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Ultraviolet sensitivity of three cone types in the aphakic observer determined by chromatic adaptation
VisionRes. Vol. 34, No. 11,pp. 1457-1459, 1994
Pergamoa
Copyright @ 1994 Eisevier Science Ltd Printed in Great Britain. All rights reserve-d 0042~6989/94 $7.00 + 0.00
0042_6989(93)EOO764
Ultraviolet Sensitivity of Three Cone Types in the Aphakic Observer Determined by Chromatic Adaptation WILLIAM
S. STARK,* ROBERT H. WAGNER,?
CAROLYN
MARTIN
GILLESPIE:
Received 3 February 1993; in revised form 22 April 1993
Sensitivities at 5.5 deg off-fovea from tke apkakic observer superimposed on orange, purple and blue fields were obtained to estimate short-, middle- and long-wavelength cone spectra respectively. The middk+wavelengtbmecbaaism had a visible wavelength maximum resembling a nomogram plus an UV sensitivity fitting a &-peak. The sbort (and, to a lesser extent, the long) wavelength coae sensitivities are higber in tbe UV than expected. Ultravioiet
Cones Lens Aphakia
Visual psychophysics
AUDUBON Despite sustained interest in UV vision, relevant studies in humans are few and far between. Photopic (Stark, 198’7)and scotopic (Griswold & Stark, 1992) sensitivities of aphakic subjects, whose lenses had been removed by cataract surgery, are high in the W since the lens absorbs W. Tan (1971) partitioned the photopic sensitivity into three cone spectra by obtaining data against chromatic backgrounds and by determining increment thresholds [IE mechanisms (see Stark & Tan, 1982)]. Tan’s middle and long wavelength cone spectra were appro~mately as expected with visible wavelength maxima and W &-peaks. However, his estimate of the short wavelength cone spectrum was unusual in that it continued to increase into the W. The purpose of this investigation was to replicate this study. An abstract of this work has been published (Stark, Wagner & Martin, 1993). MA’IXRIALSAND METHODS Subjects
Three sets of data were averaged from WSS, 44 yr old who had a traumatic cataract at age 10 removed at age 12. A quartz lens, focal length of 8 cm, corrected for the hyperopia of aphakia. RW, CG and MK were normally sighted subjects, ranging in age from 18 to 22. The data *Departmf%n of Biology, St Louis University, St Louis, MO 631032010, U.S.A. tSchool of Optometry, University of Missouri, St Louis, MO 631214499, U.S.A. $k-hool of Optometry, Univ~~ity of Alabama, Birmingham, AL 35294, U.S.A.
were collected at The University of Missouri
Our t~hniques have been detailed (Griswold t Stark, 1991), and only an overview is presented here. W and visible wavelength stimuli were from a 150 W Xenon arc in a housing without a heat filter and with a quartz condenser. Mon~hromatic stimuli were from a grating monochromator in combination with 26 interference filters from 315 to 650 m. All lenses were quartz. The image of a diaphragm, back projected onto frosted UV transmitting Plexiglas, provided stimuli of 38 min arc. Intensity was attenuated logarithmically by two Nickel quartz neutral density wedges. This main beam was superimposed onto a chromatic field of 50 min arc visual angle. A GE ribbon filament TlO/IP, was filtered by a KG1 heat filter. Orange (Corning 3480, Corning, NY, > 570 nm), purple (Wratten D-35, Kodak, Rochester, NY, f 420 and > 680 nm) and blue (Corning 5030,370-490 nm) filters were used to isolate the short-, middle- and long-wavelength cone mechanisms. Pilot data on the control subjects were used to locate three spectra at equal absolute levels by attenuating the blue filter stimulus by 1.8 log units, the purple stimulus by 1.5, and the orange stimulus by 1.2. The subject viewed the stimulus through a 2mm artificial pupil and fixated on an LED to position the stimulus 5.5 deg off-fovea on the temporal retina where cones are present but macular pigments are not, To insure stimulus visibility, the subject made sure another LED 1.1 deg temporal to the stimulus was visible, Calibrations and experiments were automated by a
1457
1458
WILLIAM
computer motors.
