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G F Harding

Publications and source records attributed to G F Harding.

At least 73 records · Page 4Linked to original sources
PubMed1986

Source derivation of the visually evoked potential.

There has been much recent conjecture over the apparent source location of the visually evoked potential, particularly in considering the lateralization found in halffield pattern reversal stimulation. This paper presents an alternative by directly identifying the sinks or sources present in the scalp current flow as these correspond to the maximal current flow into or out of the cortex (Nunez, 1981). Hjorth (1975) introduced a practical procedure for source derivation which has proved useful in the interpretation of the electroencephalogram (Wallin and Stalberg, 1980). We have applied source derivaton to cortical evoked potentials and present results obtained from right and left half-field stimulation, (Clement, Flanagan and Harding, 1985) and upper and lower field stimulation in normal subjects and patients with visual deficits.

Cerebral Cortex↗
PubMed1986

Visual acuity and the flash visually evoked cortical potential in albinos.

The flash visually evoked cortical potential (VECP) was recorded in 18 human albinos. In some individuals P2 (latency 100-120 msec) formed the major component while in others P1 (latency 60-80 msec) was the largest component and dominated the response. The relative amplitude of P1 compared to P2 was calculated, and the results compared to the levels of monocular Snellen Visual Acuity measured. A relationship was found such that the greater the amplitude of P1 compared to P2, the lower the level of acuity. (Group 1: rs = -0.509, P less than 0.05. Group 2: rs = -0.536, P less than 0.05).

Adolescent↗
PubMed1985

The influence of age on the electroretinogram and visual evoked potential.

Changes in the ERG and VEP across the life span were investigated. The dark adapted and scotopic ERGs both showed a progressive increase in the implicit times of the A and B waves and a reduction in the amplitude of the AB configuration. There was also an increase in the implicit times of the oscillatory potentials of the photopic ERG. The flash and pattern onset-offset VEP both showed changes in waveform with age whilst the waveform of the pattern reversal VEP was constant. The amplitudes of the components of the flash and pattern reversal VEP were very high in the teenage group, but once reduced, were constant from the twenties onwards, showing no further consistent age changes. The latencies of the components of the pattern VEPs showed an increase with age which could be accounted for by the reduction in retinal illuminance due to the decrease in pupil diameter with age. However, the increase in the latency of the flash major positive (P2) component was greater than that expected from the decrease in retinal illuminance alone, suggesting that this is due to neural factors.

Adolescent↗
PubMed1985

Lateralisation of the flash visually evoked cortical potential in albino babies.

Flash VECPs have been reported in 3 albino babies ranging in age from 1 to 7 months. The responses were dominated by a large positive component preceded by an earlier negative. The amplitude of this positive component measured from the peak of the preceding negative wave had a greater amplitude over the contralateral hemisphere on monocular stimulation indicative of the misrouting present in the albino visual system.

Albinism↗
PubMed1985

Primary presenile dementia: the use of the visual evoked potential as a diagnostic indicator.

The use of the flash and pattern reversal visual evoked potential (VEP) in the diagnosis of primary presenile dementia was investigated. The results from 20 patients with primary presenile dementia were compared with those from a control group of normals of equivalent age and from a control group of 20 patients with cortical atrophy but no dementia. Presenile dementia caused a slowing of the major positive (P2) component of the VEP to flash stimulation. However, the VEP to pattern reversal stimulation (P100) was of normal latency. The difference between these two latencies characterises this unusual combination of results and is found to be a more specific diagnostic indicator of primary presenile dementia than the EEG or CT scan.

Aged↗
PubMed1985

Brainstem auditory evoked potentials in chronic renal failure and maintenance hemodialysis.

Brainstem auditory evoked potentials (BAEPs) were recorded in 36 chronic uremic patients maintained on hemodialysis and in 10 normal controls. Both absolute response latencies and central transmission times were affected. There were transitory increases in interpeak latencies of I-II and III-V that were reversed by dialysis. A permanent increase in wave III-V interpeak latency was not reversed by dialysis. Wave II-III interpeak latency was unaffected.

Adult↗
PubMed1984

Presenile dementia--the use of the flash and pattern VEP in diagnosis.

