Sample size in studies on diagnostic accuracy in ophthalmology:A literature survey

Aim: To assess the sample sizes used in studies on diagnostic accuracy in ophthalmology. Design and sources: A survey literature published in 2005. Methods: The frequency of reporting calculations of sample sizes and the samples' sizes were extracted from the published literature. A manual search of five leading clinical journals in ophthalmology with the highest impact (Investigative Ophthalmology and Visual Science, Ophthalmology, Archives of Ophthalmology, American Journal of Ophthalmology and British Journal of Ophthalmology) was conducted by two independent investigators. Results: A total of 1698 articles were identified, of which 40 studies were on diagnostic accuracy. One study reported that sample size was calculated before initiating the study. Another study reported consideration of sample size without calculation. The mean (SD) sample size of all diagnostic studies was 172.6 (218.9). The median prevalence of the target condition was 50.5%. Conclusion: Only a few studies consider sample size in their methods. Inadequate sample sizes in diagnostic accuracy studies may result in misleading estimates of test accuracy. An improvement over the current standards on the design and reporting of diagnostic studies is warranted.

A survey of perceived training differences between ophthalmology residents in Hong Kong and China.

Background: To study the differences in ophthalmology resident training between China and the Hong Kong Special Administrative Region (HKSAR).Methods: Training programs were selected from among the largest and best-known teaching hospitals. Ophthalmology residents were sent an anonymous 48-item questionnaire by mail. Work satisfaction, time allocation between training activities and volume of surgery performed were determined.Results: 50/75 residents (66.7 %) from China and 20/26 (76.9 %) from HKSAR completed the survey. Age (28.9 ± 2.5 vs. 30.2 ± 2.9 years, p = 0.15) and number of years in training (3.4 ± 1.6 vs. 2.8 ± 1.5, p = 0.19) were comparable between groups. The number of cataract procedures performed by HKSAR trainees (extra-capsular, median 80.0, quartile range: 30.0, 100.0; phacoemulsification, median: 20.0, quartile range: 0.0, 100.0) exceeded that for Chinese residents (extra-capsular: median = 0, p < 0.0001; phacoemulsification: median = 0, p < 0.0001). Chinese trainees spent more time completing medical charts (>50 % of time on charts: 62.5 % versus 5.3 %, p < 0.0001) and received less supervision (≥90 % of training supervised: 4.4 % versus 65 %, p < 0.0001). Chinese residents were more likely to feel underpaid (96.0 % vs. 31.6 %, p < 0.0001) and hoped their children would not practice medicine (69.4 % vs. 5.0 %, p = 0.0001) compared HKSAR residents.Conclusions: In this study, ophthalmology residents in China report strikingly less surgical experience and supervision, and lower satisfaction than HKSAR residents. The HKSAR model of hands-on resident training might be useful in improving the low cataract surgical rate in China.

Ophthalmology. 2011 Dec;118(12):2343-50. doi: 10.1016/j.ophtha.2011.06.016. Epub 2011 Sep 1. Randomized, controlled trial of an educational intervention to promote spectacle use in rural China: the see well to learn well study.

OBJECTIVE: To test an educational intervention promoting the purchase of spectacles among Chinese children. DESIGN: Randomized, controlled trial. PARTICIPANTS: Children in years 1 and 2 of all 20 junior and senior high schools (ages 12-17 years) in 3 rural townships in Guangdong, China. METHODS: Children underwent visual acuity (VA) testing, and parents of participants with presenting VA worse than 6/12 in either eye improving by more than 2 lines with cycloplegic refraction were recommended to purchase glasses. Children at 10 randomly selected schools received a lecture, video, and classroom demonstration promoting spectacle purchase. MAIN OUTCOME MEASURES: Self-reported purchase of spectacles (primary outcome) and observed wear or possession of newly purchased glasses (secondary outcome) at follow-up examinations (mean, 219 ± 87 days after the baseline visit). RESULTS: Among 15 404 eligible children, examinations were completed for 6379 (74.6%) at intervention schools and 5044 (73.6%) at control schools. Spectacles were recommended for 2236 (35.1%) children at intervention schools and for 2212 (43.9%) at control schools. Of these, 417 (25.7%) intervention schools children and 537 (34.0%, P = 0.45) control schools children reported buying glasses. Predictors of purchase in regression models included female gender (P = 0.02), worse uncorrected VA (P < 0.001), and higher absolute value of refractive error (P = 0.001). Neither the rate of self-reported purchase of glasses or observed wear or possession of newly purchased glasses differed between control schools and intervention schools in mixed-effect logistic regression models. Among children not purchasing glasses, 21.7% had better-eye VA of worse than 6/18. CONCLUSIONS: An intervention based on extensive pilot testing and focus groups in the area failed to promote spectacle purchase or wear. The high burden of remaining uncorrected poor vision underscores the need to develop better interventions. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

What motion distributions yield global transparency and spatial segmentation?

