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Biomedical subjects

N Drasdo

Publications and source records attributed to N Drasdo.

16 recordsLinked to original sources
PubMed1992

An optical stimulator for studying the topography of electrical and magnetic visual evoked responses.

Visual processing areas cover more than 50% of the cortex in primates, but in humans only about half of this area is projected on gyral crests and thus readily accessible to investigation by evoked potential mapping. However, neuromagnetometry does not reflect the activity of these radial dipoles, but instead it strongly represents tangential dipoles, which may arise from activity within the sulci. It follows that the full exploration of the visual areas can only be achieved by combining these complementary techniques. Software-controlled video stimulators are almost universally used for evoked potential recording, but they generate troublesome interference during neural magnetometry. A simple optical stimulator was therefore designed to project a stimulus into magnetically shielded rooms from a remote situation. The system is capable of providing a wide variety of visual stimuli, including pattern reversal, onset, movement and progressive adjustment of chromatic and achromatic contrast. Evoked responses to complex images and isoluminant colored gratings were studied to demonstrate the performance of this type of system.

Color Perception↗
PubMed1992

Patients' responses to retinitis pigmentosa.

Forty-eight patients with retinitis pigmentosa completed at home a questionnaire about everyday tasks, and recorded their distance visual acuities on reduced Snellen charts. Respondents were asked to assess their own abilities for seven tasks involving visual search and mobility on a 1 to 4 scale. This revealed the relative difficulty of each task for the whole group. Other questions were open-ended to allow respondents to express their individual visual problems and experiences. Several problems came to light that were not amenable to treatment with low vision aids, but should be considered as a guide to the clinician in counseling.

Activities of Daily Living↗
PubMed1992

Using a binocular field expander on a wide-field search task.

Five patients with constricted visual fields and good central acuity were fitted with an experimental binocular field expander. They undertook a laboratory visual search task in a stationary horizontal 90 degree field with and without the device. Four of five patients adapted relatively quickly to the inherent optical distortions of the field expander. When this binocular aid was worn search efficiency improved significantly with practice; search time was highly correlated (r = -0.92; p less than 0.01) with information channel capacity in the larger visual field.

Adult↗
PubMed1992

Colour, contrast and the visual evoked potential.

Visual evoked potentials exhibit interesting morphological changes when they are elicited by checkerboards of different spatial and chromatic contrast, counterphasing in the foveal and lower macula field. The characteristic, positive wave of the phase-reversal visual evoked potential, for example, is preceded by an increasingly prominent negative peak as luminance contrast progressively increases above 10% and, at isoluminance, the response to red and green checkerboards becomes a predominantly monophasic negative wave. To study the nature of the morphological change we synthesized these waveforms with a computer simulation consisting of Gaussian components. The amplitudes of positive and negative components were altered until the synthesized response was closely similar to the recorded data. These Gaussian components have response characteristics which are identified with those of magnocellular and parvocellular neurones.

Color Perception↗
PubMed1991

Electroretinography as a prognostic indicator or neovascularisation in CRVO.

A prospective study was carried out to compare the efficacy of electroretinography, fundus fluorescein angiography and clinical examination in identifying those at risk of rubeotic glaucoma following central retinal vein occlusion (CRVO). Our preliminary observations are described. The findings suggest a complementary role for electroretinography in the management of CRVO. Particularly significant were interocular differences in 30 Hz flicker latency, the ability to elicit pattern ERGs and the ratio of scotopic and photopic a:b wave amplitudes. Of the clinical measures the depth of the relative afferent pupil defect was a sensitive indicator of rubeosis. Factors of lesser statistical prognostic value included poor visual acuity and the extent of deep retinal haemorrhages. Fundus fluorescein angiography in this study had limited value in predicting those patients at risk of rubeosis.

Electroretinography↗
PubMed1991

Tinted lenses and related therapies for learning disabilities--a review.

Research relating to the use of Irlen tinted lenses and coloured overlays for underachievers is reviewed. Many of the studies were not published in refereed journals and were methodologically poor. The weaknesses of the Irlen argument are discussed, including the absence of evidence to support the claims that these tints need to be uniquely prescribed and manufactured. Syntonics is another form of visual colour therapy that has been applied to those with a learning disability. Research on this is reviewed, and is also shown to have procedural irregularities which preclude firm conclusions. Owing to the poor quality of much of this research the claims of the protagonists of these therapies cannot be proved or disproved. A proposed new therapy is normally preceded by a valid theoretical hypothesis; this has been lacking in the present topic. Recently, a feasible explanation has been proposed in terms of 'pattern glare' resulting from mild hypersensitivity to epileptogenic patterns. This, together with potential alternative theories, is discussed. In the conclusion of this review, advice is given for eye-care practitioners who may be consulted on these therapies.

Adolescent↗
PubMed1991

A simple parametric model of the human ocular modulation transfer function.

