Biomedical subjects

V Hömberg

Publications and source records attributed to V Hömberg.

At least 37 records · Page 2Linked to original sources

PubMed1995

Hemispheric asymmetry of transcallosal inhibition in man.

The transcallosal connecting fibres linking corresponding projection areas of the same muscles of the right and left primary motor cortex may play an important role in control of unilateral movements. It appears that they have mainly inhibitory effects. This was further evaluated by transcranial magnetic stimulation using two focal coils placed on the optimal positions, i.e. the positions with the lowest thresholds at the motor representation areas of the first dorsal interosseous muscle of the left and right sides. A conditioning stimulus was given to one hemisphere 10 ms prior to the test stimulus at the opposite hemisphere. The inhibition was evaluated as relative amplitude reduction. Eleven normal right-handed subjects and 11 normal left-handed subjects participated in this study. Handedness was evaluated by the Oldfield inventory. It was found that in right-handers the inhibition after stimulation of the "dominant" left hemisphere was more marked than after stimulation of the "non-dominant" right hemisphere. In contrast, the group of left-handed subjects showed inhomogeneous findings with either right- or left-side predominant inhibition. It is concluded that not handedness but hemispheric dominance contributes to the laterality of inhibition. The results point to a superior role of the language-dominant hemisphere in governing inter-hemispheric control of motor cortical connections, supporting the view that the "language-dominant" hemisphere is also "motor dominant".

Adult↗

PubMed1995

The pattern of attentional deficits in Huntington's disease.

Different aspects of attention, e.g. phasic alertness, vigilance, divided attention, response flexibility, response inhibition and intermodal integration, were investigated with a computerized test-battery in a group of 20 patients with Huntington's disease and 27 healthy controls. Huntington's disease patients are not impaired in reacting to task-contingent external stimulation in the phasic alertness task, but the self-generated maintenance of attention as measured by the vigilance task, is disturbed. The simultaneous monitoring of different input-channels in the divided attention task and the ability to operate with information given to different modalities in the intermodal integration task are severely affected. The performance of Huntington's disease patients in the response flexibility task, in which internal cued shifts are required, is impaired. Huntington's disease patients are also impaired in reacting selectively to go/no-go stimuli in the response inhibition task. It is suggested that a number of 'higher' cognitive deficits described in Huntington's disease might, at least partly, be due to basic attentional disturbances.

Adult↗

PubMed1994

Successive roles of the cerebellum and premotor cortices in trajectorial learning.

The structures of the human brain engaged during learning of unilateral trajectorial hand movements were mapped by measurements of regional cerebral blood flow. Trajectorial movement velocity accelerated moderately after short-term training p < 0.025 and increased further after long-term training p < 0.01. During the early phase of learning there was a significant activation p < 0.001 of the ipsilateral dentate nucleus. By contrast, after overlearning the premotor cortical areas in both cerebral hemispheres were maximally activated p < 0.001, while the dentate nucleus was no longer activated. It is suggested that learning of new movement trajectories involves the cerebellum, while overlearned trajectorial movements engage the premotor cortex.

Cerebellum↗

PubMed1994

Maturation of fastest afferent and efferent central and peripheral pathways: no evidence for a constancy of central conduction delays.

The maturation of the fastest afferent and efferent central and peripheral pathways was analysed using the recording of somatosensory evoked potentials of the median nerve stimulation for afferent conduction and magnetoelectrical stimulation of motor cortex to the thenar muscles for efferent conduction. Both afferent and efferent central pathways showed a prolonged maturational pattern with adult values being reached by the age of 5 to 7 years for the afferent and by the age of around 10 years for the efferent pathway. In contrast, the maturation for peripheral afferent and efferent pathways showed a similar trend with a fairly constant conduction velocity reached around the age of 3 years. There was no evidence for a constancy of central conduction delays in both afferent and efferent pathway which has been supposed as a possible mechanism for a temporal stability in feedback loops involved in motor learning processes during development. It is concluded that the prolonged maturational central conduction time precludes the usage of a fixed temporal timing pattern during development in the human sensory motor system.

