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light. Likewise differences in overall photoreceptor spectral sensitivity among otherwise identical
subjects could lead to different blue-light intensity settings. Thus, a means of eliminating the effects
of all but the MP is essential and the HFP procedure requires a second measurement. For this, the
subject directs his or her gaze toward a i xation mark to one side of the stimulus at an eccentricity
that varies among instruments from about 5° to 8°. The stimulus itself is then imaged in the para-
foveal retina, an area that is assumed to have negligible amounts of MP. Once again the subject
seeks to eliminate or minimize l icker, and the corresponding blue-light intensity setting rel ects the
degree of lens yellowing and the spectral properties of the photoreceptors, but not the MP. Because
the parafoveal retina has a lower l icker threshold than the fovea, a lower frequency is used for this
measurement than for the foveal test. The MP optical density at the blue test wavelength is given by
the log ratio of intensity settings made by the subject:
OD
=
log
(
II
/
)
(5.1)
10
fov
parafov
Acceptance of Equation 5.1 rests on the assumption that the only factor modifying the luminance
match between the blue and green lights in the fovea compared with that in the parafovea is the
attenuation of the blue light by MP in the former match. There is, however, evidence that MP opti-
cal density may not be completely negligible at the parafoveal location. It may increase with age
(Berendschot and Van Norren 2005) or as a result of supplementation with lutein or zeaxanthin
(Rodriguez-Carmona et al. 2006). The assumption would also be invalidated if the spectral sensi-
tivity of the photoreceptors in the fovea differed from that in the parafovea. Specii cally, if the ratio
of sensitivities at the blue to green wavelengths was higher in the parafovea than in the fovea, HFP
would return a value of the subject's MP optical density that was too high. Differing sensitivity ratios
across the retina could, in principle, be expected if the proportions of the three cone types (long-,
medium-, and short-wavelength-sensitive) and rods varied. Certainly rods and short-wavelength-
sensitive cones (S-cones) are not well represented in the fovea. However there is little contribution
to luminance by S-cones (Guth et al. 1980), and since S-cones have low l icker thresholds (Brindley
et al. 1966), any contribution to luminance can be minimized by the use of sufi ciently high l icker
frequencies. Additionally, the use of stimuli with luminances well above the mesopic range will
ensure that essentially only cones, and not rods, are responding. An added safeguard that has been
adopted in a number of applications is the use of a blue adapting background on which the stimulus
is superimposed (Hammond Jr. and Fuld 1992, Wooten et al. 1999). In principle, the background
will preferentially lower the sensitivity of the S-cones to the point where their contribution to the
luminance of the stimulus becomes negligible.
On the other hand, the ratio of long (L)- to medium (M)-wavelength-sensitive cones is believed
to remain reasonably constant as one moves outward from the fovea to the parafovea (Wooten and
Wald 1973). If this is the case, and in light of the arguments presented above, the effective spectral
sensitivity in the fovea will only differ from that in the parafovea because of the presence of MP
in the former region. The validity of this assumption has been put to the test by modifying HFP
so that the test wavelength can be varied throughout the wavelength range of MP absorption. In
this way, it has been possible to construct an MP optical density spectrum, which is in remarkably
good agreement with one obtained from spectrophotometric analysis of appropriate mixtures of
lutein and zeaxanthin (Bone et al. 1992). However, a recent study calls into question the assump-
tion of a constant L-cone to M-cone ratio across the retina (Bone et al. 2007b). In this study, the
test wavelength was varied not only over the absorption range of the MP, but up to a wavelength
of 680 nm. Above about 580 nm, a signii cant, generally increasing, apparent MP optical density
was observed in a number of subjects. In reality, lutein and zeaxanthin have zero optical den-
sity at these wavelengths. There is no evidence for the existence of another foveal pigment with
appropriate spectral properties. One possible explanation is a higher L-cone to M-cone ratio in
the parafovea compared with the fovea. However, when Wald measured spectral sensitivities in
these regions by the method of absolute thresholds, he found that the log-transformed curves for
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