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Biomedical subjects

S F Farmer

Publications and source records attributed to S F Farmer.

At least 19 recordsLinked to original sources
PubMed2001

Physiological tremor in human subjects with X-linked Kallmann's syndrome and mirror movements.

Human physiological tremor consists of mechanical-reflex and neurogenic components. The origin of the neurogenic component, classically detected in the frequency range 7-12 Hz, has been much debated. We have studied six subjects with X-linked Kallmann's syndrome (XKS) and mirror movements. In these subjects unilateral magnetic brain stimulation results in abnormal bilateral EMG responses. Furthermore, abnormal sharing of central nervous inputs between the left and right motoneurone pools results in both abnormal motor unit synchronisation between left and right EMGs and abnormal sharing of long but not short-latency cutaneomuscular reflexes. XKS subjects with mirror movements thus provide a model for studying the central origin of physiological tremor. During sustained co-extension of the left and right index fingers, simultaneous finger tremor and extensor indicis (EI) EMGs were recorded and cross-correlated. The tremor and EMG signals were also subjected to time and frequency domain analysis.Results of frequency domain analysis between ipsilateral finger tremor and EI EMG were similar for both control and XKS subjects. However, in contrast to the controls, short-term synchronisation of left and right EI motor unit activity and significant coherence between left and right EMG, left and right tremor, left EMG and right tremor and right EMG and left tremor were found in XKS subjects. The frequency range (6-40 Hz) and coherence values between left and right were similar to ipsilateral coherence values of rectified EMG and tremor. These data provide strong evidence to support the hypothesis that the neurogenic component of physiological tremor is supraspinal in origin and ranges from 6 to 40 Hz.

Adult↗
PubMed2001

How broad is the phenotype of Hallervorden-Spatz disease?

Magnetic resonance imaging (MRI) has enabled ante mortem diagnosis of Hallervorden Spatz disease (HSD). Childhood-onset cases are the most common type and usually present with progressive dystonia and dementia. The duration of illness is 15 to 20 years, leading to death. Presentation in adulthood and infancy have also been reported, however again the progression is usually inexorable. We present a 30-year-old woman who developed cognitive and motor developmental delay from the age of 8 months. There was further cognitive decline in her late teenage years with seizures and then more recent motor decline with dystonia. The imaging appearance was of iron deposition in the globus pallidus and substantia nigra leading to a diagnosis of HSD. The increased availability of MRI has allowed more cases of HSD to be diagnosed in life but as our case illustrates classification of the disease may need to be further examined.

Adult↗
PubMed2001

Isolated familial hypomagnesaemia with novel neurological features: causal link or chance concurrence?

We report a patient with isolated familial hypomagnesaemia with hypocalciuria, a rare congenital disorder of magnesium metabolism. During adolescence the patient developed neurological and ophthalmological features not hitherto reported in this condition, including seizures, myoclonus, and retinal pigmentary degeneration. These suggested the phenotype of mitochondrial disease, which has been occasionally reported in association with hypomagnesaemia, but subsequent investigations of mitochondrial function were normal. The pathogenesis of this unusual neurological and ophthalmological syndrome therefore remains uncertain.

Adult↗
PubMed2000

Coherence between low-frequency activation of the motor cortex and tremor in patients with essential tremor.