Adolescent↗

PubMed1994

Control of posture in patients with neurologically asymptomatic HIV infection and patients with beginning HIV-1-related encephalopathy.

OBJECTIVE: Stance control measurements (sway velocity, sway area, and postural reflexes) were performed in 36 patients with neurologically asymptomatic human immunodeficiency virus infection (HIV) and in 10 patients with beginning HIV type 1 (HIV-1)-related encephalopathy. All recordings were performed using a movable platform system. OBSERVATIONS: Static posturography and postural reflexes revealed pathologic results in patients with beginning HIV-1-related encephalopathy and in about 25% of patients with nonsymptomatic HIV infection in comparison with age- and sex-matched control subjects. CONCLUSIONS: Postural control is well preserved in early HIV infection; thus, it is not an appropriate measure for detecting subclinical deficits, but disturbances of postural control seem to be one of the first neurological abnormalities in patients with beginning HIV-1-related encephalopathy.

AIDS Dementia Complex↗

PubMed1994

Muscle tone in Huntington's disease.

Tone was evaluated quantitatively in 39 patients with Huntington's disease (HD) by measuring resistance to passive elbow flexion and extension movements on a hydraulic testing device. A velocity dependent increase in resistance to passive elbow flexion and to a lesser extent in resistance to passive extension was found in 10 patients. Increase in muscle tone was related to disease duration but not to the age of the patients nor to the severity of choreatic movements or the amount of cognitive impairment. A hypertonic pattern could be observed also in patients without any medication, and seemed not to be related to neuroleptic treatment. The pattern of tone change in HD is compared to the pattern of hemispasticity in upper motor neuron syndrome and to that of Parkinson's disease. It appears that an increase in muscle tone, with a velocity dependent resistance to flexion more than extension movements, is a frequent feature in advanced stages of Huntington's disease.

Adult↗

PubMed1994

Right neglect following right hemisphere damage?

From a sample of 90 stroke cases showing visual inattention following right hemisphere brain damage, 17 cases were identified who showed more inattention on the right than the left side on some tests. Eight of these subjects had CT scan-confirmed unilateral right hemisphere damage and one of these eight had MRI scan-confirmed unilateral right brain damage. A number of hypotheses are examined to explain this "paradoxical" right inattention. The most parsimonious proposes that right inattention is produced by an interaction of a compensatory left sided scanning strategy in association with a non-lateralised attentional loss.

Adult↗

PubMed1994

Conditional associative learning is impaired in cerebellar disease in humans.

Eight patients with lesions restricted to the cerebellum were compared with a total of 25 age-matched controls on a reaction time (RT) task allowing the recording of simple and choice RTs as well as RTs to abstract visual patterns signifying the particular movement to be performed. In all conditions the actual movements required (either a left or a right button press) remained the same, but the cognitive requirements of the task varied. In the abstract patterns condition, the significance of the various patterns with regard to the required movement had to be learned by the subjects. The patients with cerebellar lesions were particularly impaired in this condition. It is concluded that the cerebellum is involved not just in the timing of movements but also in the decision process as to which movement should be performed under particular circumstances.

Adult↗

PubMed1993

Spinal and supraspinal mechanisms contribute to the silent period in the contracting soleus muscle after transcranial magnetic stimulation of human motor cortex.

In the voluntarily activated muscle, transcranial magnetic stimulation (TMS) of motor cortex produces subsequently to the motor evoked potential (MEP) a silent period (SP) in the electromyogram. We studied the time course of soleus motoneuron (MN) pool excitability after conditioning TMS by Hoffmann reflex (HR) testing, to determine whether inaccessibility of MNs after corticospinal input contributes to the SP. Coincidently with the early part of the SP, and only in the contracting soleus, MN depression was obtained that covaried with the degree of preinnervation, and with the size of the preceding MN discharge. However, MN excitability recovered significantly prior to the end of the SP. It is concluded that in the contracting soleus spinal mechanisms (most likely Renshaw inhibition and MN afterhyperpolarization) contribute to the early part of the SP, while the late part of the SP is supraspinal (probably cortical) in origin.