and an interface,
a shutter,
S. STARK
et al.
and stepping
7c
1
6-
Calibrations
The neutral density wedge was calibrated using a photomultiplier and a photodiode which had been crosscalibrated with other instruments (Stark, 1987; Griswold & Stark, 1992). After each experiment, the threshold intensity at each wavelength was calibrated using the photodiode and an oscilloscope. Although it is difficult to calibrate broad band chromatic backgrounds, we approximated these intensities by assigning “average” wavelengths for orange, purple and blue stimuli of 585, 420 and 480 nm respectively. The stimuli were about 5.4, 5.5 and 6.4 x 10” quanta/cm’ . set respectively. The face validities of these values were confirmed from metameric matches with calibrated narrow band monochromatic stimuli.
* @ ‘5 = <
5-
4-
5 v,
3-
: -
2-
300
7
400
500 Wa*gth
Procedure
A staircase method was used to determine thresholds. For each trial, a tone alerted the subject who then pressed a button signalling that both LEDs were visible. An audible shutter then presented a 200 msec stimulus. The subject’s response interactively drove an intensity change of 0.66 log units in the appropriate direction until the response reversed, then by 0.54 log units in the opposite direction, etc. with each change 0.12 log units less than the previous one until only a 0.06 log unit intensity change was made. After the subject alternated his response to three consecutive stimuli, the wedge angle was recorded to calculate threshold. Photopic stimuli which should look colored (Stark, 1987), UV stimuli
600
700
800
(M)
FIGURE 2. Corresponding aphakic spectral sensitivities plotted on the same scale as in Fig. 1 (from Tan, 1971; Stark & Tan, 1982). Open circles, triangles and squares with dotted lines are averaged from two subjects to estimate short-, middle- and long-wavelength mechanisms determined as we did, i.e. sensitivity against orange, purple and blue chromatic fields respectively. Solid circles, triangles and squares with solid lines (which overlap the above data substantially) are from one subject for the corresponding rrr (short-wavelength), nq’ (middlewavelength) and ns’ (long-wavelength) cone mechanisms. Ordinate is as in Tan (1971).
looking blue (Tan, 1971; Stark & Tan, 1982), were noted just by detection in this difficult threshold task.
RESULTS
-15; 250
.
, 350
-
I
450 W~velongkh
-
I.
550
I
650
.
i
750
(nm)
FIGURE 1. Aphakic photopic spectral sensitivities against orange (circles, short-wavelength cone mechanism), purple (triangles, middlewavelength cone mechanism) and blue (squares, long-wavelength cone mechanism) backgrounds. Data correct for ocular media other than the lens (Boettner & Wolter, 1962). Ordinate is inverse (hence the negative values) threshold in log,, (quanta/cm2 set) calibrated just beyond the Plexiglas screen.
The three chromatically adapted photopic spectral sensitivities at 5.5 deg off-fovea for the aphakic observer are shown in Fig. 1. Figure 2 presents Tan’s corresponding data together with Tan’s data from Stiles’s (e.g. 1959) increment threshold determinations (1971; see Stark & Tan, 1982). Figures 1 and 2 estimate receptor spectra since the cornea, aqueous humor, and vitreous humor absorptions of human eyes (Boettner & Wolter, 1962) are factored out. Figure 3 shows data of Schnapf, Kraft, Nunn and Baylor (1988) converted from a wave number to a wavelength scale, from the photocurrent of single cones of Mucacafasciculuris as well as the corresponding nomogram curves from Ebrey and Honig (1977) for vitamin Al based visual pigments peaking at 430, 530 and 561 nm, wavelengths selected from Schnapf et al. (1988). All three figures have the same log units and wavelength per unit length for comparison. Near the visible wavelength maxima, all curves can be superimposed. From the visible wavelength maxima to the lower spectral limits, our data (Fig. 1) match Tan’s (Fig. 2) closely. The Schnapf et al. (1988) data extend to shorter wavelengths than most other data and were selected for comparison because they are recent, physiological, necessarily corrected to receptor spectra by the method,
1459
HUMAN UV CONE SPECTRA
*r -*.2: > ?S __ 3-
t
at tn -4$
_
-5 -6-
-7’ 350
450
550
650
750
650
Wavelength (nm) FIGURE 3. Data for short- (circles, dotted line), middle- (triangles, dotted line) and long- (squares, dotted line) wavelength cones of the rhesus monkey (Schnapf et al., 1988) are plotted on the same scale as in Figs 1 and 2. The corresponding Ebrey and Honig (1977) nomogram curves for vitamin A, based visual pigments (solid lines) are plotted on top of the data. Ordinate in negative log units with the maxima at zero.
