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V Hömberg

Publications and source records attributed to V Hömberg.

At least 73 records · Page 4Linked to original sources
PubMed1987

Differential effects of changes in mechanical limb properties on physiological and pathological tremor.

The effect of changes in mechanical limb properties on the peak frequency of different tremor forms was analysed. Wrist tremor was recorded by an accelerometer fixed to the dorsum of the hand and demodulated surface EMG was recorded from the wrist extensors, while the extended hand was loaded with successively heavier weights. Physiological tremor was characterised by flat EMG spectra and a gradual decrease in tremor peak frequency with increasing load, as would be expected from the properties of a passive spring-mass-system. Also the peak frequency of activated physiological tremor characterised by increased synchronisation between motor units decreased in frequency with increasing loads. EMG spectra showed clear peaks of activity at the various mechanically determined tremor frequencies. In contrast, in two pathological tremor forms, the postural tremor in Parkinsonian patients and essential tremor, peak frequency tended to remain stable irrespective of changes in load. The method therefore allows a simple distinction between physiological and these two pathological tremors.

Adolescent↗
PubMed1986

Colour and brightness coding in the central nervous system: theoretical aspects and visual evoked potentials to homogeneous red and green stimuli.

We designed visual evoked potentials experiments to study the differential aspects of colour and brightness coding in man. The substitution of equally bright red and green stimuli for a background yellow was investigated and compared with different luminance increments and decrements of red and green. A dominant N87 component was found for a colour change from yellow to brighter red colours, which was less pronounced for green and absent for yellow luminance changes. It is also absent for pure red luminance increments and green luminance changes, but reappears with red luminance decrements or red-offset. The data are discussed within the framework of a new concept of how the visual system fuses red-green information and black-white border information. Retinal X-cells can transmit colour and high spatial frequency achromatic information simultaneously by encoding only the presence of edges (a.c.) for the black-white stimuli and the presence of both edges (a.c.) and uniform areas of colour (d.c.) for red-green stimuli. Phylogenetically this kind of information transmission enables colour vision to be implemented in a retina such as the cat's by adding only a second class of cones. Barlow's economy principle will be violated for colour in the periphery, but restored early in the striate cortex where there is an early decoding of the combined chromatic and achromatic information by the concentric double opponent cells. The N87 behaviour correlates with the proposed discharge of peripheral X-type cells, but not with the discharge of cortical double opponent concentric or simple cells, which no longer respond to homogeneous colour stimuli. It is suggested that N87 may be generated by geniculate afferents in the dendritic arborization of cortical cells, reflecting the behaviour of peripheral units, and thus the violation of the economy principle, rather than the next step in cortical processing. The early cortical restoration of the economy principle is supported by the absence of any further dissociated behaviour for colour and brightness in later components.

Adult↗
PubMed1986

The muscle activity spectrum: spectral analysis of muscle force as an estimator of overall motor unit activity.

Starting from the observation that the part above 6 Hz of the power spectrum of force tremor during isometric contractions can be related to the unfused twitches of motor units firing asynchronously, an attempt was made to study the usefulness of force tremor spectral analysis as a global descriptor of motoneurone pool activity. To compensate for the mechanical low-pass filter characteristic of skeletal muscle which leads to increased damping of mechanical ripples at higher frequencies, a numerical compensation rule was derived from data obtained by electrical microstimulation of small ensembles of motor units (MUs). Hidden line plots of consecutive partially overlapping spectra allowed visualization of changes in spectral composition during ongoing muscle activity. The resulting muscle activity spectra (MAS) showed broad peaks according to the range of onset firing rates in normal subjects. These tended to shift to higher frequencies with increasing force. Under conditions with increased synchronization of the MUs these broad peaks were replaced by sharp peaks corresponding to the burst repetition rate. Patients with different motor dysfunctions were selected to illustrate how alterations of MU activity are reflected in the MAS. Decreased or increased firing rates were observed as well as abnormal states of synchronization. It is concluded that the MAS provides useful information about some aspects of the discharge characteristics of ensembles of MUs and therefore represents a method to monitor some qualitative 'image' of the MU activities within a muscle.

