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Macular pigment assessment by motion photometry

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Abstract

A Moreland anomaloscope was modified to measure macular pigment optical density (MPOD) profiles by motion photometry. A grating (spatial frequency 0.38 c deg−1), whose alternate bars were filled, respectively, with 460 nm (maximum MP absorption) and 580 nm (zero MP absorption) lights, drifted steadily at 37° s−1. The subject adjusted the 580 nm radiance to minimise perceived motion (equiluminance between 460 and 580 nm). Five or more settings were made for two foveal fields (0.9° and 2.2° diameter) and 11 extrafoveal annular fields (0.8°–7.5° eccentricity). Twenty subjects made measurements for both eyes: some with replications. MPOD profiles varied in scale (0.18–0.75 for the 0.9° foveal field) and in shape. A mean profile was derived. Foveal data were optimally aligned with annular data in that profile when plotted at 0.71 of the foveal field radius. Factors that limit precision were identified, such as fixation errors foveally and Troxler’s effect parafoveally.

Section snippets

Materials and methods

A Moreland anomaloscope [18] was modified to provide a drifting square wave grating [4], [5]. The grating, spatial frequency 0.38 c deg−1, was generated by an equiangular-spiral mirror. This mirror, forming part of the photometer head of the instrument, was rotated in its own plane about the centre of the spiral. The grating, viewed through a horizontally eccentric stop, was seen as though moving horizontally across the field [1] at a constant (radial) velocity of 37° s−1: the motion on the retina

Results

All MPOD retinal profiles were characterised by a central peak ranging from 0.18 to 0.75 for the 0.9° foveal field: values typical of the large scalar variance obtaining in a normal healthy population. Individual profiles could be categorised in three ways: bell shaped (fit by a logistic function: 8 subjects), cusp shaped (fit by a 1st or 2nd order exponential function: 9 subjects), and cusp shaped with a parafoveal plateau (fit by a 4th order polynomial: 3 subjects). In several cases, the

Discussion

The minimum motion technique requires little training. However, in common with other psychophysical procedures such as flicker photometry and colour matching [12], [14], [16], [18], results with satisfactory precision and reproducibility are obtained by attending to problems that require the subject’s concentration. These are:

  • (1)

    Troxler’s effect: a local adaptation that reduces the perception of motion. The effect is particularly noticeable parafoveally where adaptation is rapid. A strategy of

Acknowledgments

The author thanks Tony Robson for stimulating discussions and two anonymous referees for help in clarifying several aspects of this paper.

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