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Biomedical subjects

N Drasdo

Publications and source records attributed to N Drasdo.

At least 19 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↗
PubMed1989

Distortion of spatial selectivity by pattern onset stimulation.

Spatial selectivity of pattern evoked potentials has been thought to provide evidence of lateral inhibition. However, spatial tuning functions may be distorted by pattern onset stimulation that is applied repeatedly to the same area of retina so an after-image is formed. This only applies at low spatial frequencies because of the randomizing effects of eye movements. Low-frequency attenuation may therefore be exaggerated. Pattern reversal stimulation has the opposite effect and this is reflected in the literature by fewer reports of bandpass functions. A new method has therefore been devised to provide the true spatial response function. The spatial phase is reversed after every two consecutive presentations. By combining this paradigm with a correction for the optical transfer function of the eye, the true neural response function is obtained. Ten subjects participated in this study to evaluate the distortion of spatial selectivity in the pattern electroretinogram. The new stimulus paradigm reduced the low spatial frequency attenuation to a barely significant level giving an almost flat amplitude response for the + ve and - ve transients of the pattern electroretinogram for check sizes from 222' to 7' angular subtense. However, correction for optical degradation produces bandpass curves, which closely correspond to those predicted from recent data on receptive fields of primate retinal ganglion cells.

Adult↗
PubMed1989

The effect of 0.5% thymoxamine on the pattern-onset electroretinogram.

Thymoxamine constricts the pupil without significant effect on accommodation and was used in this study because of the extreme sensitivity of the pattern electroretinogram (PERG) to defocus at high spatial frequencies. Pilocarpine, used in previous studies, can cause an unpredictable spasm of accommodation. A reduction in pupil size has two effects on the PERG due to illuminance and optical degradation. Illuminance differences can be compensated by neutral density filters. This allows the effects of optical degradation on the PERG to be examined under conditions of constant adaptation. Our studies on a group of 10 subjects have shown the PERG to be proportional to contrast. As contrast degradation is reduced with a smaller pupil, a larger PERG amplitude is predicted. However the tuning of neural elements under constant adaptation should not alter. Pattern onset ERGs were recorded from a further 10 subjects in response to checkerboards of various spatial frequencies under two conditions, a) with pupils moised by thymoxamine hydrochloride 0.5% and b) with natural pupils. Adaptation level was equated by a 0.4 neutral density filter before the natural pupils. The group-averaged PERGs were analyzed into retinal illuminance and pattern-specific responses. A paired t-test showed the pattern-onset ERG amplitudes to be significantly different in the two conditions, but the computed pattern-specific responses were virtually identical. These results therefore support the theory and method used for the extraction of the pattern-specific response.

Contrast Sensitivity↗
PubMed1989

The effect of stimulus contrast on the latency and amplitude of the pattern electroretinogram.

Our studies have verified that a linear relationship exists between the amplitude of the pattern onset ERG and retinal contrast, but indicate that there is no systematic variation in peak latency with contrast. The optical degradation of high spatial frequency patterns cannot therefore provide an explanation for the variation in peak latency with spatial frequency.

Contrast Sensitivity↗
PubMed1989

Do visibility and colour recognition isopters relate to the distribution of P alpha and P beta ganglion cells of the human retina?

Early perimetrists recognized that the threshold stimulus diameters for visibility and colour recognition differ markedly in peripheral vision. However, we observed that these diameters are identical at the fixation point. This means that these threshold diameters, when plotted against eccentric visual angle, have unequal slopes, perhaps related to recently reported data on estimated magnocellular and parvocellular neural sampling intervals. We therefore collected perimetric data on eleven normal observers, using red stimuli on the Zeiss Jena Type 1 Bowl Perimeter. It was found that the slopes were steeper than those for supposedly corresponding neural data. This could be explained by taking into consideration that (a) the slow rate of stimulus movement allowed parvocellular contribution to visibility; and (b) the possibility of an enhanced foveal representation in the cortical colour processing areas. The clinical significance of these findings is briefly discussed.

Adult↗
PubMed1988

Patterns and contrasts in ophthalmic investigation.

The Snellen test has been the most popular clinical measurement of spatial vision for over a century, but it does not fully express the visual ability of an individual. For more analytical purposes the information capacity of the visual system may be assessed by tests of contrast sensitivity and peripheral vision. The visual system selectively reduces the spatial information content of the visual field to avoid overloading the limited capacity for perception and decision making in the brain. The ways in which this reduction occurs and the processing of spatial information is of interest in many disciplines, and theoretical knowledge has been accelerated by the study of artificial intelligence. These processes may be investigated in human subjects by psychophysical and electrophysiological techniques. This provides additional information for diagnostic purposes and will form the basis of new systems of clinical investigation.

Evoked Potentials, Visual↗
PubMed1988

Clinical experience with preferential looking acuity tests in infants and young children.

Early preferential looking methods for the assessment of infant visual acuity relied upon formal psychophysical procedures that were lengthy and only easily applicable to laboratory studies. Two clinically appropriate techniques evaluated in this study both maintain a forced-choice testing protocol and are administered by a single examiner. One method employs a specially constructed optical projection system to present gratings in a dark-room. The second method relies upon a commercially available acuity card test, used with good room lighting. Paediatric clinical patients and normal infants and young children were examined using both techniques. Findings demonstrate that suitably adapted preferential looking methods can provide rapid and valid estimates of visual acuity in infants and young children. The tests appear sufficiently robust to be appropriate for routine clinical use.

Age Factors↗
PubMed1987

Computation of the luminance and pattern components of the bar pattern electroretinogram.

Pattern onset electroretinograms (PERGs) were recorded from four normal subjects. Square-wave gratings of 75% contrast were presented in three approximately contiguous, concentric zones of outer angular radius, 5.1 degrees, 12.6 degrees, and 23.6 degrees. The zones were calculated to give equal numbers of ganglion cell receptive fields. The recorded PERGs were considered to include luminance and pattern components which have low and bandpass spatial tuning functions respectively. These components combine in the PERG to produce a broad spatial tuning characteristic. The amplitude of PERGs in response to low spatial frequency stimuli is widely reported to be linearly related to contrast. The retinal illuminance response at every spatial frequency was computed from the eye's modulation transfer function. This function characterizes the reduction in contrast that occurs because of optical degradation. The computed retinal illuminance response was subtracted from the PERG waveform and a pattern-specific response was revealed. The latter had a highly tuned bandpass function which peaked at higher spatial frequencies than the PERG at corresponding peripheral angles.

Electroretinography↗