Adolescent↗
PubMed1986

Event-related potentials in patients with Huntington's disease and relatives at risk in relation to detailed psychometry.

Event-related potentials (ERPs) were studied in patients with Huntington's disease (HD) and their offspring at risk in a simple auditory oddball paradigm requiring the counting of the rarer of two stimulus categories. Group statistical analysis revealed prolongation of latencies of components P2, N2 and especially P3 in HD patients and to a lesser extent in at-risks. In a large population of normals the age-latency relationship for component P3 showed a non-linear shape with increasing slope and scatter in the older age groups. A bipartate linear regression analysis splitting the normal population at age 50 was used for detection of abnormalities of P3 latency in individual cases. Abnormal P3 latencies were present in the majority of HD patients and also in 25% of clinically normal at-risks. Correlation analysis of ERP components with detailed psychometry revealed a particularly high association of P3 latencies with measurements requiring speeded information processing in non-verbal tasks. P3 amplitude did not covary with performance scores but unlike P3 latency showed association with depression and psychosis scores. From the results it appears that analysis of ERPs is a useful electrophysiological tool for an objective assessment of cognition both in clinically definite and subclinical stages.

Adolescent↗
PubMed1985

Evoked potentials in patients with Huntington's disease and their offspring. II. Visual evoked potentials.

Visual evoked potentials (VEPs) to full-field and half-field stimulation were recorded in a large sample of patients with Huntington's disease (n = 36) and subjects at risk (first-order offspring, n = 55) using a checkerboard pattern reversal stimulus. Whereas a clear diminution of the amplitudes found confirmed earlier findings in patients with Huntington's disease, further alterations of VEP wave forms consisted in the absence of asymmetric hemisphere lateralization to half-field stimulation (i) and severe distortion with amplification of the early NPN/PNP complexes (ii). Considering at least two different abnormal parameters necessary to represent a reliable indicator of the disease, 24 patients (67%) and 14 (25%) of the persons at risk exhibited pathological results. In contrast, only 2 among 36 patients (5%) but 30 (55%) among the offspring exhibited normal results. The diagnostic and predictive value of the investigation of VEPs in Huntington's disease is discussed.

Adolescent↗
PubMed1984

Colour and brightness components of foveal visual evoked potentials in man.

The different effects of colour and brightness on the transient visual evoked potentials to a foveal stimulus have been investigated in a psycho-physically controlled stimulus set-up, in which equally bright red, orange and green stimuli were substituted for a standard yellow stimulus. These colour-evoked responses were compared with the responses to additional brightness increments and decrements of each of the colour stimuli. An initial component of small amplitude, P55, was followed by a colour-dominated component, N87, and a luminance-dominated component, P120, with a maximum at the occipital electrode. Both N87 and P120 showed a decline in amplitude at the parietal electrode and P120 had a reversed polarity at FZ. These results indicate that the responses to equally bright green and red stimuli have closely similar wave forms, but that this changes rapidly with additional brightness differences. Comparison with the reports of subdural recording of colour-evoked potentials in the macaque striate cortex suggests that P55 corresponds with the primary excitation via geniculo-cortical fibres and that N87 and P120 represent later stages of cortical processing.

Color Perception↗
PubMed1983

[Immunogenetics of myasthenia gravis. Significance of HLA-, complement and Cm gene systems for clinical and immunologic parameters].