Flash, pattern reversal and pattern onset-offset VEPs were recorded in 17 patients with a clinical diagnosis of presenile dementia. The results were compared with those from two control groups--17 normal volunteers of equivalent ages and 17 patients from the same psychiatric hospital with affective disorders but no clinical evidence of dementia. The flash P2 component was found to be significantly delayed in the group of patients with dementia, but the latency of the P1 component and pattern VEP components were all within normal limits. This unusual finding presents a useful index for the diagnosis of presenile dementia.

Aged↗
PubMed1984

Lateralization of the flash visual-evoked cortical potential in human albinos.

Investigations were undertaken to determine which particular components of the flash visual-evoked cortical potential (VECP) lateralize on monocular stimulation in human albinos. Our results demonstrate that only the major positive component shows clear lateralization. Statistical analysis revealed that the latency lateralization was significant (P less than 0.001) while the amplitude lateralization was not.

Adolescent↗
PubMed1981

Visual and auditory evoked potentials in a case of Marchiafava Bignami Disease.

Brainstem auditory evoked potentials and visual evoked potentials to both flash and pattern reversal stimulation were recorded in a case of clinically diagnosed Marchiafava Bignami Disease (M.B.D.) A CT scan revealed dilation of the supratentorial ventricular system and fairly extensive cortical atrophy. The brainstem potentials showed a non-specific abnormality of the N4 component, (in the region of the caudal pons) and were inconclusive. The visual evoked potentials to flash stimulation were delayed while pattern reversal responses were within normal limits.

Adult↗
PubMed1981

The visually evoked subcortical potential: is related to the electroretinogram?

The visually evoked subcortical potential (VESP) of mean latencies P21-N28-P36 has previously been recorded at an electrode site around the mastoid process. An initial topographical study of the potential indicated that it was independent of the electroretinogram (ERG), and monocular stimulation showed bilateral reduction, which suggests that the VESP is of postchiasmal origin. A more detailed topographical study of the scalp and facial distribution of the ERG and its oscillatory potentials has been carried out, with simultaneous recording of the VESP. Two monocular fields of ERG activity have been demonstrated, each having a wide area of distribution and interacting medially. Remnants of the ERG b-wave have been detected at many electrode sites, but they were of different amplitude and morphology from the VESP. Results are also presented from two atypical control subjects and two patients, providing further evidence of the separate genesis of the ERG and VESP.

Adult↗
PubMed1980

The scalp topography of the human visually evoked subcortical potential.

Stimulus and analysis parameters have been adjusted to provide optimum conditions for producing and recording the early components of flash visual evoked potentials. A visual evoked subcortical potential (VESP) of mean latency P23-N28-P34 has been recorded in 86% of subjects. The triphasic wave was maximal at an electrode position slightly posterior to the Rolandic/sylvian fissure and topographically separate from the lid electroretinogram and the visual evoked cortical potential. Monocular stimulation shows bilateral reduction of the amplitude of the VESP, indicating that the wave is independent of the retina and optic nerve and must be arising from a postchiasmal site.

Adult↗
PubMed1979

Television epilepsy--the role of pattern.

Patients with photosensitive epilepsy were asked to view normally functioning 625-line televisions while the EEG was monitored. In the first of two studies paroxysmal EEG activity was reliably induced by television at a viewing distance related to a patient's sensitivity to intermittent photic stimulation (IPS); patients who were sensitive to diffuse IPS at 50 Hz were sensitive to the television at greater viewing distances than those who were not. No such relationship was obtained with patterned IPS. On the other hand, patterned IPS was generally more epileptogenic than diffuse IPS with the same luminance. In the second study, where the angular subtense of the television screen and the subtense of its lines were manipulated independently, the convulsive response was found to be a function of both factors, the relative contribution of each depending on the viewing distance at which the patient was sensitive. For patients sensitive at normal viewing distances, where 50 Hz diffuse flicker appeared to be responsible for the induction of paroxysmal activity, the probability with which paroxysmal activity was induced was closely related to the subtense of the screen. For patients sensitive only at closer viewing distances the probability was influenced not by the subtense of the screen but by the subtense of its lines, suggesting that the paroxysmal activity was induced by the 25 Hz pattern alternation produced by the scan. A television with a small screen was considerably less epileptogenic than one with a large screen for all patients, presumably due to the reduced contribution of both diffuse flicker and pattern alternation.

Adolescent↗