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Biomedical subjects

M S Fowler

Publications and source records attributed to M S Fowler.

16 recordsLinked to original sources
PubMed2003

Hemispace differences in the visual perception of size in left hemiParkinson's disease.

The visual perception of size in different regions of external space was studied in Parkinson's disease (PD). A group of patients with worse left-sided symptoms (LPD) was compared with a group with worse right-sided symptoms (RPD) and with a group of age-matched controls on judgements of the relative height or width of two rectangles presented in different regions of external space. The relevant dimension of one rectangle (the 'standard') was held constant, while that of the other (the 'variable') was varied in a method of constant stimuli. The point of subjective equality (PSE) of rectangle width or height was obtained by probit analysis as the mean of the resulting psychometric function. When the standard was in left space, the PSE of the LPD group occurred when the variable was smaller, and when the standard was in right space, when the variable was larger. Similarly, when the standard rectangle was presented in upper space, and the variable in lower space, the PSE occurred when the variable was smaller, an effect which was similar in both left and right spaces. In all these experiments, the PSEs for both the controls and the RPD group did not differ significantly, and were close to a physical match, and the slopes of the psychometric functions were steeper in the controls than the patients, though not significantly so. The data suggest that objects appear smaller in the left and upper visual spaces in LPD, probably because of right hemisphere impairment.

Aged↗
PubMed2002

Population dynamic consequences of allee effects.

We take a well-known dynamic model of an isolated, unstructured population and modify this to include a factor that allows for a reduction in fitness due to declining population sizes, often termed an Allee effect. Analysis of the behaviour of this model is carried out on two fronts - determining the equilibrium values and examining the stability of these equilibria. Our results point to the stabilising effect on population dynamics of the Allee effect and an unexpected increase in stability with increased competition due to the interaction between competitive and Allee effects.

Ecology↗
PubMed2002

Dopamine and the representation of the upper visual field: evidence from vertical bisection errors in unilateral Parkinson's disease.

It has been suggested that dopamine is an important neurotransmitter in the brain mechanisms which represent the upper visual field. This idea was tested with a vertical line bisection task in unilateral Parkinson's disease. Stimuli of a range of lengths were presented on a large screen in three positions (left, centre and right) and at two viewing distances (0.6 and 1.5m). The patients, who were compared with a group of normal age-matched controls, comprised 16 sufferers from predominantly unilateral disease, 8 with more severe left-sided symptoms (LPD) and 8 with more severe right-sided symptoms (RPD). The LPD group consistently set the bisecting cursor below the midpoint of the stimulus lines, and their bisection error became larger as the length of the line increased. In contrast, the controls set the cursor above the midpoint of the line, an error which also increased with line length. The settings of the RPD group were similar to those of the controls. The results suggest altitudinal neglect in left unilateral PD, and support the hypothesis of dopaminergic involvement in the coding of upper visual space, with the proviso that the perceptual component of this involves the right hemisphere in humans.

Aged↗
PubMed2001

Disruption of estimation of body-scaled aperture width in Hemiparkinson's disease.

A group of patients with left-sided symptoms of Parkinson's disease (LPD) was compared with a group of patients with right-sided symptoms (RPD) and with a group of healthy age-matched controls on body-scaled judgements of aperture width. Participants judged whether or not they would fit through a life-sized schematic doorway shown on a large screen. A staircase technique was used to find the door width for which 50% of the judgements were positive. The ratio between this measure and the width of the participant's body at the shoulders (the aperture to shoulder - A/S ratio) was calculated. The A/S ratio was approximately 1.5 in the LPD group, approximately 0.9 in the RPD group, and approximately 1.1 in the control group, suggesting that the visual representation of the doorway (or that of its relationship to perceived body-size) is compressed in LPD (and perhaps expanded in RPD). The A/S ratios were invariant with viewing distance (0.6 or 1.5 m), the contrast polarity of the display (white on dark, or vice versa) and the doorway surround (blank, or vertical or horizontal stripes). The findings are discussed with reference to the neural representation of external space and of the body, and to the motor problems of Parkinson's disease.

Aged↗
PubMed2001

Evidence from a line bisection task for visuospatial neglect in left hemiparkinson's disease.

The perception of extrapersonal space in Parkinson's disease was examined with two line bisection tasks. One was a conventional pencil and paper test, the line bisection section of the Behavioural Inattention Test. In the other, the stimuli were displayed on a large (2x2.4 m) screen and varied in length (48-480 mm) and also in location on the screen (left, centre and right). They were presented at two viewing distances (0.6 and 1.5 m). Subjects remotely adjusted the position of a cursor until it appeared to bisect the stimulus line, using two push-buttons, one in each hand. The PD participants (n=18) had a marked asymmetry of motor symptoms. They were divided into two groups, those with predominantly left-sided motor symptoms (LPD, n=9), and those with predominantly right-sided motor symptoms (RPD, n=9). The control group (n=9) were all right-handed. No significant differences between the groups were found on the BIT bisection task. In contrast, when the stimuli were presented on the screen, LPD subjects showed a significant rightward bias in their settings of the cursor, particularly for lines on the left and centre of the screen, which was greater, the longer the stimulus line. The RPD group bisected lines slightly to the left, in common with the control group (pseudo-neglect). In a second experiment, Parkinsonian subjects repeated this task, but with the buttons reversed between the hands, so that the cursor was moved to the left by the right hand, and vice versa, but the pattern of results was the same as in the first experiment. The data suggest a small but reliable neglect in left hemiparkinson's disease, which is contralateral to the non-dominant (and probably worse affected) hemisphere. The dissociation between the response and the bisection error suggests a visuospatial impairment in LPD.

