Neville Drasdo

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.

Investigation of accommodative and binocular function in dyslexia

The visual correlates of dyslexia are the subject of controversy, and much evidence suggests that they may include some aspects of binocular and accommodative function. These factors were investigated in 43 control and 39 dyslexic children, who were matched for age, sex and performance intelligence quotient. The dyslexic group exhibited significantly lower positive and negative vergence reserves, and vergence instability when the eyes were dissociated at near. Their amplitudes of accommodation also were significantly reduced. However, other measures including dissociated and associated heterophoria and accommodative lag and facility were similar in both groups. The stability of motor ocular dominance, as assessed with a modified Dunlop test, was similar in both groups. The results of a simulated reading visual search task suggested that the vergence and accommodative dysfunction were not a major cause of the dyslexia. Further analyses, using reading‐age matched groups, suggested that these ocular motor correlates were not attributable to the better reading performance in the control group. The most likely remaining explanation is that they are, in most cases, non‐causal correlates of the dyslexia.

Tinted lenses and related therapies for learning disabilities - a review

Research relating to the use of Irlen tinted lenses and coloured overlays for underachieves is reviewed Many of the studies were not published in refereed journals and were methodologically poor. The weaknesses of the Irlen argument arc discussed, including the absence of evidence to support the claims that these tints need to he 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 lo 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.

Receptive field densities of the ganglion cells of the human retina

In a recent contribution on the topography of the fovea1 cone mosaic in the living human eye, Williams (1988) provides convincing evidence of the separations of fovea1 cones and their distribution in a triangular matrix. However, the data show a significant flattening of the gradient of row separation in the central half degree of the fovea. This is at variance with anatomical and psychophysical observations and may relate to the extended sampling area required for the moire technique, resulting in a slight over estimate of central cone separation. Neglecting this possibility the projected density would be 10,888 cones per solid degree, whereas a continuation of the gradient to the very centre would result in an estimate of the order of 15,000 cones per solid degree. Although it was formerly supposed that the numbers of ganglion cells serving these central fovea1 cones were related to them approximately on a 1: 1 ratio (Polyak, 1941; Missoten, 1974), several independent studies have recently indicated that there are at least two ganglion cells to each of the most central cones (Schein, 1988; Wassle, 1988). This should not cause surprise in the light of current concepts of retinal processing. We might expect that the information acquired by these specialised central cones should be used to the maximum extent, providing the input for the receptive field centre of one “on” and one “off” p /I ganglion cell, a smaller fraction of that for an “on” and an “off” p a cell, and for other forms of ganglion cell. This must inevitably result in an overall ratio in excess of two ganglion cells to one central fovea1 cone. It is difficult to avoid the conclusion that the projected density of ganglion cell receptive fields (0,) for central vision in humans probably exceeds 30,000 receptive fields per solid degree. This density is important in relation to theories of neural representation and to certain psychophysical (Rovamo & Virsu, 1979) and electrophysiological (Drasdo, Thompson, Thompson & Edwards, 1987) experiments.

A COMPARISON OF THE BRITISH NUMBER PLATE AND SNELLEN VISION TESTS FOR CAR DRIVERS

Abstract The British number plate lest and the Snellen lest of visual acuity are compared. The observations of 60 subjects. 30 of whom have failed (he official number plate test, are analysed statistically. It is found possible to csiiiiunt1 the probability of a subject having a specified Snellen acuity, reading a number plate of the 3 1/2 (80 mm) symbol type, at 67 ft, (‐20.5m). It is considered, however, that the probability of passing the official number plate test is a different value and this is also shown to be estimated from the Snellen visual acuity. The calculations based on the sample of candidates who had failed the number plate test suggest that a driving test candidate with a Snellen decimal acuity of 0.55, or approximately ft/122 in clinical notation, has a 50% chance of passing the number plate test, but the level of visual acuity which would fail the same number of candidates as the number plate test is deduced from an approximate population model, to be 0,6, or 6/9+2. Many other quantitative estimates relating the two tests are given in the text and accompanying tables and figures.

Parvocellular Neurons Limit Motion Acuity in Human Peripheral Vision

It is generally believed that the perception of moving targets is mediated by the magnocellular (M) pathway in primate vision, but evidence is emerging that the parvocellular (P) pathway may also play a role in motion perception. Human peripheral vision is susceptible to anomalous motion perception because of spatial aliasing, and in this study we used this fact to determine if the P pathway can mediate information about low- and high-velocity stimuli. Psychometric functions relating visual performance to stimulus spatial frequency were measured for the directional discrimination of drifting sinusoidal gratings presented at 40° eccentricity. Applying the sampling theorem to our results, we estimated that the Nyquist frequency of the limiting sampling array for directional discrimination is 1.7 cycles per degree. This result was compared with the Nyquist limit and spatial filtering properties of M and P ganglion cells in the human peripheral retina, calculated from histological data on their density and dendritic field size. Our results provide evidence to suggest that the reversed motion illusion in human peripheral vision is due to spatial aliasing by the P ganglion cell mosaic. We conclude that the sampling density of P ganglion cells limits veridical motion acuity in human peripheral vision, even for high-velocity targets. This provides further evidence that the P pathway is involved in processing information about motion.

Dyslexia: the link with visual deficits

Some research reports suggest that visual anomalies may have a causative role in dyslexia, and on this basis certain forms of therapy have been proposed. Recently, we have published the initial results of a matched group study which found dyslexia to be associated with binocular instability, reduced amplitude of accommodation, and reduced contrast sensitivity for both low spatial frequencies and uniform field flicker. The binocular instability was best identified by measuring the vergence amplitude: a modified Dunlop test failed to differentiate reliably between the two groups. Here, we report a significant correlation between flicker threshold and binocular instability, thus linking sensory and motor visual correlates of dyslexia. We also present further new analyses on the interaction between optometric variables and the psychometric measurement of coding skills. The results of these analyses disagree with a recent claim that binocular vision anomalies might cause poor performance at coding tasks. Our studies indicate that visual characteristics are not the major aetiological factors in specific reading difficulty.

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.

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.

The magnocellular deficit hypothesis in dyslexia: a review of reported evidence

Many reports suggest that the majority of dyslexic children have a measurable disorder of the fast processing pathway of the visual system. This pathway is believed to extend from the retina to the occipital and parietal areas of the brain, and is referred to as the magnocellular (M) or transient pathway. Evidence in support of the magnocellular deficit theory comes from several sources, but is not totally consistent. Histological studies have revealed shrinkage and disorganisation of M cells in the lateral geniculate nucleus of dyslexic subjects. Psychophysical investigations of visual persistence, contrast sensitivity to moving sine wave gratings and flicker sensitivity, report atypical results in dyslexic children, reflecting an apparent deficiency in the transient system, although not all psychophysical studies have demonstrated such a deficiency. Visual evoked potential responses to a wide range of stimuli have been reported to be deficient in dyslexic subjects, but again there are dissenting papers. These reports have been subject to critical review and analysis, new techniques specifically to stimulate the magnocellular pathway are described, and the relevance of this research to tinted lens therapies and clinical reading problems is discussed.