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Biomedical subjects

G F Harding

Publications and source records attributed to G F Harding.

At least 55 records · Page 3Linked to original sources
PubMed1994

Biomagnetic methodologies for the noninvasive investigations of the human brain (MAGNOBRAIN).

Magnetoencephalography (MEG) non-invasively infers the distribution of electric currents in the brain by measuring the magnetic fields they induce. Its superb spatial and temporal resolution provides a solid basis for the 'functional imaging' of the brain provided it is integrated with other brain imaging techniques. MAGNOBRAIN is an applied research project that developed tools to integrate MEG with MRI and EEG. These include: (1) software for MEG oriented MRI feature extraction; (2) the Brain Data Base (BDB) which is a reference library of information on the brain used for more realistic and biologically meaningful functional localisations through MEG and EEG; and (3) a database of normative data (age and sex matched) for the interpretation of MEG. It is expected that these tools will evolve into a medical informatics environment that will aid the planning of neurosurgical operations as well as contribute to the exploration of mental function including the study of perception and cognition.

Brain↗
PubMed1993

The involvement of taurine in the action mechanism of sodium valproate (VPA) in the treatment of epilepsy.

Several lines of evidence have shown that sodium valproate (VPA) mechanism of action in the therapy of epilepsy is based on the phenomena of its interaction with neurotransmitters (GABA), receptor sites and ion channels (1). However, there is no conclusive evidence to show the extent of VPA interactions with other neurotransmitters in the brain. Based on this fact, taurine (an amino acid 'neurotransmitter') found distributed in the brain the visual system may probably be involved in the drug action mechanism of VPA. The application of taurine in experimental and human epilepsy started over thirty years ago (2,3) and it has been known to possess some mild anticonvulsant activity in both humans and experimental animal models (4). This review, therefore, will attempt to draw together all the available information on the involvement of taurine in epilepsy and its possible association with the action mechanism of VPA in suppressing epileptic seizures. Structural and physiological distribution of taurine in the brain will be discussed. Its association with the phenomena of VPA action in epilepsy will be cited. Its neurotransmitter candidacy, involvement in ocular pathology, receptor sites and modulatory activity will be dealt with in relation to valproate action in the therapy of epilepsy.

Brain↗
PubMed1992

Visual evoked electrical and magnetic response to half-field stimulation using pattern reversal stimulation.

The visual evoked magnetic response to half-field stimulation using pattern reversal was studied using a d.c. SQUID coupled to a second order gradiometer. The main component of the magnetic response consisted of a positive wave at around 100 ms (P100M). At the time this component was present the response to half-field stimulation consisted of an outgoing magnetic field contralateral and extending to the midline. When the left half field was stimulated the outgoing field was over the posterior right visual cortex and when the right half field was stimulated it was over the left anterior visual cortex. These findings would correctly identify a source located in the contralateral visual cortex. The orientation of the dipoles was not that previously assumed to explain the paradoxical lateralization of the visual evoked potential. The results are discussed in terms of both electrical and magnetic models of the calcarine fissure.

Adult↗
PubMed1991

Topographic mapping and source localization of the pattern reversal visual evoked magnetic response.

The topography of the visual evoked magnetic response (VEMR) to pattern reversal stimulation was studied in four normal subjects using a single channel BTI magnetometer. VEMRs were recorded from 20 locations over the occipital scalp and the topographic distribution of the most consistent component (P100M) studied. A single dipole in a sphere model was fitted to the data. Topographic maps were similar when recorded two months apart on the same subject to the same stimulus. Half field (HF) stimulation elicited responses from sources on the medial surface of the calcarine fissure mainly in the contralateral hemisphere as predicted by the cruciform model. The full field (FF) responses to large checks were approximately the sum of the HF responses. However, with small checks, FF stimulation appeared to activate a different combination of sources than the two HFs. In addition, HF topography was more consistent between subjects than FF for small check sizes. Topographic studies of the VEMR may help to explain the analogous visual evoked electrical response and will be essential to define optimal recording positions for clinical applications.

Adult↗
PubMed1991

Visual evoked magnetic fields to flash and pattern in 100 normal subjects.

The practicality of recording visual evoked magnetic fields in 100 subjects 15-87 yr of age using a single channel d.c. SQUID second order gradiometer in an unshielded environment was investigated. The pattern reversal response showed a major positive component between 90 and 120 msec (P100M) while the response to flash produced a major positive component between 90 and 140 msec (P2M). Latency norms of the P100M were more variable than the corresponding P100 and P2 visual evoked potentials. The latency of the P100M may show a steep increase with age in most subjects after about 55 yr whereas only a small trend of latency with age was detected for the flash P2M.

