An innovative instrument for the psychophysical measurement of macular pigment optical density using a CRT display

P. West, Cambridge Research Systems Ltd, Rochester, Kent, UK

J. Mellerio, Rayne Institute of Ophthalmology, Kings College, London, UK

Abstract

We describe a new instrument for psychophysical measurement of Macular Pigment Optical Density (MPOD) designed to overcome many of the difficulties usually encountered when performing subjective photometry on naïve subjects.

The system employs a CRT monitor for stimulus presentation and incorporates an optical filter that overcomes the usual limitations of the phosphors of a CRT monitor. Part of the broad spectral emission of each of the three CRT phosphors is absorbed by the macular pigment. Therefore when employed as a stimulus for heterochromatic flicker or motion nulling photometry there is a systematic and significant underestimate in MPOD. To overcome this limitation we have designed a band blocking filter that blocks light between 460-640 nm. When viewed through the filter the spectra of the red and blue phosphors do not overlap and the blue component is absorbed by the MP whilst the red is not. Thus the subjective photometric measurements made using this configuration are close to those made with monochromatic lights.

Test Stimuli are consecutively presented in concentric arcs between 0 and 8 degrees, with central fixation, thus allowing a MPOD profile to be measured. The design brief was that the instrument should measure MPOD in large groups of subjects, for example for screening or drug trials. Therefore we have incorporated two further novel features that improve subject performance and measurement robustness.

We employ the motion nulling grating method of Cavanagh and Anstis (1983). A chromatic blue-red grating is displayed sequentially with a luminance grating. The gratings appear to drift either up or down. The direction of drift is dependent upon the relative perceived luminance of the red and blue component. This stimulus is easy to see and setting luminance for a motion null is easily to perform: it is ideally suited to the forced choice staircase psychophysical paradigm that we employ.

The quality of any psychophysical MPOD estimate relies on knowing the exact retinal eccentricity that is being measured. We use a video based gaze tracking system to ensure that correct central fixation is maintained. The stimulus presentation sequence is inhibited unless the subject is accurately maintaining central fixation. Subject feed back is provided by a visual cue when fixation is correct.

We report MPOD profile measurements made with the device and compare them to results in the literature using near-monochromatic lights (Mellerio, et al, 2002; Moreland et al, 2004).

References

• Anstis, S. & Cavanagh, P. (1983) A minimum motion technique for judging equiluminance.
• Mollon, J.D. & Sharpe, L.T. (Eds.), Colour vision: Psychophysics and physiology. London, Academic Press, pp.155-166
• Mellerio, J., Ahmadi-Lari, S., van Kuijk, F. J. G. M.,Pauleikhoff, D., Bird, A. C. & Marshall, J. (2002) A portable instrument for measuring macular pigment with central fixation. Cur. Eye Res. 25:37–47
• Moreland, J.D. Macular pigment assessment by motion photometry. (2004) Archives of Biochemistry and Biophysics. 430:143–148

Related product information

Metropsis: Macular Pigment Optical Density Test

Metropsis MPOD test incorporates three key innovations: a unique optical filter that eliminates CRT phosphor problems, an undemanding test stimulus that radically improves test performance and measurement repeatability, and integrated eye tracking that ensures that the correct retinal locations are tested.

 

Copyright © Cambridge Research Systems Ltd. Click for details.

^ Back to top