may contribute to stimulus detectability, and the judgment is difficult (Methods). Although the UV peak of the middle-wavelen~h cone mechanism is about as high as the visible wavelength peak, we think this spectrum is as expected, with a UV sensitivity fitting a cis peak or p-band (Stark & Tan, 1982). By contrast, data for sensitivity mediated by the short-, and perhaps the long-, wavelength cones suggest UV sensiti~ties beyond those expected from the cone rhodopsin’s cis peak (Stark & Tan, 1982) and beyond the corresponding cones from Mucuca (Schnapf et al., 1988). Fluorescence mediation of UV sensitivity was discussed by Tan (197 1). Fluorescence of ocular media can be virtually omitted as a possible mechanism since these media absorb little, and the distances from receptors would necessitate remarkable quantum efficiencies. Furthermore, UV stimuli apear crisp, not blurred as would be expected from fluorescent emission from media far from receptors. Fluorescence within receptor cells was considered “a matter of definition”, and the possibility of a sensitizing pigment which fluoresces or transfers energy to the visual pigment by inductive resonance (Stark & Tan, 1982) cannot be eliminated.
REFERENCES and from a relevant species. Our data (Fig. 1) for the middle-wavelength cone mechanism match those of Schnapf et al. (Fig. 3) quite well, but our shortand long-wavelength mechanisms (Fig. 1) show higher sensitivities than those obtained from the rhesus monkey. Especially relevant is our confirmation (Fig. 1) of Tan’s finding (Fig. 2) that the short-wavelength mechanism does not even trace out a visible wavelength maximum but, instead, continues to climb into the UV.
DISCUSSION We used one of two established techniques and replicated the surprising and little appreciated finding that there may be mechanisms enhancing the UV sensitivities of short-, and perhaps long-, wavelength cones in humans. Determining thresholds as a function of background intensities, the Stiles’s (e.g. 1959) A mechanisms (Fig. 2), produced similar curves to those derived by our threshold approach (Figs 1 and 2) (Tan, 1971; Stark & Tan, 1982). This threshold method, the asymptote of the increment threshold technique in which spectra are obtained against bright chromatic fields, was used, for instance, by Wald (1968) to isolate cone spectra from normal and color blind subjects. This methodology is simple but may be complicated in that differing colors
Boettuer, E. A. & Wolter, J. R. (1962). Transmission of the ocular media. Investigative Ophthalmology and Visual Science, 1, 776783. Ebrey, T. G. & Honig, B. (1977). New wavelength dependent visual pigment nomograms. Vision Research, 17, 147-l 5 1. Griswold, M. S. & Stark, W. S. (1992). Scotopic spectral sensitivity of phakic and aphakic observers extending into the near ultraviolet. Vision Research,
32, 1739-I 743.
Schnapf, J. L., Kraft, T. W., Nnnn, B. J. & Baylor, D. A. (1988). Spectral sensitivity of primate photoreceptors. Visuul Neuroscience, I, 255-261.
