A two-stage neural spiking model of visual contrast detection in perimetry.

Perimetry is a commonly used clinical test for visual function, limited by high variability. The sources of this variability need to be better understood. In this paper, we investigate whether noise intrinsic to neural firing could explain the variability in normal subjects. We present the most physiologically accurate model to date for stimulus detection in perimetry combining knowledge of the physiology of components of the visual system with signal detection theory, and show that it requires that detection be mediated by multiple cortical cells in order to give predictions consistent with psychometric functions measured in human observers.

Modeling the sensitivity to variability relationship in perimetry.

PURPOSE: Studies in glaucoma patients show that standard automated perimetry results increase in variability as sensitivity decreases. However, the reasons for this change are unclear. This study presents the principle of Divergent Dysfunction as a possible explanation for this change in variability, and incorporates it into a model that can be used to simulate perimetry. METHODS: A computer program was written to simulate visual field test results based on the model, using a Full Threshold testing strategy. The validity of the simulation was tested by comparing it with normal sensitivity values, and with test-retest data from 63 participants evaluated five times each over the course of 1 month. The effect on the simulated data of varying parameters of the model was investigated, such as changing the magnitude of variability and the percentages of false positive and negative responses. RESULTS: The correlation between subject and simulated test-retest data was 0.987. Several factors were found to affect the sensitivity-variability relationship for the simulated data, most notably the rate of sensitivity decline, the percentage of false positives, and the starting luminance of the test procedure. CONCLUSIONS: The principle of Divergent Dysfunction presented here provides a plausible explanation for the sensitivity-variability relationship for standard automated perimetry in glaucomatous eyes. The model and resultant simulation program aim to provide an intuitive demonstration of the principle, which can also be used to examine the effectiveness of different testing strategies. These findings have great implications for future clinical research.

Frequency of testing for detecting visual field progression.

AIMS: To investigate the effect of frequency of testing on the determination of visual field progression using pointwise linear regression (PLR). METHODS: A "virtual eye" was developed to simulate series of sensitivities over time at a given point in the eye. The user can input the actual behaviour of the point (for example, stable or deteriorating steadily), and then a configurable amount of noise is added to produce a realistic series over time. The advantage of this over using patient data is that the actual status of the eye is known. Series were generated using different frequencies of testing, and the diagnosis that would have been made from each series was compared with the true status of the eye. A point was diagnosed as progressing if the regression line for the series showed a deterioration of at least 1 dB per year, significant at the 1% level. From these results, graphs were produced showing the number of points correctly or incorrectly diagnosed as progressing. RESULTS: With the virtual eye deteriorating at a rate of 2 dB/year, it was found that the point was determined to be progressing quicker when more tests were carried out each year. With a stable virtual eye, it was found that increasing the frequency of testing increased the number of series that were falsely labelled as progressing during the first 3 years of testing. CONCLUSIONS: As the frequency of testing increases, the sensitivity of PLR increases. However, the specificity decreases; possibly meaning more unnecessary changes in treatment. Three tests per year provide a good compromise between sensitivity and specificity.

HRT-3 Moorfields reference plane: effect on rim area repeatability and identification of progression.

AIMS: To assess the effect of the Moorfields Reference Plane on Heidelberg Retina Tomograph (HRT) rim area repeatability and its effect on progression rates using an event analysis. METHODS: The HRT reference plane (RP) defines structures above as "rim" and below as "cup." The Moorfields RP applies the Standard RP (located 50 microm posterior to the temporal disc margin) at baseline and maintains the distance between the Standard RP and the reference ring (located in the image periphery) for follow-up images. The Moorfields RP was applied to an HRT test-retest dataset, and rim area repeatability coefficients were calculated. Repeatability coefficients were compared between the Moorfields, Standard and 320 (located 320 microm posterior to the reference ring) RPs. The Moorfields RP was applied to HRT images from 198 ocular hypertensives, acquired over 6 years. HRT progression required rim area baseline/follow-up differences exceeding the repeatability coefficient in two or more sectors, with confirmation in at least one of two consecutive images. Field progression was assessed using Advanced Glaucoma Intervention Study criteria. RESULTS: The Moorfields RP improved rim area repeatability compared with the Standard RP; repeatability was similar between the Moorfields and the 320 RP. The frequency of identified progression using Moorfields RP was 40% compared with 28% for the 320 RP. There was a greater percentage with concurrent field progression -15.1% (Moorfields RP) compared with 12.1% (320 RP). CONCLUSIONS: Although rim area repeatability was similar using the 320 RP and the Moorfields RP, the latter resulted in greater rates of detection of change.