Stimuli were generated by a slightly modified version of the tabletop macular densitometer previously described.
9 This instrument utilizes light-emitting diodes (LEDs) and electronic controls. For the present study, the following features were added to the device: (1) The aperture and the diffuser forming the test field were mounted on a motorized filter wheel so that the size and shape of the test field could be changed easily by the examiner. (2) Another LED was added to the test source to make available an auxiliary blue-green test stimulus in addition to the blue stimulus near the spectral peak of MP. (3) A small red LED was added to serve as a fixation point to place the test field at the reference locus in the parafoveal retina. (4) An ophthalmic lens was placed in front of the subject’s test eye. This lens was designed to focus an emmetropic subject at the plane of the test aperture. Another lens was sometimes used to match the refractive correction of the subject (described later).
Four test field configurations—two solid disks and two annuli—were used to derive a spatial profile of MPOD in the right eye. With solid disks, it has been shown that the measure of MPOD is obtained at the edge of the disk.
2 5 10 Thus, the subject was instructed to fixate at the center of the following targets: (1) A solid disk of 15 minarc radius; (2) a solid disk of 0.5° arc radius; (3) an annulus with inner radius 50 minarc, outer radius 70 minarc (to derive a measure at 1° eccentricity); and (4) an annulus with inner radius 90 minarc, outer radius 120 minarc (to derive a measure at 1.75° eccentricity). A small black dot was present at the center of the solid disks as a fixation aid. A fixation target of 5 minarc radius was also centered within each annulus.
For the reference measurement, the subject viewed a test target of 1° radius centered at 7° temporal eccentricity while looking at the red fixation LED. A spatial profile was not derived in the left eye; only two test targets were used (at 0.5° and 1° radius), and the reference was the 1° target at 7° nasal eccentricity. In all cases, the test target was superimposed on the center of a blue background field of 3° radius.
The peak wavelength, full bandwidth at half maximum, and luminance of the stimuli were respectively as follows: background, 468 nm, 30.6 nm, 2.6 cd/m2; yellow-green test, 564 nm, 32.5 nm, 1.9 cd/m2; blue test, 460 nm, 22.5 nm bandwidth, luminance adjusted by subject; blue-green test, 488 nm, 33 nm bandwidth, luminance adjusted by subject. Note that these luminance values are for each wavelength band in isolation. In the experiment, the test stimuli were superimposed on the background, so that the luminance of the test target was added to the luminance of the background.
In this article, MPOD calculations are based on the MP spectrum for fresh macaque retina measured by the late Paul Brown.
11 The absorption of MP for each test stimulus was computed by integrating over the wavelength band of the relevant LED. MPOD is referenced to the peak wavelength, 460 nm for the blue test, and to 500 nm for the blue-green auxiliary test.