Adult↗

PubMed1993

Ontogeny is different for explicit and implicit memory in humans.

This study was aimed to clarify in how far ontogenetic development is different for declarative/explicit compared to procedural/implicit learning. These two memory subsystems are differentially affected in a number of pathological conditions. Sixty-one children between the age of 5 and 10 years were tested on a verbal story recall and non-verbal pictorial recall task, representing explicit memory. To test for implicit memory, a motor mirror tracking task and a non-motor tower-of-hanoi puzzle were used. Both explicit memory tasks showed a clear developmental profile with an increase in the number of recalled items for immediate, late and delayed recall with adult values not reached before the age of 9-10 years. In contrast, there were no age differences for both implicit memory tasks. The study demonstrates that explicit and implicit memory follow different maturational trends and further corroborate the notion of two different memory systems.

Adolescent↗

PubMed1993

Posthypoxic amnesia: regional cerebral glucose consumption measured by positron emission tomography.

Using positron emission tomography (PET), the regional cerebral metabolic rate of glucose consumption (rCMRGlc) was measured in 7 subjects suffering from chronic posthypoxic amnesia and in 12 controls. In 6 of the 7 patients regional decreases in rCMRGlc below normal limits were found involving the thalamus in 3 cases, the medial temporal cortex in 2 cases, and the caudate nucleus and cerebellum in one case each. No significant decrease in rCMRGlc was observed in any of the neocortical regions in any of the patients studied. Significant differences between group means of rCMRGlc were found only for the thalamus and the medial temporal cortex. These data suggest that posthypoxic amnesia stems from damage to the medial temporal cortex and its thalamic projection areas and not from neocortical dysfunction, confirming recent theories on the genesis of amnesia in man.

Adult↗

PubMed1993

Pre- and postcentral somatosensory evoked potentials in Huntington's disease: effects of stimulus repetition rate.

We recorded frontal, central and parietal somatosensory evoked potentials (SEPs) to median nerve stimulation in 20 patients with Huntington's disease (HD) and in a group of normal controls. Two stimulus repetition rates, 1 Hz and 5 Hz, were employed. In HD patients the early cortical potentials (latency range 20-30 ms) at all 3 recording locations were replaced by a widespread, broadly configured N20-25 deflection, while later potentials at 40-80 ms did not significantly differ from those of normals. In contrast to the early P22, P27 and N30 potentials in normals, the N20-25 potential in the patients was not significantly modified by changing the stimulus repetition rate. At 40-80 ms the stimulus rate effects were similar in the patients and normals. The results show that early pre- and postcentral SEPs are both pathological in HD, while later frontal and parietal components can be totally preserved. The early N20-25 in HD is possibly a subcortical potential, seen due to unmasking in the absence of early cortical deflections.

Adult↗

PubMed1993

Motor training in the therapy of Parkinson's disease.

Motor training techniques may be a helpful adjunct to medical treatment in improving motor function in patients with Parkinson's disease. Rational physical therapy should be based on a detailed understanding of the underlying nature of the motor deficit in Parkinson's disease. The design of training techniques should take into account the known neurophysiologic aspects of motor impairment in Parkinson's disease, such as the inability to initiate movement (akinesia), the inability to perform sequential movements, impairments in the pacing of rhythmic movements, and impairments in the predictability of movements. Training techniques are definitely necessary to prevent the sequelae, especially of the later stages of the disease, including contractures and muscle spasm. To increase voluntary motor activity, physiotherapeutic approaches must take into account specific aspects of the motor control deficit in Parkinson's disease. For example, physical therapy should emphasize large-amplitude movements and truncal activities and make use of cuing as a major facilitator in achieving rhythmic movement patterns. In addition, the psychosocial integration of the patient may be improved by physical exercises, particularly if these are provided in group therapy sessions.

Humans↗

PubMed1992

Intact conduction of fastest corticospinal efferents is not sufficient for normal voluntary muscle strength: transcranial motor cortex stimulation in patients with tetraplegia.

