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Biomedical subjects

D B Henson

Publications and source records attributed to D B Henson.

At least 37 records · Page 2Linked to original sources
PubMed1995

Establishing the threshold prior to single and multiple stimulus supra-threshold strategies.

The purpose of this study was to establish whether the routines used to establish the threshold at the beginning of single and multiple stimulus supra-threshold strategies yield the same values and whether data from one strategy can, therefore, be compared with those from the other without having to compensate for a difference in test intensities. Single stimulus and multiple stimulus supra-threshold strategies were conducted on 79 subjects using a Henson 4000 perimeter. On average the multiple stimulus strategy gave a value 0.095 dB higher than the single stimulus strategy. The differences between the two strategies was so small that results from the two strategies can be compared without having to compensate for any changes in the supra-threshold test intensity.

Adolescent↗
PubMed1995

Correcting lens system for perimetry.

A lens system to correct for both refractive error and perimeter test distance is frequently needed for accurate perimetry. Patients often lack appropriate near vision corrections and full aperture trial case lenses, when they are available, barely cover the central 30 degrees and hence frequently produce lens rim artefacts. A special system of correction lenses has been developed for perimetry which overcomes these problems. This system is head mounted, being based upon the Essilor presbyopia demonstration set, and is suitable for use with most perimeters.

Equipment Design↗
PubMed1995

Detecting progressive visual field loss.

Involvement in shared care projects requires the optometrist to make a series of decisions one of which is an assessment of the visual field data to see if there has been any progression. To aid the optometrist in making this decision, the relationship between inter-test variability in the visual field and extent of loss has been established for the Henson CFS2000 perimeter. In this study, visual field data were collected, with a supra-threshold strategy, from 174 eyes (89 patients) over a period of 2 years, there being a total of 595 visual field records. The sample included patients with both normal and glaucomatous visual fields. The results show that analysis based upon the number of stimuli missed at 5 or more dB above the threshold estimate reveals an increasing variability with extent of loss. These data can be used by optometrists to establish the significance of any change in the number of missed stimuli from one visit to the next.

Glaucoma↗
PubMed1994

Visual field analysis using artificial neural networks.

There have been several reports on the application of artificial neural networks (ANNs) to visual field classification. While these have demonstrated that neural networks can be used with good results they have not explored the effects that the training set can have upon network performance nor emphasized the unique value of ANNs in visual field analysis. This paper considers the problem of differentiating normal and glaucomatous visual fields and explores different training set characteristics using field data collected from a Henson CFS2000 perimeter. Training set properties including size, balance between normals and glaucomas, extent of field loss and the spatial location of glaucomatous defects are explored. A multilayer network with 132 input nodes, 20 hidden layer nodes and 2 output nodes in trained using an error backpropagation algorithm. Both sensitivity and specificity are measured during testing. The results demonstrate that large random sets are better than small random sets since sensitivity improves with size and specificity is not adversely affected. The variability in performance also reduces as training set size increases. In addition, sets that are biased towards glaucoma examples are more sensitive and less specific, while sets biased with normal examples are more specific and less sensitive than balanced sets. Thus large training sets with class balance are generally desirable for good sensitivities and specificities. The actual glaucoma examples contained in the set are also important. A training set deficient in examples has no detrimental effect on sensitivity or specificity. The spatial distribution of defects is also crucial. Spatially biased sets are unable to recognize defects that occur in locations where no previous defect has been presented while more balanced sets lead to better performance. In conclusion the 'ideal' training set should contain many examples of early defects that represent the full range of locations where these defects may occur.

Algorithms↗
PubMed1993

Influence of proximal, accommodative and disparity stimuli upon the vergence system.

The most important cues to the normal vergence response are retinal disparity, accommodative effort and perceived distance (proximal cue). In the past, proximal cues have been thought to contribute little to the total vergence response but more recent studies have suggested that they play an important role. The relative contributions of the proximal, accommodative and disparity cues to the vergence response were assessed by measuring the effect upon the vergence system when each of these cues were altered. The effect upon the vergence system was measured by the change in 'phoria, the change of fixation disparity and the amount of prism adaptation after 210 s of binocular viewing. The results indicate that the greatest effect of cue disharmony occurred when the proximal and disparity cues were altered. A finding which signifies the importance of the proximal cue to the overall vergence response.

Accommodation, Ocular↗
PubMed1993

Effect of uncorrected refractive errors upon central visual field testing.