Adolescent↗
PubMed1991

The influence of age on the pattern and flash visual evoked magnetic response (VEMR).

The visual evoked magnetic response (VEMR) was measured over the occipital cortex to pattern and flash stimuli in 86 normal subjects aged 15-86 years. The latency of the major positive component (outgoing magnetic field) to the pattern reversal stimulus (P100M) increased with age, particularly after 55 years, while the amplitude of the P100M decreased more gradually over the lifespan. By contrast, the latency of the major positive component to the flash stimulus (P2M) increased more slowly with age after about 50 years, while its amplitude may have decreased in only a proportion of the elderly subjects. The changes in the P100M with age may reflect senile changes in the eye and optic nerve, e.g. senile miosis, degenerative changes in the retina or geniculostriate deficits. The P2M may be more susceptible to senile changes in the visual cortex. The data suggest that the contrast channels of visual information processing deteriorate more rapidly with age than the luminance channels.

Adolescent↗
PubMed1990

Visual evoked potential monitoring of optic nerve function during surgery.

A study was made with intra-operative flash--visual evoked potentials (VEP) monitored using a fibre-optic/contact lens photo stimulator in 57 patients undergoing intra-orbital surgical procedures with potential risk to the optic nerve. The VEPs recorded under enflurane and nitrous oxide anaesthesia did not differ significantly in latency or amplitude from the pre-operative recordings. Transient abolition of the VEP was seen under many circumstances and did not correlate with the outcome of surgery, but absence of a previously normal VEP for more than four minutes during surgical manipulation within the orbit did show a correlation with post operative impairment of vision. The technique provides early warning to the surgeon of threats to the integrity of the optic nerve.

Adolescent↗
PubMed1989

The pattern reversal VEP in short-gestation infants on taurine or taurine-free diet.

The visual evoked potential to flash stimulation has been studied in premature infants for many years. There have been no studies of the response to pattern reversal stimuli during the pre-term period, however. Twenty babies all born prematurely between 32 and 35 weeks post menstrual age had pattern reversal visual evoked potentials recorded on at least two occasions between the ages of 32.5 and 52 weeks post menstrual age with the first recording prior to reaching 40 weeks post menstrual age. The stimulator consisted of a small hand-held television with a 55 x 40 mm screen, and responses were averaged by a Cadwell 5200 averager. The check size was 7 X 7 mm and the contrast was 78%. The stimulator was held at a distance of 20 cm from the eyes. The angular subtense of the TV screen was 15.3 degrees X 11.31 degrees. On each occasion flash visual evoked potentials were obtained for comparison, as were flash electroretinograms recorded with a DTL electrode. The infants were randomly assigned to receive breast milk substitute with or without taurine. Pattern reversal responses consisted of a positive component with a mean latency of 320 msec at 32.5 weeks post menstrual age. The negative relationship between age at recording and the latency of the major positive component of the pattern reversal visual evoked potential was significant at p less than 0.0001. There was no apparent difference in the visual responses of the two groups.

Double-Blind Method↗
PubMed1989

The topography of the P1 component of the flash visual evoked response.

The scalp topography of the P1 component of the human flash visual evoked response was investigated by means of the biologic brain mapping system. Thirty subjects, ranging in age from 21 to 84 years, had flash visual evoked responses recorded using the standard 10-20 electrode positions referred to a balanced noncephalic reference. The subjects were divided by age into three groups: young, middle and old. A P2 component was recorded over the occipital region for all three groups and a frontal negative component was found to occur concurrently with the P2 occipital component. Neither the young or the middle age groups showed an identifiable P1 component. However this component was clearly present in the older group at 76 msec. The distribution of this component was more widespread anteriorly than the P2 component. In both the middle and the older age groups an earlier frontal negative component was present at around 75 msec; no such component was recordable in the young. This work suggests that the development of the P1 component during middle age is preceded by the development of a frontal negative component of around the same latency.

Adult↗
PubMed1988

Multi-channel visual evoked potentials in early compressive lesions of the chiasm.

The sensitivity of transient, pattern-reversal visual evoked potentials in the detection of early compressive lesions of the chiasm is controversial in the literature. There have been claims that the technique is capable of detecting an abnormality in the absence of any demonstrable visual field loss, and conversely that VEPs are not reliable for the detection of chiasmal lesions even when a bitemporal hemianopsia is clearly recordable. Using nine patients with pituitary adenoma we attempted to quantify the extent of visual field loss and correlate the diagnostic capabilities of topographically recorded potentials following full- and half-field stimulus presentations of various field and check sizes. Differential light thresholds were measured and quantified according to one investigator's graticule for the neural representation of visual space. Results show a strong correlation between the degree of information loss and the diagnostic value of the visual evoked potential. The technique was, however, capable of detecting abnormality in the absence of recordable field loss when large field and check sizes were used.