In six patients with chronic severe tetraparesis, caused by closed head injury (3 cases), basilar thrombosis (2 cases) or global hypoxia (1 case), responses of suprathreshold transcranial magnetic stimulation of motor cortex to thenar and abductor hallucis muscles on both sides were studied. Results showed in most patients normal thresholds and latencies (17 out of 24 latencies within normal +/- 2 S.D., 20 thresholds within normal +/- 1 S.D.) and amplitudes of even higher than normal values. Latencies or thresholds did not correlate with severity of pareses and were in marked contrast to the severe pareses of the muscles under investigation. Despite different etiology CT-scans showed diffuse atrophy of cerebral white matter as a common finding. It is concluded that intact descending corticospinal motor pathways are a necessary but not a sufficient condition for voluntary motor function.

Adolescent↗

PubMed1992

Development of speed of repetitive movements in children is determined by structural changes in corticospinal efferents.

This study was aimed to determine the relationship between the maturation of corticospinal efferents, determined by transcranial stimulation of motor cortex, and the development of fastest repetitive voluntary motor activity in children. The development of fastest repetitive voluntary motor activity was assessed for 3 different types of movements including fastest repetitive tapping movements, aiming movements and a pegboard transportation task. These 3 motor activities were chosen as they were different as to their dependence on detailed sensory guidance. Despite these differences the speed of all 3 movements showed a very similar developmental profile, which was matched, however, by the developmental slope of the fastest cortico-motoneuronal efferents. Hence the development of central conduction times determines the speed of repetitive movements in children. In contrast, we could not observe significant effects of repetitive training on speed of these movements. We show for the first time that the development of fastest voluntary movements is a structure-bound phenomenon, being independent from learning.

Adolescent↗

PubMed1992

Cerebral visual motion blindness: transitory akinetopsia induced by transcranial magnetic stimulation of human area V5.

The perception of visual motion can be selectively and reversibly compromised by transcranial magnetic stimulation (TMS) of a small region of cortex, roughly 1 cm in diameter and corresponding in position to human area V5. The reversible inactivation of a small and specialized visual area which receives its predominant input from area V1 and sends a powerful return (re-entrant) input back to it allowed us to study for the first time the backward influence of area V5 on area V1. In contrast to the complete and temporary visual motion blindness which occurs during stimulation of V5, a less-prominent interference with the perception of visual motion occurs at 70-80 ms after the onset of the visual stimulus when TMS is applied to V1. Because V5 is critical for the perception of coherent motion, and because an intact re-entry of signals from V5 to V1 is essential for the conscious perception of visual motion, the results obtained with stimulation of V1 must be caused by a disruption of the re-entrant signals from V5 to V1.

Blindness↗

PubMed1992

Reversible conduction block in human ischemic neuropathy after ergotamine abuse.

Conduction block [a significant reduction in compound muscle action potential (CMAP) amplitude after proximal compared to distal stimulation] is often found in demyelinating neuropathies, including inflammatory neuropathies and degenerative neuropathies, such as "liability to pressure neuropathy." There is experimental evidence that a transient conduction block can occur in rats after ischemic lesions of peripheral nerves are induced either by ligation of arterial vessels supplying nerve trunks, or by injection of arachidonic acid into peripheral arterial vessels. Conduction block has also recently been described in cases with necrotizing vasculitis. To date, however, no example of a reversible conduction block has been reported in human ischemic neuropathy.

Adult↗

PubMed1992

Magnetic stimulation of motor cortex in relation to fastest voluntary motor activity in neurologically asymptomatic HIV-positive patients.

Forty-two HIV-positive patients of various CDC stages without clinically evident neurological deficits were examined with transcranial magnetoelectrical stimulation (TMS). Cortical as well as cervical and lumbar root stimulation was performed after excluding peripheral neuropathies in comparison to an age- and sex-matched control group. Whereas central conduction times were normal, conduction between cervical or lumbar roots and muscle was prolonged. Results were correlated to those of a motor test battery, which revealed slowing of fast alternating movements similar to findings in extrapyramidal disorders. Data indicate that proximal parts of the peripheral nervous system and extrapyramidal structures are subclinically involved in early HIV infection whereas the fastest corticospinal projections remain intact.

Adult↗