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OCR for page 31
Photosensitivity of Primate
Photoreceptors
JULIE L. SCHNAPF
Our ability to discriminate different wavelengths of light depends on
the different spectral sensitivities of the three kinds of cones in our reti-
nas. These spectral functions have been determined in rods and cones of
macaque and human retinas by recording their photocurrents. The spectra
obtained in these experiments provide the receptoral basis for human color-
matching functions and luminosity functions. Results have been described
in detail by Baylor et al. (1984, 1987) and Schnapf et al. (1987, 1988~.
METHODS
Using a suction pipette, photocurrent was recorded from single rods
and cones of the monkey Macaca fasciculans. Psychophysical experiments
(DeValois et al., 1974) suggest that this monkey's photoreceptors are similar
to those of humans. A limited number of electrical recordings were also
made in cones of a single human eye (Schnapf et al., 1987), and the results
support that suggestion.
Photoreceptors were illuminated by light incident perpendicular to the
long axes of their outer segments so that pigment self-screening was neg-
ligible. Assuming then that the quantal efficiency of bleaching is constant
for the wavelengths used, the spectra obtained here should be proportional
to the probability that a single photopigment will absorb light as a function
of wavelength.
Spectral sensitivity was obtained by the method of criterion response
(Naka and Rushton, 1966; Baylor et al., 1984~. Brief flashes of light
were applied at a test wavelength and a reference wavelength (500 nm) to
determine the relative amount of light at the two wavelengths required to
elicit responses of criterion size. The reciprocal of the flash photon densities
31
OCR for page 32
32
JULIE Lo SCHNAPF
of the two lights gave the relative spectral sensitivity. The responses of the
photoreceptors appeared to obey the "principle of spectral univariance,"
depending only on the number of photons absorbed and not the wavelength
(Naka and Rushton, 1966~. Results were obtained from 51 rods and cones
in the macaque retina and 6 cones in a human retina.
RESULTS
Spectral Sensitivity
Spectral sensitivities of the macaque photoreceptors are shown in
Figure 1. The symbols plot the normalized average values obtained from
the blue (o), green Gil, and red cones (o) and the rods (~) of 12 macaque
monkeys. The curves near the cone spectra are drawn by eye, while the
rod curve is the Dartnall nomogram with a peak sensitivity at 491 nm. The
peaks of the cone spectra lie at about 430, 530, and 560 nm.
Spectra obtained from human cones are shown in Figure 2. The points
plot the average normalized spectra from five red cones (A) and one green
cone (B). The smooth curves are the same as those drawn through the
corresponding macaque spectra of Figure 1. Spectra obtained from the
two species are indistinguishable. The amplitude and waveform of the light
responses are also very similar for the two species.
The spectra of individual cones varied little for cells within a class. For
a given class of cone, the standard deviation of the positions of the spectra
along the abscissa was less than 1.5 nm. (Spectral position was estimated
by fitting a straight line to the descent of sensitivity at low wavenumber.)
This amount of variability is similar to that estimated from human color-
matching experiments (MacLeod and Webster, 1983; Neitz and Jacobs,
1986~.
When plotted on a normalized wavenumber scale, where wavenumber
is divided by the wavenumber of maximum absorption, the absorption
spectra of the four macaque photoreceptors have a common shape. The
action spectra shown here confirm this observation, although the normalized
rod spectrum is slightly broader than that of the cones. It is not known
what, if any, physical mechanism might be implied by this scaling principle.
It appears, however, that the shape of the normalized spectrum is species
specific, as the normalized action spectrum of photoreceptors of the ground
squirrel differs significantly from that of the macaque (Kraft, 1988~.
Color Matching
How can the action spectra of the cones be compared to human color
vision? Since the electrical response in a cone outer segment depends
OCR for page 33
PHOTOSENSITIVITY OF PRIMATE PHOTORECEPTORS
Wavenumber (pm~~)
2.6 2.4 2.2 2.0 1.8
33
1.6 1.4 1.2
1 1 1 1 1 1 1 1
o
-1
Log S
—2
3
4
- 5
—6
_~] ~
8=~
, Irk ~~
\
400 500 600 700 800 900
Wavelength (nary)
FIGURE 1 Spectral sensitivities of macaque photoreceptor. Points plot the average
normalized sensitivities of 5 blue cones (o), 20 green cones God, 16 red cones (o), and 10
rods God, as a function of wavenumber. Wavelength scale above. SOURCE: Schnapf et al.
(1988~.