BACKGROUND: In healthy people, rhythmic activation of the motor cortex in the 15-30 Hz frequency range accompanies and contributes to voluntarily-generated postural contractions of contralateral muscle. In patients with Parkinson's disease, an abnormal low-frequency activation of the motor areas of the cortex occurs and has been directly linked to the characteristic 3-6 Hz rest tremor of this disease. We therefore investigated whether the motor cortex is involved in the transmission of the rhythmic motor drive responsible for generating essential tremor. METHODS: Non-invasive recordings of activity from the hand area of the motor cortex were made from six patients with essential tremor by magnetoencephalography. The recordings were made simultaneously with the electromyogram recorded from contralateral finger muscles during periods of postural tremor. A statistical spectral analysis was done to determine at which frequencies the two signals were correlated. FINDINGS: Spectral analysis of the electromyogram signals showed a significant low-frequency component at the frequency of the tremor bursts. However, there was no coherence between magnetoencephalogram and electromyogram recordings at the tremor frequency, indicating that no correlation existed between the tremor signal and low-frequency activity recorded from the primary motor cortex in individuals with essential tremor. Coherence at frequencies higher than the tremor frequency was similar to that in healthy individuals performing voluntary postural contractions. INTERPRETATION: The absence of significant coherence between the magnetoencephalogram and electromyogram at tremor frequencies suggests that in essential tremor the tremor is imposed on the active muscle through descending pathways other than those originating in the primary motor cortex. These findings challenge the model widely used to explain the efficacy of neurosurgical treatment of essential tremor, are in contrast to those of previous studies of parkinsonian rest tremor, and highlight an important difference in the pathophysiology of essential and parkinsonian tremor.

Adolescent↗
PubMed2000

Myelopathy following influenza vaccination in inflammatory CNS disorder treated with chronic immunosuppression.

We report a patient who developed a transverse myelitis with Brown-Séquard syndrome following a prophylactic influenza vaccination, despite being chronically immunosuppressed for a steroid-responsive optic neuropathy. Although influenza vaccination is recommended in patients receiving chronic immunosuppression, its use may on occasion be associated with neurological complications previously reported in immunocompetent individuals.

Adult↗
PubMed1999

The unilateral and bilateral control of motor unit pairs in the first dorsal interosseous and paraspinal muscles in man.

1. The discharges of two motor units were identified in an intrinsic hand muscle (first dorsal interosseous, FDI) or an axial muscle (lumbar paraspinals, PSP) in ten healthy subjects. Each motor unit was situated in the homologous muscle on either side of the body (bilateral condition) or in the same muscle (ipsilateral condition). The relationship between the times of discharge of the two units was determined using coherence analysis. 2. Motor unit pairs in the ipsilateral FDI showed significant coherence over the frequency bands 1-10 Hz and 12-40 Hz. Motor units in the ipsilateral PSP were significantly coherent below 5 Hz. In contrast there was no significant coherence at any frequency up to 100 Hz in the bilateral FDI condition and only a small but significant band of coherence below 2 Hz in the bilateral PSP condition. 3. Common drive to motor units at frequencies of < 4 Hz was assessed by cross-correlation of the instantaneous frequencies of the motor units. A significantly higher coefficient was found in the ipsilateral FDI, ipsi- and bilateral PSP compared with shifted, unrelated data sets. This was not the case for the bilateral FDI condition. 4. The presence of higher frequency coherence ( > 10 Hz) in the ipsilateral FDI condition and its absence in ipsilateral PSP is consistent with a more direct and influential cortical supply to the intrinsic hand muscles compared with the axial musculature. The presence of low frequency drives (< 4 Hz) in the bilateral PSP condition and its absence in the bilateral FDI condition is consistent with a bilateral drive to axial, but not distal, musculature by the motor pathways responsible for this oscillatory input.

Adult↗
PubMed1999

Common 3 and 10 Hz oscillations modulate human eye and finger movements while they simultaneously track a visual target.

1. A 10 Hz range centrally originating oscillation has been found to modulate slow finger movements and anticipatory smooth eye movements. To determine if an interaction or linkage occurs between these two central oscillations during combined visuo-manual tracking, frequency and coherence analysis were performed on finger and eye movements while they simultaneously tracked a visual target moving in intermittently visible sinusoidal patterns. 2. Two different frequencies of common or linked oscillation were found. The first, at 2-3 Hz, was dependent on visual feedback of target and finger tracking positions. The second, at around 10 Hz, still occurred when both target and finger positions were largely obscured, indicating that this common oscillation was generated internally by the motor system independent of visual feedback. Both 3 and 10 Hz oscillation frequencies were also shared by the right and left fingers if subjects used these together to track a visual target. 3. The linking of the 10 Hz range oscillations between the eyes and finger was task specific; it never occurred when eye and finger movements were made simultaneously and independently, but only when they moved simultaneously and followed the target together. However, although specific for tracking by the eyes and fingers together, the linking behaviour did not appear to be a prerequisite for such tracking, since significant coherence in the 10 Hz range was only present in a proportion of trials where these combined movements were made. 4. The experiments show that common oscillations may modulate anatomically very distinct structures, indicating that single central oscillations may have a widespread distribution in the central nervous system. The task-specific manifestation of the common oscillation in the eye and finger suggests that such mechanisms may have a functional role in hand-eye co-ordination.