In 82 non-related patients with myasthenia gravis the following immunogenetic marker systems were investigated: HLA-A, -B, -C, -DR antigens, complement polymorphisms for Bf, C3 and C4 as well as immunoglobulin allotypes G1m, G3m and Km. In 78 patients the level of circulating acetylcholin receptor autoantibody was measured. The HLA antigens showed no significant differences in frequency when compared with healthy controls. This was also true for the HLA-bound Bf and C4 polymorphism, the non-HLA-bound C3 polymorphism and the immunoglobulin allotypes. A significant (P less than 0.005) increase in heterozygote frequency of 39.7% in the Gm allotype combination G1m (1), (17); (3) when compared with an expected frequency as calculated from the gene frequency of 19.8% was found. HLA-B8 and/or HLA-DR3 positive myasthenia gravis patients showed a significantly (P less than 0.025 and less than 0.005) earlier average onset of the disease whereas in HLA-DR2 positive patients the average onset was later (P less than 0.05). The same HLA antigens correlated with quantitative differences in the acetylcholine receptor autoantibodies: HLA-B8 and/or -DR3 positive patients had on average higher antibody levels than HLA-B8 or -DR3-negative patients. The difference for HLA-DR3 was statistically significant (P less than 0.05). On the other hand patients with HLA-DR2 antigens had significantly lower antibody levels (P less than 0.025). Increased mean antibody levels were also seen in G1m(1), (17) allotype (P less than 0.005) and G3m(21) (P less than 0.05). As the HLA antigens and Gm allotype genes are situated on different chromosomes (C6 and C14) two distinct gene regions are identified in myasthenia gravis. These are associated with quantitative differences in acetylcholine receptor antibody titres.

Autoantibodies↗
PubMed1981

Cerebral potentials during skilled slow positioning movements.

Movement-related potentials recorded from the scalp of man were investigated in two skilled positioning tasks, requiring flexions at different joints of the upper extremities. The average response time was approximately 1 sec. Subjects paced their movements themselves and performed without visual control or other external cues. After each trial a delayed visual feedback was given. It was found that the negative potential shift prior to the EMG onset, the 'Bereitschaftpotential', is followed by a persisting negativity during the execution of the action until the target position is reached. Approximately at this point a positive-going deflection appears. This 'goal-directed movement potential' is composed of at least two components: (a) a widely distributed, centrally dominant negativity, and (b) a smaller negative wave over the sensorimotor cortex contralateral to the responding limb. Small variations in response force do not influence the amplitude of the potentials. A negative shift in anticipation of the visual feedback has a topography different from the movement-related potentials, being predominant over the right hemisphere independent of the hand used

Adult↗
PubMed1979

Cerebral potentials during smooth goal-directed hand movements in right-handed and left-handed subjects.

During smooth goal-directed hand movements a negative potential shift can be recorded from the human scalp. This goal-directed movement potential (GDMP) is preponderant over central areas with a maximum at the vertex and, over the motor cortex, contralaterally larger than ipsilaterally to the moving hand. In 11 right-handed and 6 left-handed subjects, the morphology and distribution of these potential shifts were studied in relation to task differences, moving hand and handedness. The results show a functional differentiation of two components of the GDMP: (1), a lateralized, slow negative wave, restricted to the precentral area and selectively varying with the hand used, and (2), a widespread, bilateral symmetrical component, selectively influenced by task-specific and individual factors. Two effects reflect an influence of the subject's handedness on the GDMP: (A) averaged for both hand conditions, there are larger amplitudes over the hemisphere contralateral to the dominant hand, and (b), averaged for both hemispheres, larger amplitudes result from using the non-dominant hand.

Adolescent↗
PubMed1979

Relationships between the late component of the contingent negative variation and the bereitschaftspotential.

The purpose of this study was to determine whether the late component of the contingent negative variation (CNV) in a S1-S2-respond paradigm shares critical features with the Bereitschaftspotential (BP) prior to self-paced voluntary movements. In a group of 8 subjects, the late CNV and the BP exhibited: (a) similar effects of response speed variation, (b) corresponding influences of subjective factors, and (c) a similar scalp distribution with the exception that the BP was much more lateralized. In contrast, no such relationships were found between early CNV and BP.

Brain↗