Optometry and Vision Sciences - Research Publications

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    Scaling the size of perimetric stimuli reduces variability and returns constant thresholds across the visual field
    Bedggood, P ; Prea, SM ; Kong, YXG ; Vingrys, AJ (ASSOC RESEARCH VISION OPHTHALMOLOGY INC, 2020-10)
    The conventional stimulus for standard automated perimetry is fixed in size, giving elevated contrast thresholds and reduced test reliability in the periphery. Here, we test the hypothesis that appropriate scaling of the size of perimetric stimuli will return fixed thresholds and reduced variability across the visual field. We derived frequency-of-seeing (FOS) curves in five healthy subjects at central (3 degrees) and peripheral (27 degrees) locations with a method of constant stimuli (MOCS) using a desktop LCD display. FOS curves for a Goldmann III (GIII) stimulus were compared with those for size scaled spots. To consider clinical translation, we tested a further five healthy subjects (22-24 years) with the Melbourne Rapid Fields (MRF) tablet perimeter at several locations spanning 1 degree to 25 degrees from fixation, deriving FOS curves (MOCS) and also conducting repeated adaptive clinical thresholding to assess intra- and interobserver variability. We found that GIII contrast thresholds were significantly elevated in the periphery compared with the parafovea, with concomitant reduction of FOS slope. Using appropriately size scaled spots, threshold and slope differences between these locations were significantly reduced. FOS data collected with the tablet perimeter confirmed that size scaling confers broad equivalence of the shape of the FOS curve across the visual field. Repeated adaptive thresholding with size scaled stimuli gave relatively constant intra-observer variability across the visual field, which compares favorably with published normative data obtained with the GIII stimulus. The reduced variability will improve signal-to-noise ratio for correct classification of normal visual field test results, whereas the lower contrast thresholds yield greater dynamic range, which should improve the ability to reliably monitor moderate defects.
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    Are you sure? The relationship between response certainty and performance in visual detection using a perimetry-style task
    Bedggood, P ; Ahmad, A ; Chen, A ; Lim, R ; Maqsudi, S ; Metha, A (ASSOC RESEARCH VISION OPHTHALMOLOGY INC, 2020-08)
    Conventional psychophysical methods ignore the degree of confidence associated with each response. We compared the psychometric function for detection with that for "absolute certainty" in a perimetry-style task, to explore how knowledge of response certainty might aid the estimation of detection thresholds. Five healthy subjects performed a temporal 2-AFC detection task, indicating on each trial whether they were "absolutely certain." The method of constant stimuli was used to characterize the shape of the two psychometric functions. Four eccentricities spanning central and peripheral vision were tested. Where possible, conditions approximated those of the Humphrey Field Analyzer (spot size, duration, background luminance, test locations). Based on the empirical data, adaptive runs (ZEST) were simulated to predict the likely improvement in efficiency obtained by collecting certainty information. Compared to detection, threshold for certainty was 0.5 to 1.0 dB worse, and slope was indistinguishable across all eccentricities tested. A simple two-stage model explained the threshold difference; under this model, psychometric functions for detection and for certainty-given-detection are the same. Exploiting this equivalence is predicted to reduce the number of trials required to achieve a given level of accuracy by approximately 30% to 40%. The chances of detecting a spot and the chances of certainty-given-detection were approximately the same in young, healthy subjects. This means, for example, that a spot detected at threshold was labeled as "certainly" detected approximately half the time. The collection of certainty information could be used to improve the efficiency of estimation of detection thresholds.