The ocular modulation transfer function (MTF) has an important role in many investigations into human vision. It can be approximated by the equation MT = exp [-(SF/SFc)n], where MT, SF, SFc and n are the modulation transfer, spatial frequency, frequency constant and MTF index respectively. In a theoretical study, the parameters n and SFc were determined by a mathematical method from previously published data. A general equation was deduced expressing the MTF as a function of pupil size for diameters of 1.5-7.5 mm. For pupil diameters below 1.5 mm different equations providing MTFs as a function of pupil diameter have been produced. These are based on the assumption that for these pupil diameters diffraction and retinal scatter determine the optical degradation. The use of these models is described in studies on spatial vision, where they have widespread applications and the effects of optical degradation are frequently neglected.

Contrast Sensitivity↗
PubMed1990

Review of ophthalmic factors in dyslexia.

Dyslexia and the effect of visual problems on reading ability have long been subjects of controversy. This paper critically reviews the literature on the relationship between reading disability and visual acuity, refractive error, binocular vision anomalies, eye movements, ocular pathology and the transient and sustained visual systems. The limitations of much of the work in this field are described, with particular reference to the problems of subject selection and group matching. Tentative conclusions, with deference to the complex nature of dyslexia, are drawn and suggestions are made for future research.

Accommodation, Ocular↗
PubMed1990

Efficiency in reading with closed-circuit television for low vision.

A closed circuit television (CCTV) magnifier was used to measure the reading rates of nine fluent low-vision subjects. Performance was found to be dependent on field width, character size, and the number of characters displayed in line. A field width of 70 degrees and a mean of 24 characters were commensurate with the fastest reading rate. The optimal character size that elicited the fastest reading was determined for each subject. In seven of the nine subjects, this differed from the character size selected by chance at their initial CCTV sessions, prior to these experiments. A clinical procedure is recommended, which avoids inappropriate choice, and guides the examiner in achieving directly the most efficient CCTV reading conditions for each individual patient.

Adult↗
PubMed1990

The influence of age on the pattern-reversal electroretinogram.

Pattern reversal electroretinograms (PERGs) in response to 30' checks and 6 degree checks were recorded from 60 eyes of 30 normal healthy individuals aged between 20 and 78 years. Results were assessed to show age changes in the amplitudes and latencies of the PERGs, the ratio of 30' to 6 degree PERG amplitude, the 'N95/P50' ratio and the inter-eye ratio of the 30' PERG. The positive component of the PERG decreases in amplitude and increases in latency with increasing age. In addition, it was found that the 30' PERG/6 degree PERG ratio decreases, the N95/P50 ratio increases and the inter-eye ratio shows a greater difference between the two eyes in the older subjects.

Adult↗
PubMed1978

Electroretinography with noncorneal and corneal electrodes.

The noncorneal ERG is essential for certain patients in which corneal recording is contraindicated. A preliminary report is presented investigating ERGs from three noncorneal montages and their correlation with the corneal ERG. Statistical analysis showed that an electrode at the medial canthus combined with medial viewing gave the closest correlation to the corneal ERG. Repeatability was good for all montages. The inner canthus electrode was considered preferable for noncorneal recording because it was less affected by other time-locked activity; it correlated well with the corneal ERG and can be used in conjunction with the electro-oculogram.

Adolescent↗
PubMed1978

A pilot study on the use of visual field expanders.

The nature and magnitude of the visual difficulties of five volunteer patients with severely contracted visual field and high visual acuities was investigated by several techniques. These comprised a questionnaire providing a quantitative score of visual impairment, a specially devised visual search task, and an arbitrary method of estimating visual efficiency. Many aspects of visual impairment resulting from contract visual fields were revealed and discussed. Performance scores on the search task indicated that the field expander would probably be useful to a small number of cases for continuous wear, usually as a binocular bioptic system. Other cases might benefit from a clip-on or hand-held device for occasional use. Methods of manufacture and dispensing and difficulty in locating suitable patients were the major limiting factors in this study. The methods of assessment, and the optical system described in this study are worthy of further investigation and development for trial on a larger number of patients.

Eyeglasses↗
PubMed1976

Visual field expanders.

Diminishing telescope systems designed to widen the field of view of patients with tunnel vision are described. Theoretical analysis suggests that telescope systems will greatly improve visual search times and diminish the difficulties of such patients. This can be considered quantitatively, and it is suggested that the optimum amount of minifying power can be calculated by applying established formulations on visual efficiency. The inadequacies of previous designs have inhibited development, and further improvements to bioptic systems may be necessary before the final assessment of the clinical value of these devices can be made.

Eyeglasses↗
PubMedDate not supplied

Coordinate systems for evoked potential topography.

Two coordinate systems for specifying the position of a point on the scalp are described. The coordinates resemble those of conventional cartography, but are related to irregular solids by using proportional curvilinear surface units instead of solid angular geometry. Cylindrical projections convert the coordinate lines into rectangular graticules for isopotential maps of sensory evoked responses. Visual and somatosensory data are presented to illustrate the techniques. The practical advantages of these principles applied to contemporary developments in colour contour displays are discussed.

Brain Mapping↗