We have examined the ability of observers to parse bimodal local-motion distributions into two global motion surfaces, either overlapping (yielding transparent motion) or spatially segregated (yielding a motion boundary). The stimuli were random dot kinematograms in which the direction of motion of each dot was drawn from one of two rectangular probability distributions. A wide range of direction distribution widths and separations was tested. The ability to discriminate the direction of motion of one of the two motion surfaces from the direction of a comparison stimulus was used as an objective test of the perception of two discrete surfaces. Performance for both transparent and spatially segregated motion was remarkably good, being only slightly inferior to that achieved with a single global motion surface. Performance was consistently better for segregated motion than for transparency. Whereas transparent motion was only perceived with direction distributions which were separated by a significant gap, segregated motion could be seen with abutting or even partially overlapping direction distributions. For transparency, the critical gap increased with the range of directions in the distribution. This result does not support models in which transparency depends on detection of a minimum size of gap defining a bimodal direction distribution. We suggest, instead, that the operations which detect bimodality are scaled (in the direction domain) with the overall range of distributions. This yields a flexible, adaptive system that determines whether a gap in the direction distribution serves as a segmentation cue or is smoothed as part of a unitary computation of global motion.

Continuity - based and discontinuity - based segmentation in transparent and spatially segregated global motion

The mechanisms underlying the parsing of a spatial distribution of velocity vectors into two adjacent (spatially segregated) or overlapping (transparent) motion surfaces were examined using random dot kinematograms. Parsing might occur using either of two principles. Surfaces might be defined on the basis of similarity of motion vectors and then sharp perceptual boundaries drawn between different surfaces (continuity-based segmentation). Alternatively, detection of a high gradient of direction or speed separating the motion surfaces might drive the process (discontinuity-based segmentation). To establish which method is used, we examined the effect of blurring the motion direction gradient. In the case of a sharp direction gradient, each dot had one of two directions differing by 135°. With a shallow gradient, most dots had one of two directions but the directions of the remainder spanned the range between one motion-defined surface and the other. In the spatial segregation case the gradient defined a central boundary separating two regions. In the transparent version the dots were randomly positioned. In both cases all dots moved with the same speed and existed for only two frames before being randomly replaced. The ability of observers to parse the motion distribution was measured in terms of their ability to discriminate the direction of one of the two surfaces. Performance was hardly affected by spreading the gradient over at least 25% of the dots (corresponding to a 1° strip in the segregation case). We conclude that detection of sharp velocity gradients is not necessary for distinguishing different motion surfaces.

Directional performance in motion transparency

Motion transparency provides a challenging test case for our understanding of how visual motion, and other attributes, are computed and represented in the brain. However, previous studies of visual transparency have used subjective criteria which do not confirm the existence of independent representations of the superimposed motions. We have developed measures of performance in motion transparency that require observers to extract information about two motions jointly, and therefore test the information that is simultaneously represented for each motion. Observers judged whether two motions were at 90 to one another; the base direction was randomized so that neither motion taken alone was informative. The precision of performance was determined by the standard deviations (S.D.s) of probit functions fitted to the data. Observers also made judgments of orthogonal directions between a single motion stream and a line, for one of two transparent motions against a line and for two spatially segregated motions. The data show that direction judgments with transparency can be made with comparable accuracy to segregated (non-transparent) conditions, supporting the idea that transparency involves the equivalent representation of two global motions in the same region. The precision of this joint direction judgment is, however, 2–3 times poorer than that for a single motion stream. The precision in directional judgment for a single stream is reduced only by a factor of about 1.5 by superimposing a second stream. The major effect in performance, therefore, appears to be associated with the need to compute and compare two global representations of motion, rather than with interference between the dot streams per se. Experiment 2tested the transparency of motions separated by a range of angles from 5 to 180 by requiring subjects to set a line matching the perceived direction of each motion. The S.D.s of these settings demonstrated that directions of transparent motions were represented independently for separations over 20. Increasing dot speeds from 1 to 10 deg/s improved directional performance but had no effect on transparency perception. Transparency was also unaffected by variations of density between 0.1 and 19 dots/deg2

