Chemistry Reference
In-Depth Information
his subjects were parallel above 578 nm (Wald 1945). This is consistent with the L-cone to M-cone
ratio in the fovea and parafovea being the same. Additional work in this area is needed to resolve
the issue.
HFP has also been used to measure the proi le of MP optical density across the retina rather
than a single, central optical density measurement (Hammond Jr. et al. 1997, Bone et al. 2004). In
order to make such a measurement, a set of i xation marks is provided to one side of the stimulus
so that it can be imaged at various distances from the center of the fovea. In addition, a number
of researchers have exploited the “edge hypothesis” for the same purpose (Werner et al. 1987,
Hammond Jr. et al. 1997, Beatty et al. 2000a, Hammond and Caruso-Avery 2000, Werner et al.
2000, Delori et al. 2001, Snodderly et al. 2004). This hypothesis states that for a circular stimulus,
l icker sensitivity is enhanced at the edge of the stimulus. Thus, when a subject achieves a l icker
null, it is because the luminances of the blue and green components of the stimulus are equalized at
an eccentricity from the fovea equal to the stimulus radius. By using stimuli of different radii, one
can, according to the hypothesis, obtain MP optical density measurements at several eccentrici-
ties and thereby obtain a proi le. However, the validity of the edge hypothesis has been questioned
(Bone et al. 2004).
5.2.2 M
INIMUM
M
OTION
AND
A
PPARENT
M
OTION
P
HOTOMETRY
Minimum motion photometry is a close cousin of HFP. The stimulus in this case consists of a grat-
ing of alternately colored bars that move across a circular, centrally viewed visual i eld. As with
HFP, the two colors are selected for maximum absorption by the MP and nearly zero absorption.
In Moreland's apparatus (Robson et al. 2003, Moreland 2004), wavelengths of 460 nm (blue) and
580 nm (orange) were chosen, and the stimulus superimposed on a 450 nm pedestal in order to satu-
rate S-cones. The subject adjusts the luminance of the orange bars until the perception of motion
of the bars across the i eld is minimized. This minimization condition occurs when the luminances
of the bars are equal for the subject. Proponents of the method claim that i nding this null point is
easier than the corresponding task in HFP. Once again it is necessary to make a reference measure-
ment with the stimulus imaged in an MP-free region of the retina in order to account for the factors
other than MP mentioned in the previous section. For this measurement, the visual i eld is in the
shape of an annular arc with the i xation point at the center of curvature. Proi les of the MP across
the retina can be obtained using arcs of different radii.
In apparent motion photometry, colored bars appear to move across the i eld of view but their
direction of motion reverses as the subject passes through the equiluminance condition (Anstis and
Cavanagh 1983). The illusion is achieved on a CRT monitor by presenting a repetitive sequence of
four square-wave gratings, each phase advanced by a quarter cycle from the previous one (West
and Mellerio 2005). The i rst and third gratings are composed of blue and red bars; the second
and fourth of light gray and dark gray bars. If the blue bars are brighter than the red, the subject
will associate their luminance with that of the light gray bars of the following grating that are, for
example, phase-shifted a quarter cycle to the right. When the third grating appears, its blue bars will
appear at yet another quarter cycle to the right, so the subject's perception is of the pattern of bars
moving continuously to the right. If, on the other hand, the red bars are brighter than the blue, their
luminance will be associated with the light gray bars of the following grating that are phase-shifted
to the left, and the perception is of motion to the left.
For the central measurement, the i eld of view is rectangular (e.g., 0.3 by 1.25°) and for other
eccentricities, the i eld is an annular arc similar to that provided in minimum motion photometry.
The use of a CRT monitor introduces the same problem as the use of LEDs in HFP, namely, the
broadband nature of the screen phosphors, and a correction must be made before reporting the peak
MP optical density. A system that could employ lamps and i lters instead of a CRT monitor would
be difi cult to design because of the complexity of the visual stimulus.
Search WWH ::
Custom Search