Adenoma↗
PubMed1987

A contact lens photostimulator for surgical monitoring.

A haptic contact lens stimulator has been developed to allow monitoring of the visual evoked potential to flash stimulation during orbital surgery. The contact lens stimulator is fitted with a small plastic prism into which a fibre-optic cable is inserted at right angles to the axis of the contact lens. This allows both the stimulator and the fibre-optic to be permanently in place throughout orbital surgery without interfering with the surgeon's vision or dexterity. Using this technique continuous evoked potentials can be obtained allowing immediate feedback to the surgeon of any condition threatening the integrity of the optic nerve. The technique has now been used on over 20 eyes and is commercially available.

Contact Lenses↗
PubMed1987

Pathology of the optic nerve and visual association areas. Information given by the flash and pattern visual evoked potential, and the temporal and spatial contrast sensitivity function.

Spatial and temporal contrast sensitivity functions (CSF) were compared with visual evoked potentials (VEP) in two groups of patients with pathology selectively affecting different parts of the visual pathway. These consisted of 10 patients with a selective delay of the pattern VEP due to demyelination of one optic nerve (unilateral optic neuritis) and 11 patients with a selective delay of the flash VEP due to pathology of the visual association areas (primary presenile dementia, or Alzheimer's disease). The results led to the following conclusions. Comparison of the VEP and CSF results indicated that VEP latency was more closely associated with the temporal CSF than the spatial CSF. Pathology of the visual association areas which selectively increased the latency of the flash P2 component also reduced sensitivity at low and medium temporal frequencies. In demyelination of the optic nerve, a highly significant correlation was found between the delay of the pattern reversal VEP and sensitivity at high temporal frequencies. Comparison of the results in these two types of pathology suggests that the pattern VEP and flash P1 component, the spatial CSF, and temporal CSF at high temporal frequencies are all processed in the geniculostriate pathway. The flash P2 component and temporal CSF at low and medium frequencies were affected differently from the other measures. The possibility that these processes are transmitted by nongeniculate pathways is discussed.

Adult↗
PubMed1986

Serial visual evoked potential recordings in Alzheimer's disease.

Primary presenile dementia slows the major positive component of the visual evoked potential to flash stimulation but does not affect the visual evoked potential to patterned stimulation. The progressive effect of Alzheimer's disease was followed in a 58 year old woman over three and a half years from the development of the earliest symptoms to complete mental incapacity. The pattern reversal visual evoked potential remained normal, but the flash visual evoked potential gradually slowed from 129 ms in 1981 to 153 ms in 1984. The severity of the abnormality of the flash visual evoked potential thus reflected the severity of the dementia. Electroencephalography, computed tomography, and psychometric tests indicated generalised cortical disease, but the results were not specific to dementia. The combination of a slowed flash and normal pattern visual evoked potential seems to be specific to Alzheimer's disease and supports the use of flash and pattern visual evoked potentials in routine diagnostic testing for this condition.

Alzheimer Disease↗
PubMed1986

Visual evoked cortical and subcortical potentials in human albinos.

Albinism is a congenital condition in which hypopigmentation occurs. In addition to this abnormality there is a misrouting of the optic nerve fibers, with some fibers from the temporal retina following a crossed route at the chiasma and terminating in the contralateral cortical hemisphere. This contralateral preponderance of fibers from each eye should be recognizable from recording the visual evoked cortical potential over each hemisphere on monocular stimulation. Such a technique should produce evidence of responses of either increased amplitude or shorter latency over the contralateral hemisphere. Twenty-five human albinos (twenty-three oculocutaneous, two ocular) have been examined. Pattern appearance-disappearance visual evoked cortical potentials were used, but only on bioccipital derivations did these show clear lateralization. With the flash response the P2 component has a consistently shorter latency over the contralateral hemisphere to the eye stimulated. The visual evoked subcortical potential shows contrasting lateralization. The implications of these findings are discussed.

Adolescent↗
PubMed1986

The flash and pattern VEP as a diagnostic indicator of dementia.

Ninety-one patients with forgetfulness, confusion and depression were referred for visual evoked potential and electroencephalographic investigation. The patients were subdivided, on clinical symptoms alone, into a group of 41 with evidence of dementia and a patient control group of 50 with effective disorders. A second control group of 30 normal volunteers of equivalent age was used. In dementia the P2 component of the flash visual evoked potential is delayed, while the pattern reversal P100 component is of normal latency. This unusual combination of results from the two different visual stimuli is shown to be more specific than either the electroencephalogram or computerised tomography in the diagnosis of dementia.

Aged↗