Stark, W. S. (1987). Photopic sensitivities to ultraviolet and visible wavelengths and the effects of the macular pigments in human aphakic observers. Current Eye Research, 6, 631638. Stark, W. S. & Tan, K. E. W. P. (1982). Ultraviolet light: Photosensitivity and other effects on the visual system. Photochemistry and Photobiology,
36, 371-380.
Stark, W. S., Wagner, R. & Martin, C. (1993). Ultraviolet sensitivity of three cone types in aphakic observers determined by chromatic adaptation. Investigative Ophthalmology and Visual Science, 34, 751. Stiles, W. S. (1959). Colour vision: The approach through increment threshold sensitivity. Proceedings of the National Academy of Sciences, U.S.A, 75, 100-114. Tan, K. E. W. P. (1971). Vision in the ultraviolet Ph. D., Utrecht. In University of Missouri Library (call No. QP48l.Tl6). Wald, G. (1968). Molecular basis of visual excitation. Science, 162, 230-239.
Acknowledgenzenrs-Supported by NIH grant ROl EY07192 and by an undergraduate fellowship to Carolyn Martin Gillespie from a Howard Hughes Medical Institute grant to The University of Missouri&Columbia where the data were collected. We thank M. Kirks for assistance with data collection and for serving as a control (phakic) subject.
Pergamoa
Copyright @ 1994 Eisevier Science Ltd Printed in Great Britain. All rights reserve-d 0042~6989/94 $7.00 + 0.00
0042_6989(93)EOO764
Ultraviolet Sensitivity of Three Cone Types in the Aphakic Observer Determined by Chromatic Adaptation WILLIAM
S. STARK,* ROBERT H. WAGNER,?
CAROLYN
MARTIN
GILLESPIE:
Received 3 February 1993; in revised form 22 April 1993
Sensitivities at 5.5 deg off-fovea from tke apkakic observer superimposed on orange, purple and blue fields were obtained to estimate short-, middle- and long-wavelength cone spectra respectively. The middk+wavelengtbmecbaaism had a visible wavelength maximum resembling a nomogram plus an UV sensitivity fitting a &-peak. The sbort (and, to a lesser extent, the long) wavelength coae sensitivities are higber in tbe UV than expected. Ultravioiet
Cones Lens Aphakia
Visual psychophysics
AUDUBON Despite sustained interest in UV vision, relevant studies in humans are few and far between. Photopic (Stark, 198’7)and scotopic (Griswold & Stark, 1992) sensitivities of aphakic subjects, whose lenses had been removed by cataract surgery, are high in the W since the lens absorbs W. Tan (1971) partitioned the photopic sensitivity into three cone spectra by obtaining data against chromatic backgrounds and by determining increment thresholds [IE mechanisms (see Stark & Tan, 1982)]. Tan’s middle and long wavelength cone spectra were appro~mately as expected with visible wavelength maxima and W &-peaks. However, his estimate of the short wavelength cone spectrum was unusual in that it continued to increase into the W. The purpose of this investigation was to replicate this study. An abstract of this work has been published (Stark, Wagner & Martin, 1993). MA’IXRIALSAND METHODS Subjects
Three sets of data were averaged from WSS, 44 yr old who had a traumatic cataract at age 10 removed at age 12. A quartz lens, focal length of 8 cm, corrected for the hyperopia of aphakia. RW, CG and MK were normally sighted subjects, ranging in age from 18 to 22. The data *Departmf%n of Biology, St Louis University, St Louis, MO 631032010, U.S.A. tSchool of Optometry, University of Missouri, St Louis, MO 631214499, U.S.A. $k-hool of Optometry, Univ~~ity of Alabama, Birmingham, AL 35294, U.S.A.
