occipital cortex, which extended into the right temporo-parietal cortex and left sensorimotor cortex. A significant interaction of hemispace and modality of target presentation observed in right temporo-parietal cortex resulted from an increase in neural activity with kinesthetic target presentation in right hemispace. The data support an important role for the right temporo-parietal area in visuospatial processing and suggest a specific role of the right hemisphere in kinesthetic spatial processing.

Adult↗
PubMed2003

Body scheme gates visual processing.

We used functional magnetic resonance imaging (fMRI) to explore how guidance of motor acts is influenced by the visually perceived body scheme. We found that when subjects view their hand as their opposite hand, i.e., the right hand is seen as the left hand and vice versa, activation in the visual cortex was lateralized opposite to the seen hand. This demonstrates for the first time that our body scheme to which vision relates our environment is already represented at the level of visual cortex.

Adult↗
PubMed2003

Mirror apraxia affects the peripersonal mirror space. A combined lesion and cerebral activation study.

Mirror apraxia is a condition in which patients with lesions of the posterior parietal cortex have deficits in reaching to objects presented through a mirror. The aim of the present study was to investigate possible mechanisms underlying this disorder. First, we addressed the question of whether mirror apraxia is exhibited to the same extent in peripersonal and in body space. Four patients with lesions of the posterior parietal lobe on either side and with marked mirror apraxia were required to reach for objects that were presented to them through a mirror and located either in body space (i.e. on the body surface) or in peripersonal space (i.e. in the reaching distance). Whereas reaching for objects located in body space was flawless in all patients, the performance deteriorated when the same objects were transferred to the peripersonal space. Although the objects were located only a few centimetres above the body surface, the patients reached towards the virtual object in the mirror. Based on these results we suggest that mirror apraxia may originate from a dissociation between the representations of body schema and peripersonal space and that objects located on the body surface become integrated into the body schema. In the second part of the study, using positron emission tomography study (PET), we studied the cerebral activation pattern during reaching to objects presented through a mirror in the peripersonal space in healthy subjects. The results show that increased neural activity in the anterior part of the intraparietal sulcus and in the dorsal premotor cortex was bound to the transformation of the target position from the mirror space to the real space. In contrast, the activity related to object localization in the mirror occurred at the parieto-occipital junction. Both mirror and arm transformation involved the medial posterior part of the superior parietal lobule, putatively area V6a. The results demonstrate that acting through a mirror is processed in a number of cortical areas of the dorsal stream.

Aged↗
PubMed2003

Femtosecond photodesorption of small molecules from surfaces: a theoretical investigation from first principles.

A microscopic model for the excitation and relaxation processes in photochemistry at surfaces is developed. Our study is based on ab initio calculations and the surrogate Hamiltonian method treating surface electron-hole pairs as a bath of two-level systems. Desorption probabilities and velocities in the experimentally observed range are obtained. The excited state lifetime is calculated, and a dependence of observables on pulse length is predicted.

Journal Article↗
PubMed2003

Deep brain stimulation of the subthalamic nucleus enhances emotional processing in Parkinson disease.

BACKGROUND: High-frequency electrical stimulation of the subthalamic nucleus is a new and highly effective therapy for complications of long-term levodopa therapy and motor symptoms in advanced Parkinson disease (PD). Clinical observations indicate additional influence on emotional behavior. METHODS: Electrical stimulation of deep brain nuclei with pulse rates above 100 Hz provokes a reversible, lesioning-like effect. Here, the effect of deep brain stimulation of the subthalamic nucleus on emotional, cognitive, and motor performance in patients with PD (n = 12) was examined. The results were compared with the effects of a suprathreshold dose of levodopa intended to transiently restore striatal dopamine deficiency. Patients were tested during medication off/stimulation off (STIM OFF), medication off/stimulation on (STIM ON), and during the best motor state after taking levodopa without deep brain stimulation (MED). RESULTS: More positive self-reported mood and an enhanced mood induction effect as well as improvement in emotional memory during STIM ON were observed, while during STIM OFF, patients revealed reduced emotional performance. Comparable effects were revealed by STIM ON and MED. Cognitive performance was not affected by the different conditions and treatments. CONCLUSIONS: Deep brain stimulation of the subthalamic nucleus selectively enhanced affective processing and subjective well-being and seemed to be antidepressive. Levodopa and deep brain stimulation had similar effects on emotion. This finding may provide new clues about the neurobiologic bases of emotion and mood disorders, and it illustrates the important role of the basal ganglia and the dopaminergic system in emotional processing in addition to the well-known motor and cognitive functions.