Adult↗
PubMed1999

Coherent cortical and muscle discharge in cortical myoclonus.

There is increasing evidence in man that the cortical drive to motor neurons is rhythmic. This oscillatory drive may be exaggerated in patients with cortical myoclonus. Spectral analysis of surface bipolar EEG and EMG activity was performed in eight such patients. Only three cases had evidence of giant cortical evoked potentials or a cortical correlate on back-averaging at the time of study. In six subjects, significant coherence between contralateral and vertex EEG and EMG was observed in ranges similar to that previously reported for normal subjects (15-30 and 30-60 Hz). Three out of these six subjects also had significant coherence at higher frequencies (up to 175 Hz). All eight patients had a correlate in the cumulant density estimate between EEG and contralateral EMG. EMG lagged EEG by about 14, 25 and 35 ms for the muscles of the forearm, hand and foot, respectively. These delays were estimated from the slope of the phase curves and the timing of the peaks in the cumulant density estimates, and are appropriate for conduction in fast pyramidal pathways. The results provide clear evidence of a cortical drive synchronizing muscle discharge over a broad range of frequencies in patients with cortical myoclonus. Fourier analysis is a promising technique in the diagnosis and investigation of such patients.

Adolescent↗
PubMed1999

Load-independent contributions from motor-unit synchronization to human physiological tremor.

This study describes two load-independent rhythmic contributions from motor-unit synchronization to normal physiological tremor, which occur in the frequency ranges 1-12 Hz and 15-30 Hz. In common with previous studies, we use increased inertial loading to identify load-independent components of physiological tremor. The data consist of simultaneous recordings of tremor acceleration from the third finger, a surface electromyogram (EMG), and the discharges of pairs of single motor units from the extensor digitorum communis (EDC) muscle, collected from 13 subjects, and divided into 2 data sets: 106 records with the finger unloaded and 84 records with added mass from 5 to 40 g. Frequency domain analysis of motor-unit data from individual subjects reveals the presence of two distinct frequency bands in motor-unit synchronization, 1-12 Hz and 15-30 Hz. A novel Fourier-based population analysis demonstrates that the same two rhythmic components are present in motor-unit synchronization across both data sets. These frequency components are not related to motor-unit firing rates. The same frequency bands are present in the correlation between motor-unit activity and tremor and between surface EMG activity and tremor, despite a significant alteration in the characteristics of the tremor with increased inertial loading. A multivariate analysis demonstrates conclusively that motor-unit synchronization is the source of these contributions to normal physiological tremor. The population analysis suggests that single motor-unit discharges can predict an average of 10% of the total tremor signal in these two frequency bands. Rectified surface EMG can predict an average of 20% of the tremor; therefore within our population of recordings, the two components of motor-unit synchronization account for an average of 20% of the total tremor signal, in the frequency ranges 1-12 Hz and 15-30 Hz. Our results demonstrate that normal physiological tremor is a complex signal containing information relating to motor-unit synchronization in different frequency bands, and lead to a revised definition of normal physiological tremor during low force postural contractions, which is based on using both the tremor spectra and the correlation between motor-unit activity and tremor to characterize the load-dependent and the load-independent components of tremor. In addition, both physiological tremor and rectified EMG emerge as powerful predictors of the frequency components of motor-unit synchronization.

Adult↗
PubMed1998

Rhythmicity, synchronization and binding in human and primate motor systems.