OCR for page 34
34
JULIE Lo SCHNAPF
A
Wavenumber (pm')
t.2
1 1 1 1 1 1 1 ~
_ In, RED
_°~ ~
1 1 1 1 1 1 1 1
2.6 2.4 2.2 2.0 1.8 1.6 1.4
n
—1
Log S
—2
3
4
5
B
Wavonumber (lim-')
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2
1 1 1 1 1 1 1 1
of
400 500 600 700 800 400
Wavelength (nary)
~0 GREEN
1 1 1 1 1 1 1 1
1 1 1 1 1
500 600 700 800
Wavelength tnm)
_
FIGURE 2 Spectral sensitivity of human cones. Points plot the average normalized
sensitivities of five red cones in A and one green code B as a function of wavenumber,
wavelength above. The bars in A are the standard deviations. The curves are the same as
those for the corresponding spectra in Figure 1. SOURCE: Schnapf et al. (1987~.
Only on the number of photons absorbed and not wavelength, it follows
that stimuli comprised of different combinations of wavelengths will be
indistinguishable to us if the number of quantal adsorptions evoked in our
cones is identical. The intensities of the component wavelengths required
to equate quantal adsorptions elicited by physically different stimuli may
be calculated from the spectral sensitivity functions of the three cones.
The color-matching experiments of Stiles and Burch (1955) were used
for the calculations. In these psychophysical experiments, observers ad-
justed the intensities of long-, middle-, and short-wavelength "primaries"
so that when the three lights were superimposed the stimulus looked iden-
tical to a single "test" wavelength of unit intensity. The smooth curves
in Figure 3 show the intensities of the primaries (incident on the cornea)
plotted as a function of the wavelength of the test light. The symbols
indicate the expected color-matching functions calculated from the spectral
sensitivity measurements of the macaque cones of Figure 1. The macaque
spectra were first adjusted to take into account differences in the way the
light was applied in the two kinds of experiments. Specifically, the effects
of light absorption by the human lens and macular pigment were taken
into account as well as photopigment self-screening within the cone outer
segments in the psychophysical experiment. The forms of the preretinal ab-
sorption spectra were taken from Wyszecki and Stiles (1982~; self-screening
was assumed to follow Beer's Law. The absolute magnitude of the lens,
OCR for page 35
PHOTOSENSlT~ITY OF PRIMATE PHOTORECEPTORS
4
3
Relative
Intensi ty
2
o
35
o
_ ~ N~ t: _ ~
400 500 600 700
Wavelength (nary)
FIGURE 3 Color-matching functions. The smooth curves show the relative amount of
light required of the three pnmanes to match a test light of unit intensity as a function
of the wavelength of the test light (Stiles and Burch, 1955~. The points plot the predicted
results based on the spectral sensitivity functions of the macaque cones in Figure 1 after
co~Tectio~i for absorption by the lens and macular pigment and photopigment self-screening.
The correction factors used were: macular pigment density at 460 nm, 0.29; lens density at
400 nm, 1.22; and peak axial photopigment density, 0.27. the wavelengths of the pIimanes
are 444 (a), 526 (o) and 645 (o) nm. SOURCE: Baylor et al. (1987~.
macular, and photopigment densities (given in Figure 3's caption) were
chosen to give the best fit between the two sets of data. A good correspon-
dence was obtained between the psychophysical and adjusted physiological
measurements, supporting the idea that color-matching functions may be
derived simply from the absorption characteristics of the cone pigments
and that the macaque and human cone pigments are very similar.
OCR for page 36
36
JULIE L SCHNAPF
Luminosity
Another reflection of the spectral sensitivity functions as seen in psy-
chophysical experiments is luminosity. Luminosity describes the efficiency
of light (incident on the cornea) in stimulating the visual system as a func-
tion of wavelength. In light-adapted conditions the photopic luminosity
function is thought to reflect a linear sum of the excitation of the red
and green cones. Under dark-adapted conditions the scotopic function
is thought to depend on the excitation of the rods. These two functions
are given by the smooth curves in Figure 4. The continuous line is the
photopic function of Vos (1978~. The open circles plot the weighted sum
of the "corrected" spectral sensitivities of the red and green cones of the
human retina (Figure 2~; the corrections were the same as those applied
in Figure 3. The corrected spectra were weighted so that the green spec-
trum contributed only about half as much (0.52) to the sum as did the
red spectrum. This weighting was chosen to maximize the fit between the
calculated and measured functions.
The scotopic luminosity function of Crawford (1949) is shown by the
dashed curve, and the filled symbols plot the "corrected" spectral sensitivity
function of the macaque rods (Figure 1~. The correction factors here (given
in Figure 4's caption) were chosen to give the best fit between the points
and the curve. A good match was obtained between the two luminosity
functions and the corresponding physiological results, consistent with the
idea that the scotopic function is set by the filtered rod spectrum and that
the photopic function is determined by the summed activity of the red and
green cones. The scaling factor (0.52) weighting the contributions of the
two cone spectra may reflect the relative numbers of the two cone types
in the human eye or the relative strengths of their signals to higher-order
neurons.