Basal Ganglia↗
PubMed2003

The cerebral oscillatory network of parkinsonian resting tremor.

Data from experiments in MPTP monkeys as well as from invasive and non-invasive recordings in patients with Parkinson's disease suggest an abnormal synchronization of neuronal activity in the generation of resting tremor in Parkinson's disease. In six patients with tremor-dominant idiopathic Parkinson's disease, we recorded simultaneously surface electromyograms (EMGs) of hand muscles, and brain activity with a whole-head magnetoencephalography (MEG) system. Using a recently developed analysis tool (Dynamic Imaging of Coherent Sources; DICS), we determined cerebro-muscular and cerebro-cerebral coherence as well as the partial coherence between cerebral areas and muscle, and localized coherent sources within the individual MRI scans. The phase lag between the EMG and cerebral activity was determined by means of a Hilbert transform of both signals. After overnight withdrawal from medication, patients showed typical Parkinson's disease resting tremor (4-6 Hz). This tremor was associated with strong coherence between the EMG of forearm muscles and activity in the contralateral primary motor cortex (M1) at tremor frequency but also at double tremor frequency. Phase lags between M1 activity and EMG were between 15 and 25 ms (M1 activity leading) at single, but also at double tremor frequency, corresponding well to the corticomuscular conduction time. Furthermore, significant coherence was observed between M1 and medial wall areas (cingulate/supplementary motor area; CMA/SMA), lateral premotor cortex (PM), diencephalon, secondary somatosensory cortex (SII), posterior parietal cortex (PPC) and the contralateral cerebellum at single tremor and, even stronger at double tremor frequency. Spectra of coherence between thalamic activity and cerebellum as well as several brain areas revealed additional broad peaks around 20 Hz. Power spectral analysis of activity in all central areas indicated the strongest frequency components at double tremor frequency. Partial coherence analysis and the calculation of phase shifts revealed a strong bidirectional coupling between the EMG and diencephalic activity and a direct afferent coupling between the EMG and SII and the PPC. In contrast, the cerebellum, SMA/CMA and PM show little evidence for direct coupling with the peripheral EMG but seem to be connected with the periphery via other cerebral areas (e.g. M1). In summary, our results demonstrate tremor-related oscillatory activity within a cerebral network, with abnormal coupling in a cerebello-diencephalic-cortical loop and cortical motor (M1, SMA/CMA, PM) and sensory (SII, PPC) areas contralateral to the tremor hand. The main frequency of cerebro-cerebral coupling corresponds to double the tremor frequency.

Adult↗
PubMed2003

Somatosensory and motor disturbances in patients with parietal lobe lesions.

Lesion studies show that a wide range of integrative sensorimotor functions can be selectively disturbed in patients with parietal lobe damage. Lesions restricted to the somatosensory representations on the anterior parietal lobe produce somatosensory deficits that resemble deafferentated states, including the secondary effects on motor control. Slightly more posterior lesions often are associated with impairment of more complex synthetic somatosensory functions similar to those observed after dorsal column lesions. Damage of the posterior parietal lobe can selectively interfere with virtually every aspect of somatosensory function. These perceptive and cognitive disturbances may or may not be associated with complex motor disturbances of the apractic type. The frequent association of astereognosia and tactile apraxia illustrate the mutual interdependence of the sensorimotor processes involved in active touch and the proximity of the respective processing modules. Parietal lobe function is critical for the control of force and posture, and for the formation of the body image and its relation to external space (the guidance of movements, including the eyes, to external objects). Imaging studies underscore the prominent role of the parietal cortex as a sensorimotor interface and provide complementary information about the interrelationship between perception and action. Action observation activates premotor cortex, but parietal cortex is also recruited whenever an action involves objects, thus emphasizing the significance of parietal cortex for object-directed motor behavior. In contrast to the ventral-dorsal route processing dichotomy in the visual system, both the perceptual-cognitive and motor aspects of somatosensory processing are compromised after parietal damage, demonstrating a different functional architecture of the two sensory systems. The preservation of the comprehension of the meaning of gestures or of object use in patients with lesions restricted to the parietal lobe reveals that the semantic aspects of motor behavior are mediated in the temporal lobe.

Animals↗