An investigation of the effects of uncorrected refractive errors upon a central visual field examination has been made with a view to establishing what the likely effects would be of omitting a refractive correction when screening the visual field with a threshold related, supra-threshold strategy on the Henson CFA3000. The results indicate that, while a linear relationship exists between the extent of threshold elevation and the product of residual refractive error and pupil size, the scatter in the results means that accurate predictions cannot be made in individual cases. The threshold elevation was found to be independent of eccentricity (within the central 21 degrees from fixation) and did not increase the variability of results. These last two findings indicate that uncorrected refractive errors are unlikely to affect the sensitivity of the threshold related, supra-threshold strategy to localized visual field defects.

Adult↗
PubMed1992

The effect of orthoptic treatment upon the vergence adaptation mechanism.

This paper is a review of the research work that has been carried out over the past few years investigating the ability of the oculomotor system to adapt to prism-induced heterophoria. Our results show that subjects with normal binocular vision can adapt to horizontal and vertical prism-induced heterophorias whether fixating at distance or near. Further studies have shown that subjects with symptomatic abnormal binocular vision have an abnormal adaptation mechanism. Finally, we have found that when orthoptic treatment results in relief from the symptoms, there is an associated improvement in the subjects' ability to adapt to prism-induced heterophoria.

Adaptation, Ocular↗
PubMed1989

Visual field screening and the development of a new screening program.

The diagnosis of chronic open-angle glaucoma requires diligent and careful screening. Intraocular pressure measures and disc evaluations, when used in isolation, give unacceptably high numbers of false positives and false negatives, while full visual field analysis, as conducted with kinetic or full threshold static perimetry, takes too long for routine use. Screening of the visual field may offer a viable alternative. The effect of changing certain parameters of a visual field screening test are evaluated. These include the location of the stimuli, the number of stimuli and their suprathreshold increment. The results of an optimized visual field screening test are then evaluated with respect to the theoretical predictions. The results indicate that high levels of sensitivity and specificity can be achieved with visual field screening tests which are short enough to be included as part of the routine optometric examination.

Glaucoma, Open-Angle↗
PubMed1988

Cluster analysis in visual field quantification.

The central visual fields of 2165 normal and 106 glaucoma eyes were measured using a threshold related suprathreshold strategy. The effects of altering the cluster radius in normals and glaucoma eyes sheds light on the nature of defects in these two groups. It is estimated that approximately 13% of normals have clusters; the great majority of these individuals have one cluster of two defects. Most clusters in normals are formed artefactually due to angioscotoma and/or physiological variations in the blind spot position. Clusters due to other factors occur rarely. Clusters are found with equal frequencies in the superior and inferior fields in normal eyes, but with a greater frequency in the superior field in glaucoma eyes. The use of clusters in quantification is both sensitive and specific. Using results from this large sample and looking at other visual field properties, it is possible to devise weighted probability indices to score visual fields.

Adult↗
PubMed1988

Clinical evaluation of the Non-Contact Tonometer Mark II.

The purpose of this investigation was to test the reliability of the American Optical Non-Contact Tonometer Mark II (NCT II) using the Goldmann Applanation Tonometer (GAT) as the validating instrument. The sample contained 102 consecutive patients from our University Eye Clinic, of whom one-half had 4 NCT II measurements first, followed by 4 GAT measurements; the other one-half had 4 GAT measurements first, followed by 4 NCT II measurements. No significant change in intraocular pressure (IOP) was noted over the measurement sequence with either instrument. There was no significant difference between paired NCT II and GAT readings when the NCT II was used first; however, a highly significant difference between paired readings was obtained when the GAT was used first, indicating that the GAT measurement produced a delayed reduction in the IOP. This effect did not occur with the NCT II. Although the NCT II is shown to have a good overall reliability when compared to the GAT in both protocols, the agreement between any two tonometers may be influenced greatly by the very process of taking a measurement and by the dynamic nature of the IOP.

Adult↗
PubMed1988

Screening for glaucomatous visual field defects: the relationship between sensitivity, specificity and the number of test locations.

The visual field results from 104 eyes with early glaucomatous loss and 1860 normal eyes have been used to derive the relationship between sensitivity, specificity and the number of test locations in a visual field screening test. A logarithmic relationship was found between sensitivity and the number of test locations and a linear relationship between specificity and the number of test locations. The results indicate that large numbers of test locations are not required to reach high levels of sensitivity and that clinically it might be better to screen all patients with a few stimuli quickly than to examine carefully those patients who meet a precondition such as raised intraocular pressure or abnormal disc appearance.