Speed tuning of direction repulsion describes an inverted U-function

Direction repulsion describes the phenomenon in which observers typically overestimate the direction difference between two superimposed motions moving in different directions (Marshak & Sekuler, Science 205(1979) 1399). Previous research has found that, when a relatively narrow range of distractor speeds is considered, direction repulsion of a target motion increases monotonically with increasing speed of the distractor motion. We sought to obtain a more complete measurement of this speed-tuning function by considering a wider range of distractor speeds than has previously been used. Our results show that, contrary to previous reports, direction repulsion as a function of distractor speed describes an inverted U-function. For a target of 2.5deg/s, we demonstrate that the attenuation of repulsion magnitude with high-speed disractors can be largely explained in terms of the reduced apparent contrast of the distractor. However, when we reduce target motion speed, this no longer holds. When considered from the perspective of Edwards et al.s (Edwards, Badcock, & Smith, Vision Research 38 (1998) 1573) two global-motion channels, our results suggest that direction repulsion is speed dependent when the distractor and target motions are processed by different globalmotion channels, but is not speed dependent when both motions are processed by the same, high-speed channel. The implications of these results for models of direction repulsion are discussed.

Test stimulus characteristics determine the perceived speed of the dynamic motion aftereffect

Using a speed-matching task, we measured the speed tuning of the dynamic motion aftereVect (MAE). The results of our Wrst experiment, in which we co-varied dot speed in the adaptation and test stimuli, revealed a speed tuning function. We sought to tease apart what contribution, if any, the test stimulus makes towards the observed speed tuning. This was examined by independently manipulating dot speed in the adaptation and test stimuli, and measuring the eVect this had on the perceived speed of the dynamic MAE. The results revealed that the speed tuning of the dynamic MAE is determined, not by the speed of the adaptation stimulus, but by the local motion characteristics of the dynamic test stimulus. The role of the test stimulus in determining the perceived speed of the dynamic MAE was conWrmed by showing that, if one uses a test stimulus containing two sources of local speed information, observers report seeing a transparent MAE; this is despite the fact that adaptation is induced using a single-speed stimulus. Thus while the adaptation stimulus necessarily determines perceived direction of the dynamic MAE, its perceived speed is determined by the test stimulus. This dissociation of speed and direction supports the notion that the processing of these two visual attributes may be partially independent.

The direction aftereffect is driven by adaptation of local motion detectors

The processing of motion information by the visual system can be decomposed into two general stages; point-by-point local motion extraction, followed by global motion extraction through the pooling of the local motion signals. The direction aftereVect (DAE) is a well known phenomenon in which prior adaptation to a unidirectional moving pattern results in an exaggerated perceived direction diVerence between the adapted direction and a subsequently viewed stimulus moving in a diVerent direction. The experiments in this paper sought to identify where the adaptation underlying the DAE occurs within the motion processing hierarchy. We found that the DAE exhibits interocular transfer, thus demonstrating that the underlying adapted neural mechanisms are binocularly driven and must, therefore, reside in the visual cortex. The remaining experiments measured the speed tuning of the DAE, and used the derived function to test a number of local and global models of the phenomenon. Our data provide compelling evidence that the DAE is driven by the adaptation of motion-sensitive neurons at the local-processing stage of motion encoding. This is in contrast to earlier research showing that direction repulsion, which can be viewed as a simultaneous presentation counterpart to the DAE, is a global motion process. This leads us to conclude that the DAE and direction repulsion reflect interactions between motion-sensitive neural mechanisms at different levels of the motion-processing hierarchy.

Retinal Pigment Epithelial Cell DNA is Damaged by Exposure to Benzo[a]pyrene, a Constituent of Cigarette Smoke.

This study examined the effect of exogenous benzo[ a ]pyrene (BaP), an important constituent of cigarette smoke, on cultured bovine retinal pigment epithelial (RPE) cells. Evidence is presented for its metabolic conversion into benzo[ a ]pyrene diol epoxide (BPDE) and the consequent formation of potentially cytotoxic nucleobase adducts in DNA. Cultured RPE cells were treated with BaP at concentrations in the range of 0–100 µm. The presence of BaP was found to cause inhibition of cell growth and replication. BaP induced the expression of a phase I drug metabolizing enzyme which was identified as cytochrome P450 1A1 (CYP 1A1) by RT–PCR and by Western blotting. Coincident with the increased expression of CYP 1A1, covalent adducts between the mutagenic metabolite BPDE and DNA could be detected within RPE cells by immunocytochemical staining. Additional support for their formation was afforded by nuclease P1 enhanced 32P-postlabelling assays on cellular DNA. Single-cell gel electrophoresis (comet) assays showed that exposure of RPE cells to BaP rendered them markedly more susceptible to DNA damage induced by broad band UVB or blue light laser irradiation. In the case of UVB, this is consistent with the photosensitization of DNA cleavage by nucleobase adducts of BPDE. Collectively, these findings imply that BaP has a significant impact on RPE cell pathophysiology and suggest mechanisms whereby exposure to cigarette smoke might cause RPE dysfunction and cell death, thus possibly contributing to degenerative disorders of the retina.