This review focuses on recent advances in our understanding of temporal pattern coding in the motor systems of animals and man. Examples of millisecond time scale rhythmic synchronization in the visual system are considered. Results of experiments that demonstrate similar phenomena in the motor system are discussed. Finally problems concerning the nature of the correlation between neurophysiological signals and the relationship of correlation to motor behaviour are explored.

Animals↗
PubMed1998

Using electroencephalography to study functional coupling between cortical activity and electromyograms during voluntary contractions in humans.

Previous studies of neuronal oscillations in sensorimotor cortex in humans and primates have observed rhythmic 15-30 Hz activity, which is correlated with motor output. In humans, this work has been limited to magnetic recordings. In the present study we investigate if similar results can be obtained using electroencephalography (EEG). EEG recordings were made from over the sensorimotor cortex of five adult subjects who performed repeated periods of maintained wrist extension and flexion. Coherence analysis between EEG and electromyogram (EMG) recordings from these muscles revealed correlation in the 15-30 Hz range, with a synchronous correlation structure which matches that previously observed in humans and in paired cortical recordings from primates. We conclude that EEG is equally efficient at investigating functional aspects of these cortical rhythms during voluntary movement in humans.

Adult↗
PubMed1998

Abnormal motor unit synchronization of antagonist muscles underlies pathological co-contraction in upper limb dystonia.

The aim of this study was to examine the pathophysiological mechanisms underlying co-contraction in patients with dystonia (n = 6) and writer's cramp (n = 5). Multi-unit needle and surface EMGs were recorded from extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles during motor tasks that elicited dystonia or writer's cramp. The EMGs from ECR and FCR were recorded simultaneously and analysed using cross-correlation analysis. Similar recordings were obtained from healthy age- and sex-matched control subjects (n = 8). Despite co-contraction of the muscles, cross-correlograms from the healthy subjects did not reveal evidence of motor unit synchronization. Cross-correlograms from the dystonic subjects revealed a central peak with a median duration of 37 ms, indicating broad-peak motor unit synchronization. Cross-correlograms from patients with writer's cramp were either flat or modulated by a 11-12-Hz tremor. Frequency-domain analysis of ECR and FCR EMGs demonstrated significant coherence in the patients with dystonia and writer's cramp. These results indicate that co-contraction in dystonia is neurophysiologically distinct from voluntary co-contraction and is produced by abnormal synchronization of presynaptic inputs to antagonist motor neuron pools. ECR and FCR co-contraction in writer's cramp may be a compensatory process under voluntary control.

Adolescent↗
PubMed1998

Upbeat nystagmus: clinicoanatomical correlation.

A patient is reported on with isolated upbeating nystagmus with a linear slow phase in whom a solitary lesion, probably inflammatory, was detected radiologically in the dorsal paramedian caudal medulla, encompassing the most caudal of the perihypoglossal nuclei, the nucleus intercalatus of Staderini. The conjunction of a vestibular pattern of nystagmus with this focal lesion runs contrary to a previous suggestion that the nucleus intercalatus may act as a neural integrator for vertical conjugate eye movements.

Adult↗
PubMed1997

A review of recent applications of cross-correlation methodologies to human motor unit recording.

This article reviews some recent applications of time and frequency domain cross-correlation techniques to human motor unit recording. These techniques may be used to examine the pre-synaptic mechanisms involved in control of motoneuron activity during on-going motor tasks in man without the need for imposed and artificial perturbations of the system. In this review we examine, through several examples, areas in which insights have been gained into the basic neurophysiological processes that bring about motoneuron firing in man and illustrate how these processes are affected by central nervous system pathology. We will demonstrate that synchronization and coherence may be revealed between human motor unit discharges and give examples that support the hypothesis that these phenomena are generated by activity in a focused common corticospinal input to spinal motoneurons. Disruption of central motor pathways due to diseases of the nervous system leads to pathophysiological alterations in the activity of these pre-synaptic motoneuron inputs that can be revealed by cross-correlation analysis of motor unit discharges. The significance of these studies and outstanding questions in this field are discussed.

Electromyography↗