were collected at The University of Missouri
Our t~hniques have been detailed (Griswold t Stark, 1991), and only an overview is presented here. W and visible wavelength stimuli were from a 150 W Xenon arc in a housing without a heat filter and with a quartz condenser. Mon~hromatic stimuli were from a grating monochromator in combination with 26 interference filters from 315 to 650 m. All lenses were quartz. The image of a diaphragm, back projected onto frosted UV transmitting Plexiglas, provided stimuli of 38 min arc. Intensity was attenuated logarithmically by two Nickel quartz neutral density wedges. This main beam was superimposed onto a chromatic field of 50 min arc visual angle. A GE ribbon filament TlO/IP, was filtered by a KG1 heat filter. Orange (Corning 3480, Corning, NY, > 570 nm), purple (Wratten D-35, Kodak, Rochester, NY, f 420 and > 680 nm) and blue (Corning 5030,370-490 nm) filters were used to isolate the short-, middle- and long-wavelength cone mechanisms. Pilot data on the control subjects were used to locate three spectra at equal absolute levels by attenuating the blue filter stimulus by 1.8 log units, the purple stimulus by 1.5, and the orange stimulus by 1.2. The subject viewed the stimulus through a 2mm artificial pupil and fixated on an LED to position the stimulus 5.5 deg off-fovea on the temporal retina where cones are present but macular pigments are not, To insure stimulus visibility, the subject made sure another LED 1.1 deg temporal to the stimulus was visible, Calibrations and experiments were automated by a
1457
1458
WILLIAM
computer motors.
and an interface,
a shutter,
S. STARK
et al.
and stepping
7c
1
6-
Calibrations
The neutral density wedge was calibrated using a photomultiplier and a photodiode which had been crosscalibrated with other instruments (Stark, 1987; Griswold & Stark, 1992). After each experiment, the threshold intensity at each wavelength was calibrated using the photodiode and an oscilloscope. Although it is difficult to calibrate broad band chromatic backgrounds, we approximated these intensities by assigning “average” wavelengths for orange, purple and blue stimuli of 585, 420 and 480 nm respectively. The stimuli were about 5.4, 5.5 and 6.4 x 10” quanta/cm’ . set respectively. The face validities of these values were confirmed from metameric matches with calibrated narrow band monochromatic stimuli.
* @ ‘5 = <
5-
4-
5 v,
3-
: -
2-
300
7
400
500 Wa*gth
Procedure
A staircase method was used to determine thresholds. For each trial, a tone alerted the subject who then pressed a button signalling that both LEDs were visible. An audible shutter then presented a 200 msec stimulus. The subject’s response interactively drove an intensity change of 0.66 log units in the appropriate direction until the response reversed, then by 0.54 log units in the opposite direction, etc. with each change 0.12 log units less than the previous one until only a 0.06 log unit intensity change was made. After the subject alternated his response to three consecutive stimuli, the wedge angle was recorded to calculate threshold. Photopic stimuli which should look colored (Stark, 1987), UV stimuli
600
700
800
(M)
FIGURE 2. Corresponding aphakic spectral sensitivities plotted on the same scale as in Fig. 1 (from Tan, 1971; Stark & Tan, 1982). Open circles, triangles and squares with dotted lines are averaged from two subjects to estimate short-, middle- and long-wavelength mechanisms determined as we did, i.e. sensitivity against orange, purple and blue chromatic fields respectively. Solid circles, triangles and squares with solid lines (which overlap the above data substantially) are from one subject for the corresponding rrr (short-wavelength), nq’ (middlewavelength) and ns’ (long-wavelength) cone mechanisms. Ordinate is as in Tan (1971).
looking blue (Tan, 1971; Stark & Tan, 1982), were noted just by detection in this difficult threshold task.
RESULTS
-15; 250
.
, 350
-
I
450 W~velongkh
-
I.
550
I
650
.
i
750
(nm)
FIGURE 1. Aphakic photopic spectral sensitivities against orange (circles, short-wavelength cone mechanism), purple (triangles, middlewavelength cone mechanism) and blue (squares, long-wavelength cone mechanism) backgrounds. Data correct for ocular media other than the lens (Boettner & Wolter, 1962). Ordinate is inverse (hence the negative values) threshold in log,, (quanta/cm2 set) calibrated just beyond the Plexiglas screen.
