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Biomedical subjects

B T Barrett

Publications and source records attributed to B T Barrett.

10 recordsLinked to original sources
PubMed1997

Grating detection and orientation discrimination in amblyopia.

PURPOSE: We examined whether the misperceptions associated with amblyopic visual perception can be revealed under natural viewing conditions by comparing the ability to detect the presence of a grating with the ability to identify the grating orientation. METHODS: Grating detection and orientation discrimination performance (horizontal versus vertical) were determined, using stimuli that consisted of sinusoidal gratings of fixed contrast (75%) but with variable spatial frequency. A total of four amblyopic subjects (two strabismic and two non-strabismic) and four age-matched normals participated in the experiment. RESULTS: Psychometric functions for grating detection and orientation identification were found to be closely matched in the normal subjects and in all four amblyopic subjects, indicating that orientation could be correctly identified at detection threshold. CONCLUSIONS: The absence of orientation uncertainty in the psychophysical data for the amblyopic observers is not consistent with the several previous reports of spatial aliasing in the central field of amblyopes. Our results suggest that non veridical visual perception in central amblyopic vision can not be revealed under natural viewing conditions by comparing the ability to detect the presence of a grating with the ability to identify its orientation. Possible reasons for the failure of this technique to reveal spatial aliasing in amblyopes are discussed.

Adult↗
PubMed1996

Assessing retinal/neural function in the presence of ocular media opacities.

Techniques currently available for investigating retinal/neural function in the presence of media opacities have traditionally been assessed in terms of their ability to predict the level of Snellen acuity which will be achieved following therapy. We propose an alternative method of assessment in which techniques are evaluated in terms of their ability to pre-operatively distinguish patients with reduced retinal/neural function from normals. In order to demonstrate this method of assessment, the data from a paper comparing the predictive abilities of a white-light and a laser interferometer in patients undergoing capsulotomy [21] have been re-analyzed. The approach enables fair comparison of the predictive abilities of the various techniques and will therefore help to resolve the issue of which technique provides the best means of assessing retinal/neural function in the presence of media opacities.

Cataract↗
PubMed1996

Centroid analysis predicts visual localization of first- and second-order stimuli.

Perceived alignment of asymmetric Gaussian-windowed stimuli was measured in an attempt to differentiate between stimulus characteristics which might underlie visual localization. These asymmetric stimuli have the advantage of being continuous in the spatial domain and of possessing well-defined spatial characteristics in which centroid, points of inflexion and peak can be separated from each other. Results for both luminance- and contrast-defined stimuli are reasonably well described on the basis that the centroid of the stimulus envelope represents the primitive which determines perceived visual location. Centroid location is inherent in the output of filters which are large enough to cover the object of interest.

Contrast Sensitivity↗
PubMed1996

Anterior chamber depth measurement in clinical practice.

This study compares the validity of anterior chamber depth estimates provided by a slit-lamp technique with those obtained from pachometry and ultrasonography. The technique provides depth estimates which are valid to within +/- 0.33 mm relative to standard pachometry, and to within +/- 0.42 mm relative to ultrasonography. The technique is simple to perform, requires no extra slit-lamp attachments, and may be carried out on any slit-lamp which has a calibrated variable length of slit.

Adult↗
PubMed1995

Clinical comparison of three techniques for evaluating visual function behind cataract.

The aim of the study was to compare the ability of displacement threshold hyperacuity to identify reduced retinal/neural function in cataract patients with that of the potential acuity meter (PAM) and laser interferometer. Fifty-two patients (mean age 72.0 +/- 10.8 years) admitted for extracapsular cataract extraction were randomly selected for inclusion in the study. Patients underwent routine refraction and logMAR acuity measurement on the eve of cataract surgery. Following pupillary dilation and in random order, the oscillatory displacement threshold (ODT; defined as the smallest detectable displacement of an object) was determined, and predictions of the post-operative acuity were obtained using the PAM and Rodenstock Retinometer (RR). Patients were re-examined approximately 7 weeks post-operatively when logMAR acuities and ODTs were remeasured. On the basis of the results of an independent fundus examination, patients were assigned to either a normal or a pathology group. Increasing cataract density adversely affected the predictions provided by the PAM (p = 0.0001) and ODT (p = 0.0001) techniques, but not the RR (p = 0.137). One-factor ANOVA revealed that pre-operative ODT (p = 0.0001), PAM (p = 0.0001) and RR (p = 0.0004) measures successfully distinguished patients with retinal/neural pathology from normals. Pre-operative cut-off levels to distinguish normal from pathology cases were retrospectively determined for each technique. The cut-off values for the PAM and ODT techniques yielded higher sensitivity and specificity values than was the case for the RR. The results suggest that the predictive abilities of the PAM and ODT techniques are similar, and superior to that of the RR. Further work involving larger numbers of patients with posterior segment pathology is necessary to reveal which technique provides the best means of assessing retinal/neural function in the presence of ocular media opacities.

Aged↗
PubMed1994

Assessing retinal/neural function in patients with cataract using oscillatory displacement thresholds.

Oscillatory displacement threshold (ODT) measurement has been proposed as a useful means of assessing retinal/neural function behind cataract. In order to examine the qualitative predictive ability of this technique, the influence of age, simulated cataract, and various posterior segment disorders upon ODT's were investigated. In a separate investigation, thresholds were measured in 30 cataract patients on the eve of surgery, and again 6 to 8 weeks postoperatively. ODT's were found to increase with age (p < 0.001) and were highly resistant to the effects of image degradation as produced by simulated cataract. In addition, thresholds were sensitive to the presence of retinal/neural disorders (p << 0.001). Although not completely independent of the presence of cataract, higher ODT's were measured in cataract patients with coexistent retinal/neural dysfunction than was the case for patients with normal posterior segments (p < 0.001). Preoperative ODT measurement may offer a means of distinguishing between the optical and retinal/neural components of visual impairment in patients with cataract.

Adolescent↗
PubMed1994

Effects of posterior segment disorders on oscillatory displacement thresholds, and on acuities as measured using the potential acuity meter and laser interferometer.

Oscillatory displacement thresholds (ODTs) were measured in 67 subjects with clear ocular media, but known posterior segment disorders. The ability of these thresholds to assess reduced visual function, as quantified by visual acuity deficits, was compared with that of the Potential Acuity Meter (PAM) and the Rodenstock Retinometer (RR). ODTs were found to be sensitive to acuity deficits which were retinal/neural in origin. Despite the fact that interferometric acuities are, unlike ODTs, a measure of resolution, the RR proved no more accurate in assessing the level of letter acuity which existed than did ODT measurement. As would be expected, the PAM provided for the most accurate means of assessing non-optical visual acuity deficits. However, the PAM proved to be less useful in eyes with: very poor levels of acuity, and certain anomalous conditions. The results have implications for the relative ability of each of the three tests to identify reduced visual function behind cataract.

Adolescent↗