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Biomedical subjects

R Harrad

Publications and source records attributed to R Harrad.

16 recordsLinked to original sources
PubMed2002

Psychosocial and clinical determinants of compliance with occlusion therapy for amblyopic children.

AIMS: The objective of this study was to determine the extent that psychosocial and clinical variables influence parental compliance with occlusion therapy (eye patching) in children with amblyopia. METHODS: Children (n = 151) receiving occlusion therapy (eye patching) for the treatment of amblyopia were recruited from five orthoptic clinics in Bristol, UK. Parents completed a questionnaire based on Rogers' (1983) Protection Motivation Theory (PMT). The parents (n = 105) were also followed up 2 months later. Clinical data, including measures of visual acuity, were also recorded. Compliance with eye patching was assessed through self-report accounts of parents. Stepwise regression analyses were used to determine the factors predictive of compliance with eye patching. RESULTS: Self-reported compliance with eye patching at study entry revealed that only 54% of parents were achieving orthoptists' recommendations to patch their child. Perceived self-efficacy was positively associated with compliance and perceived prohibition of the child's activities were negatively associated with compliance. At follow-up, past behaviour accounted for the largest proportion of explained variance in patching behaviour followed by response efficacy, and prohibition of the child's activities. CONCLUSION: The present findings may serve to inform interventions aimed at enhancing current orthoptic practice to improve compliance in amblyopic children. The importance of 'self-efficacy' and past behaviour suggests that consultations with parents exhibiting higher levels of success with patching may elicit strategies that could be shared with parents experiencing difficulties with patching their children. In addition, it is possible that the perceived efficacy of the treatment could be enhanced if orthoptists emphasised evidence of improvements in visual acuity which may, in turn, foster the maintenance of eye patching.

Amblyopia↗
PubMed1998

Different mechanisms underlie three inhibitory phenomena in cat area 17.

Recently, it has been proposed that all suppressive phenomena observed in the primary visual cortex (V1) are mediated by a single mechanism, involving inhibition by pools of neurons, which, between them, represent a wide range of stimulus specificities. The strength of such inhibition would depend on the stimulus that produces it (particularly its contrast) rather than on the firing rate of the inhibited cell. We tested this hypothesis by measuring contrast-response functions (CRFs) of neurons in cat V1 for stimulation of the classical receptive field of the dominant eye with an optimal grating alone, and in the presence of inhibition caused by (1) a superimposed orthogonal grating (cross-orientation inhibition); (2) a surrounding iso-oriented grating (surround inhibition); and (3) an orthogonal grating in the other eye (interocular suppression). We fitted hyperbolic ratio functions and found that the effect of cross-orientation inhibition was best described as a rightward shift of the CRF ('contrast-gain control'), while surround inhibition and interocular suppression were primarily characterised as downward shifts of the CRF ('response-gain control'). However, the latter also showed a component of contrast-gain control. The two modes of suppression were differently distributed between the layers of cortex. Response-gain control prevailed in layer 4, whereas cells in layers 2/3, 5 and 6 mainly showed contrast-gain control. As in human observers, surround gratings caused suppression when the central grating was of high contrast, but in over a third of the cells tested, enhanced responses for low-contrast central stimuli, hence actually decreasing threshold contrast.

Action Potentials↗
PubMed1996

Psychophysics of suppression.

In amblyopia the visual input from the amblyopic eye is suppressed. The difference in contrast sensitivity between the two eyes can lead to suppression of the amblyopic eye through dichoptic masking in anisometropic amblyopia and small angle strabismus. Binocular cells in the visual cortex may also be instrumental in bringing about suppression in small angle strabismus through fusional suppression and disparity-dependent suppression. In large angle strabismus without amblyopia suppression is very powerful and is probably a modified form of binocular rivalry suppression. Which particular mechanism is operating is dependent on the pathogenesis of the amblyopia, the depth of the amblyopia and, in strabismic amblyopia, the angle of strabismus. In any given subject more than one mechanism may be operating in the different areas of the field.

Amblyopia↗
PubMed1995

Interocular suppression in the primary visual cortex: a possible neural basis of binocular rivalry.

In an attempt to demonstrate a physiological basis for the alternating suppression of perception when the two eyes view very different contours (binocular rivalry), we studied the responses of neurons in the lateral geniculate nucleus (LGN) and area 17 of cats for drifting gratings of different orientation, spatial frequency and contrast in the two eyes. Almost half of the LGN neurons studied exhibited modest inhibitory interocular interaction, but independent of interocular differences in orientation. Monocularly driven units in layer 4 of area 17 behaved similarly. However, for the majority of binocular cortical cells, the response to a grating of optimal orientation in one eye was suppressed by a grating of very different orientation shown to the other eye, over a wide range of spatial frequency and independent of relative spatial phase. This interocular suppression exhibits a remarkable non-linearity: a grating of non-preferred orientation in one eye causes significant interocular suppression only if the neuron is already responding to an appropriate stimulus in the other eye [Sengpiel and Blakemore (1994) Nature, 368, 847-850]. We propose that the switches in perceptual dominance during binocular rivalry depend on interocular interactions at the level of binocular neurons of the primary visual cortex, which might involve intracortical inhibition between adjacent ocular dominance columns. The spontaneous alternations in perceptual suppression that occur during prolonged viewing of rivalrous patterns remain to be explained, although significant variation in the strength of neuronal suppression in such conditions was occasionally seen.