Computers↗
PubMed1986

Frequency distribution of early glaucomatous visual field defects.

The optimization of visual field screening programs for glaucoma requires precise knowledge of where glaucoma defects occur and whether any relation exists between the size of defects and the associated retinal disease. The visual field results from 109 eyes of patients with early glaucomatous damage were investigated with a Friedmann Mark II Visual Field Analyser. The frequency distribution of scotomas as a function of location demonstrated that the tested field could be divided into four zones, one where scotomas occur frequently, one where scotomas occur a reasonable number of times, one where scotomas occur rarely, and one which corresponds to the blind spot (BS) region. The distribution of scotoma size in the first three zones was found to be about the same. Differences among the distributions of defects measured in this study and those of previous studies are attributed to different examination techniques.

Glaucoma↗
PubMed1986

Effects of prolonged forced vergence upon the adaptation system.

In previous studies the ability of the oculomotor system to adapt to prism-induced heterophoria has been shown. The 'phoria that occurs immediately after a prism has been placed before one eye gradually reduces and returns to its original baseline value as the subject is allowed binocular experience while wearing the prism. Little work has been carried out to determine when the adaptation is complete. In this study we report on the results of two experiments designed to determine when adaptation is complete. In the first, the rate of decay of adaptation, i.e. loss of slow fusional vergence, is measured after varying periods of forced vergence. In the second experiment the rate of adaptation to an additional disparity is measured; this should be the same as that before the induced effect if adaptation is complete. The results of this study indicate that although adaptation may appear complete by the criterion that the 'phoria has returned to its baseline value, the adaptation is unlikely to be truly complete for a much longer period of time has passed.

Adaptation, Ocular↗
PubMed1985

Informational content of visual field location in glaucoma.

Information Theory has been applied to produce estimates of how valuable each stimulus location on the Friedmann Visual Field Analyser mk. II is in detecting early glaucomatous visual field defects. The results indicate that stimuli in the superior arcuate region of the visual field and in the inferior nasal quadrant give the maximum amount of information. Stimuli in the extreme superior field (beyond 20 degrees), the inferior temporal quadrant and around the physiological blind spot give the least amount of information. It is suggested that these results can be used to develop computerised techniques for the analysis of visual field data.

Adult↗
PubMed1985

Vergence-adaptive change with a prism-induced noncomitant disparity.

Oculomotor adaptation to a magnified motor field in one eye can occur as a result of the vergence system responding either to (1) the magnification of the field, or (2) via small adaptive fields. This was investigated by monitoring adaptation with the whole oculomotor field magnified in one experiment, and just the central motor field magnified in another experiment. It was shown that adaptation in the periphery occurred at a much slower rate when it was occluded. It is concluded that adaptation in the periphery occurs via adaptive spread from central positions and not via a direct response to the magnification of the oculomotor field.

Adaptation, Physiological↗
PubMed1985

Adaptation to lens-induced heterophorias.

In previous studies we have shown that subjects with normal binocular vision adapt to prism-induced heterophorias. In this study we have continued the work by investigating whether the oculomotor system can also adapt to lens-induced heterophorias. It was found that subjects gradually adapted to the esophoria induced when viewing a distance chart through negative lenses. Adaptation also occurred to lens-induced esophoria and exophoria at near, while at distance, where the visual feedback was unaltered, the phoria remained virtually constant. These findings demonstrate the flexibility of the oculomotor system.

Adaptation, Ocular↗
PubMed1984

Adaptive changes with prolonged effect of comitant and incomitant vergence disparities.

When comitant and incomitant vergence disparities are induced for prolonged periods of time, the adaptive state becomes more stable. When the change is complete, the adaptive ability of the oculomotor system to a further disparity should be the same as that before any induced effect. Comitant change was investigated in five subjects who wore a 2 delta base-up Fresnel prism for prolonged periods of time (1 to 3 days). Adaptation was periodically monitored with a further 2 delta base-up prism. In three subjects the rate of adaptation to the additional prism approached that which occurred before the experiment. In two subjects, both of whom suffered asthenopic symptoms with the initial disparity, adaptation to the additional prism remained slow and incomplete. Incomitant change was investigated in two subjects who wore a -3.00 contact lens-spectacle lens (CL-SL) system for 2 to 3 days. Adaptation was monitored with a further -1.50 CL-SL system at the start and the end of the prolonged wear. Both these subjects had a much faster rate of adaptation at the end of the prolonged period than they had at the beginning.

Adaptation, Ocular↗