Bleaching Cross Section
The bleaching cross-sectional area, A, of a photopigment is a measure
of the effective physical area over which a single pigment captures light.
For steady light of intensity I, the product lA gives the expected rate
of isomerization per pigment. This value has been previously estimated
for rhodopsin by measuring, in the presence of steady bright light, the
optical density of a dilute solution of rhodopsin as a function of time. The
density falls exponential in time as the fraction of unbleached rhodopsin
declines; the time constant of decay is (L4~-~. The estimated values of A
for rhodopsin (for light of optimal wavelength) were in the range of 7-10
X 1o-~7 cm2 (Dartnall, 1972~.
OCR for page 37
PHOTOSENSITIVITY OF PRIMATE PHOTORECEPTORS
o
—1
Log S
l
—2 · `
- 5
—6
37
/
/
~'—,WQ
/ oW
/
\
~ \ ~
\
\ \
\
\
\
400 500 600 700 800
Wavelength ( rim )
FIGURE 4 Luminosity functions and comparison to rod and cone spectra. The continuous
curve is the photopic luminosity function of Vos (1978~; the dashed curve is the scotopic
luminosity function of Crawford (1949~. The symbols plot the correlated spectral sensitivities
of the macaque rods (~) and the weighted sum of the human red and green cones (o).
The cone spectra were corrected as in Figure 3. The corrections for the rods were: lens
density at 400 nm, 1.54; and peak axial photopigment density, 0.35. SOURCE: Schnapf et
al. (1988~.
Similar density measurements for cone pigments are not available
because of difficulties in pigment extraction. However, measurements of
the photocurrent of single cones appear to provide an alternative method
of estimating A. In the presence of steady bleaching light, the photocurrent
was found to decline exponentially in time, with a time constant that was
inversely related to I. These observations are consistent with the idea that
the photocurrent is tracking the fraction of unbleached pigment in the cone
outer segment. If so, A may be calculated from the intensity of the light and
OCR for page 38
38
JULIE L SCHNAPF
the time constant of decay. The mean value of A calculated from several
cones was found to lie within the range of that estimated for rhodopsin.
ACKNOWLEDGMENTS
This work was supported by grant EY05750 from the National Eye
Institute and by an award from Research to Prevent Blindness, Inc.
REFERENCES
Baylor, D.N, BJ. Nunn, and J.L. Schnapf
1984 The photocurrent, noise and spectral sensitivity of rods of the monkey
Macaca fasciculans. Joumal of Physiology 357:575 607.
1987 Spectral sensitivity of cones of the monkey Macaca fasciculans Journal of
Physiology 390:145-160.
Crawford, B.H.
1949 The scotopic visibility function. Proceedings of ~hePhysical Society B62:321-
334.
Dartnall, HJ.N
1972 Photosensitivity. Pp. 122-145 in Photochemistry of Vision, HJ.A. Dartnall,
ed. New York: Springer-Verlag.
DeValois, R., H.C. Morgan, M.C. Polson, OUR. Mead, and ELI. Hull
1974 Psychophysical studies on monkey vision I. Macaque luminosity and color
vision tests. Vision Research 14:53 67.
Kraft, TW.
1988 Transduction by cones in the golden-mantled ground squirrel, Citellus lateralis.
Biophysical Joumal 53:474a.
MacLeod, D.I.N, and M.N Webster
1983 Factors influencing the colour matches of normal observed. Pp. Sl-92 in
Colour Vision, J.D. Mollon and LT. Sharpe, eds. London: Academic Press.
Naka, HI., and W.A.H. Rushton
1966 S-potentials from colour units in the retina of fish (Cypnnidae). Joumal of
Physiology 185:53~555.
Neitz, J., and G.H. Jacobs
1986 Polymorphism of the long-wavelength cone in normal human colour vision.
Nature 323:623 625.
Schnapf, J.L, T.W. Kraft, and D.N Baylor
1987 Spectral sensitivity of human cone photoreceptors. Nature 325:439-441.
Schnapf, J.L, T.W Kraft, BJ. Nunn, and D.N Baylor
1988 Spectral sensitivity of primate photoreceptors. Visual Neuroscience 1:255-261.
Stiles, W.S., and J.M. Burch
1955 Interim report to the Commission Internationale de l'Eclairage, Zurich,
1955, on the National Physical Laborato~y's investigation of colour-matching
(1955~. Optica Acta 2:168 181.
Vos, J.J.
1978 Colorimetric and photometric properties of a 2° fundamental observer.
Color Research and Applications 3:125-128.
Wyszecki, G., and W.S. Stiles
1982 Color Science: Concepts and Methods, Quantitative Data and Formulae. New
York: Wiley.
Representative terms from entire chapter:
green cones
