The effect of induced intraocular straylight on perimetric tests.

PURPOSE: To investigate the effect of intraocular straylight (IOS) on threshold measurements made by four types of perimetry: standard automated perimetry (SAP), frequency-doubling technology (FDT) perimetry, flicker-defined form (FDF) perimetry, and the Moorfields Motion Displacement Test (MDT). METHODS: Six healthy young (21-29 years) psychophysically experienced observers were tested five times with each perimeter, each time with one of five different white opacity filters (WOFs), to induce various levels of IOS. Each WOF gave rise to an increase in IOS, measured with a straylight meter, of between 10% and 200% from baseline and ranging in effect from normal ageing to significant cataract. The change in sensitivity from baseline was normalized, allowing comparison of standardized (z) scores (change divided by the standard deviation of normative values) for each instrument. RESULTS: Average SAP, FDT, and FDF thresholds were significantly affected (P < 0.001) by moderate to large increases in IOS (50%-200%), whereas MDT remained largely unaffected over the same range of straylight levels. CONCLUSIONS: The Moorfields MDT shows greater resilience to the effects of additional straylight compared with SAP, FDT, or FDF.

Exploration of the psychophysics of a motion displacement hyperacuity stimulus.

PURPOSE: To explore the summation properties of a motion-displacement hyperacuity stimulus with respect to stimulus area and luminance, with the goal of applying the results to the development of a motion-displacement test (MDT) for the detection of early glaucoma. METHODS: A computer-generated line stimulus was presented with displacements randomized between 0 and 40 minutes of arc (min arc). Displacement thresholds (50% seen) were compared for stimuli of equal area but different edge length (orthogonal to the direction of motion) at four retinal locations. Also, MDT thresholds were recorded at five values of Michelson contrast (25%-84%) for each of five line lengths (11-128 min arc) at a single nasal location (-27,3). Frequency-of-seeing (FOS) curves were generated and displacement thresholds and interquartile ranges (IQR, 25%-75% seen) determined by probit analysis. RESULTS: Equivalent displacement thresholds were found for stimuli of equal area but half the edge length. Elevations of thresholds and IQR were demonstrated as line length and contrast were reduced. Equivalent displacement thresholds were also found for stimuli of equivalent energy (stimulus area x [stimulus luminance - background luminance]), in accordance with Ricco's law. There was a linear relationship (slope -0.5) between log MDT threshold and log stimulus energy. CONCLUSIONS: Stimulus area, rather than edge length, determined displacement thresholds within the experimental conditions tested. MDT thresholds are linearly related to the square root of the total energy of the stimulus. A new law, the threshold energy-displacement (TED) law, is proposed to apply to MDT summation properties, giving the relationship T = K logE where, T is the MDT threshold, Kis the constant, and E is the stimulus energy.