MinireviewMacular pigment assessment by motion photometry
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:
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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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