Aged↗
PubMed2000

Monocular occlusion can improve binocular control and reading in dyslexics.

Developmental dyslexia is a neurodevelopmental condition which causes 5-10% of children to have unexpected difficulty learning to read. Many dyslexics have impaired development of the magnocellular component of the visual system, which is important for timing visual events and controlling eye movements. Poor control of eye movement may lead to unstable binocular fixation, and hence unsteady vision; this could explain why many dyslexics report that letters appear to move around, causing visual confusion. Previous research has suggested that such binocular confusion can be permanently alleviated by temporarily occluding one eye. The aim of the present study was therefore to assess the binocular control and reading progress of dyslexic children with initially unstable binocular control after the left eye was patched. One hundred and forty-three dyslexics were studied. They were selected from children aged 7-11 years referred to a learning disabilities clinic if they were dyslexic and had unstable binocular control. They were randomly assigned to wear yellow spectacles with or without the left lens occluded, and were followed for 9 months. Significantly more of the children who were given occlusion gained stable binocular fixation in the first 3 months (59%) compared with children given the unoccluded glasses (36%). This advantage was independent of IQ or initial reading ability. Furthermore, at all the 3-month follow-ups, children were more likely to have gained stable binocular control if they had been wearing the occluded glasses. Gaining stable binocular control significantly improved reading. The children who did so with the help of occlusion improved their reading by 9.4 months in the first 3 months, compared with 3.9 months in those who were not patched and did not gain stable fixation. Over the whole 9 months, children who received occlusion and gained stable fixation nearly doubled their rate of progress in reading compared with those who remained unstable. At all the follow-ups the reading of those given occlusion was significantly better than that of those not occluded. Thus monocular occlusion helped children with unstable binocular control to gain good binocular fixation. If they gained stability, they made significantly faster reading progress. The progress made by the children who gained stable fixation was much greater than that achieved with other remedial techniques.

Child↗
PubMed1996

Squints and diplopia seen after brain damage.

The aim of this study was to investigate the incidence of squint after brain damage. We performed an observational study on 239 consecutive patients admitted to a specialist neurological rehabilitation unit: 129 with stroke, 84 with head injury and 26 with other conditions. Standard orthoptic measures, including visual acuity, cover test, eye movement recording and tests of binocular function were performed. Of all the patients, 89 (37%), were found to have squints, but only 32 of these (36%) experienced double vision. Brain stem lesions causing peripheral ocular motor impairment were found in a high proportion of patients after head injury (56%). Squints were found in 27 of 95 (28%) patients with cortical strokes, many with no other signs of brain stem involvement. Left was just as likely as right hemisphere damage to be associated with squint, but right-sided lesions seemed to protect against diplopia. We conclude that squint is common after brain damage, even if the brain stem is not obviously affected, but only a minority of these patients with acquired squint suffer diplopia.

Brain Damage, Chronic↗
PubMed1991

Yellow spectacles to improve vision in children with binocular amblyopia.

Yellow spectacles were given to 20 children who had binocular amblyopia. Immediately, their vision improved. Furthermore, in the younger children, after they had worn the yellow lenses for up to nine months, this improvement persisted even when they were not wearing the spectacles. Yellow filters may help these children with binocular amblyopia because they remove the blue fringes that are due to chromatic aberration of the eye.

Adolescent↗
PubMed1990

Spatial discrimination in children with poor vergence control.

In a 1987 study of children with reading difficulties, Stein, Riddell, and Fowler observed a relationship between poor vergence control, as indicated on a synoptophore vergence test, and the children's problems with learning to read. They suggested that poor vergence control led to impaired accuracy of spatial localisation and that this impeded learning to read. Here we have compared the accuracy of spatial localisation on a nonlinguistic computer game by children having good and poor vergence control. The children with poor vergence control made significantly more errors when locating targets than children with good vergence control. These results lend further support to the hypothesis that some children do not learn to read because they are unable to determine accurately the positions of letters in words.

Attention↗
PubMed1987

Fine binocular control in dyslexic children.

Dyslexic children often complain that letters seem to move around. The hypothesis advanced here is that this is a symptom of immature vergence control which leads to an unstable sense of visual direction. Evidence is presented that (1) sixty-seven per cent of dyslexic children exhibit poor dynamic control of vergence movements in response to a small fusion stimulus, (2) most good readers have good vergence control, (3) children with poor vergence control have reduced stereoacuity, (4) six months monocular occlusion for reading and close work assisted 51 per cent of dyslexics with unstable vergence control to improve; thereafter their reading improved rapidly also. It is concluded that defective vergence control is an important, though not the only, cause of dyslexics' problems.

Child↗
PubMed1986

The Dunlop test and reading in primary school children.

Using the Dunlop synoptophore test we have examined the reliability of vergence control for small fusion targets in 753 primary school children aged 7-11, and we have compared these results with the reading performance of 451 of them. 30% of the total sample of children had unstable responses in the Dunlop test. The proportion decreased with age, ranging from 49% of 5-year-olds to only 11% of 10-year-olds. The reading of children who had developed accurate vergence control was on average 6.3 months in advance of those who had not. Those with unstable Dunlop test responses were much more likely to be backward or low normal readers than children with stable responses. We conclude that in experienced hands the Dunlop test is a useful indicator of the development of vergence control and that immaturity of vergence control may contribute to children's reading problems.

Aging↗