Action Potentials↗
PubMed1994

The value of the short tau inversion recovery sequence in magnetic resonance imaging of thyroid eye disease.

22 patients with thyroid eye disease were examined with magnetic resonance imaging (MRI) and the results compared with 10 controls. Imaging was performed on a 1.0 T scanner using a head coil. All patients were examined using both T1W and short tau inversion recovery (STIR) sequences. The relative signal intensity (SI) of individual extraocular muscles were quantified by comparison with SI from the adjacent temporalis muscle to give a signal intensity ratio (SIR). The results were compared with clinical disease activity assessed by the Werner grading system. Visual evaluation of muscle size and calculated SIRs showed an increase when compared to normals in 15 of the 22 patients. This difference was significant in patients with high grade (4-6) clinical disease. The known histological changes in this disease suggest that this increase in signal is caused by oedema secondary to acute inflammation. STIR sequences suppress the retro-orbital fat and thus enhance these changes both in the muscles and in the fat itself. The MR findings suggest that the STIR sequence can be used to predict those patients that will respond to anti-inflammatory treatment.

Adult↗
PubMed1994

Interocular suppression in the visual cortex of strabismic cats.

Strabismic humans usually experience powerful suppression of vision in the nonfixating eye. In an attempt to demonstrate physiological correlates of such suppression, we recorded from the primary visual cortex of cats with surgically induced squint and studied the responses of neurons to drifting gratings of different orientation, spatial frequency, and contrast in the two eyes. Only 1 of 50 apparently monocular cells showed any evidence of remaining, subliminal excitatory input from the "silent" eye when the two eyes were stimulated with gratings of similar orientation, and even among the small proportion of cells that remained binocularly driven, very few exhibited facilitation when stimulated binocularly. The majority of cells from both exotropes and esotropes, even those that could be independently driven through either eye, displayed nonspecific interocular suppression: stimulation of the nondominant eye with a drifting grating of any orientation depressed the response to an optimal grating being presented to the dominant eye. This phenomenon exhibited a gross nonlinearity in that it was dependent on the temporal sequence of stimulus presentation: stimulation of the nondominant eye caused significant suppression only if the neuron was already responding to an appropriate stimulus in the dominant eye, but not when onset of stimulation in the two eyes was simultaneous. Interocular suppression was always independent of the relative spatial phase of the two grating stimuli, and usually broadly tuned for the spatial frequency of the suppressive stimulus. Suppression may depend on inhibitory interaction between neighboring ocular dominance columns, combined with the loss of conventional disparity-selective binocular interactions for matched stimuli in the two eyes. The similarity of interocular suppression in strabismic cats and that caused by orthogonal gratings in the two eyes in normal cats (Sengpiel and Blakemore, 1994; Sengpiel et al., 1994) suggests that strabismic suppression and binocular rivalry depend on similar neural mechanisms.

Animals↗
PubMed1993

Can second eye cataract extraction be justified?

A prospective study was carried out to test the null hypothesis that there is no additional benefit to be gained from second eye cataract extraction. Twenty-nine patients with unilateral cataract but contralateral pseudophakia completed a questionnaire enquiring into their visual disability and underwent testing of monocular and binocular visual acuity, contrast sensitivity, glare disability, fusion and stereopsis. These tests were repeated 4 months after second eye cataract extraction and lens implantation. Patients universally considered their vision to have been improved by second eye surgery and the prevalence of all symptoms were significantly reduced by this procedure. Normal binocular summation and stereopsis were restored following second eye cataract extraction. Symptomatic patients may benefit from second eye cataract extraction with lens implantation.

Aged↗
PubMed1992

Contrast sensitivity and glare in cataract using the Pelli-Robson chart.

There is a need for a convenient, clinically applicable test of glare disability which can be used in the preoperative evaluation of patients with cataract. In this study, contrast sensitivity (using the Pelli-Robson letter chart), near vision, and visual acuity were compared, with and without the introduction of a glare source in 70 patients with cataract, 15 with intraocular lenses, and 19 controls. A disposable pen torch was shone at the pupillary margin to induce glare. Contrast sensitivity demonstrated the most marked reduction during glare testing. Cortical cataracts were most affected followed by posterior subcapsular opacities. The glare disability was significantly less in pseudophakic patients and was absent from patients with non-cataractous phakic eyes. Glare testing with a disposable pen torch and a Pelli-Robson contrast sensitivity letter chart provides a rapid test of glare disability which can be easily incorporated into the clinical appraisal of patients with cataract.

Aged↗