The three chromatically adapted photopic spectral sensitivities at 5.5 deg off-fovea for the aphakic observer are shown in Fig. 1. Figure 2 presents Tan’s corresponding data together with Tan’s data from Stiles’s (e.g. 1959) increment threshold determinations (1971; see Stark & Tan, 1982). Figures 1 and 2 estimate receptor spectra since the cornea, aqueous humor, and vitreous humor absorptions of human eyes (Boettner & Wolter, 1962) are factored out. Figure 3 shows data of Schnapf, Kraft, Nunn and Baylor (1988) converted from a wave number to a wavelength scale, from the photocurrent of single cones of Mucacafasciculuris as well as the corresponding nomogram curves from Ebrey and Honig (1977) for vitamin Al based visual pigments peaking at 430, 530 and 561 nm, wavelengths selected from Schnapf et al. (1988). All three figures have the same log units and wavelength per unit length for comparison. Near the visible wavelength maxima, all curves can be superimposed. From the visible wavelength maxima to the lower spectral limits, our data (Fig. 1) match Tan’s (Fig. 2) closely. The Schnapf et al. (1988) data extend to shorter wavelengths than most other data and were selected for comparison because they are recent, physiological, necessarily corrected to receptor spectra by the method,
1459
HUMAN UV CONE SPECTRA
*r -*.2: > ?S __ 3-
t
at tn -4$
_
-5 -6-
-7’ 350
450
550
650
750
650
Wavelength (nm) FIGURE 3. Data for short- (circles, dotted line), middle- (triangles, dotted line) and long- (squares, dotted line) wavelength cones of the rhesus monkey (Schnapf et al., 1988) are plotted on the same scale as in Figs 1 and 2. The corresponding Ebrey and Honig (1977) nomogram curves for vitamin A, based visual pigments (solid lines) are plotted on top of the data. Ordinate in negative log units with the maxima at zero.
may contribute to stimulus detectability, and the judgment is difficult (Methods). Although the UV peak of the middle-wavelen~h cone mechanism is about as high as the visible wavelength peak, we think this spectrum is as expected, with a UV sensitivity fitting a cis peak or p-band (Stark & Tan, 1982). By contrast, data for sensitivity mediated by the short-, and perhaps the long-, wavelength cones suggest UV sensiti~ties beyond those expected from the cone rhodopsin’s cis peak (Stark & Tan, 1982) and beyond the corresponding cones from Mucuca (Schnapf et al., 1988). Fluorescence mediation of UV sensitivity was discussed by Tan (197 1). Fluorescence of ocular media can be virtually omitted as a possible mechanism since these media absorb little, and the distances from receptors would necessitate remarkable quantum efficiencies. Furthermore, UV stimuli apear crisp, not blurred as would be expected from fluorescent emission from media far from receptors. Fluorescence within receptor cells was considered “a matter of definition”, and the possibility of a sensitizing pigment which fluoresces or transfers energy to the visual pigment by inductive resonance (Stark & Tan, 1982) cannot be eliminated.
REFERENCES and from a relevant species. Our data (Fig. 1) for the middle-wavelength cone mechanism match those of Schnapf et al. (Fig. 3) quite well, but our shortand long-wavelength mechanisms (Fig. 1) show higher sensitivities than those obtained from the rhesus monkey. Especially relevant is our confirmation (Fig. 1) of Tan’s finding (Fig. 2) that the short-wavelength mechanism does not even trace out a visible wavelength maximum but, instead, continues to climb into the UV.
DISCUSSION We used one of two established techniques and replicated the surprising and little appreciated finding that there may be mechanisms enhancing the UV sensitivities of short-, and perhaps long-, wavelength cones in humans. Determining thresholds as a function of background intensities, the Stiles’s (e.g. 1959) A mechanisms (Fig. 2), produced similar curves to those derived by our threshold approach (Figs 1 and 2) (Tan, 1971; Stark & Tan, 1982). This threshold method, the asymptote of the increment threshold technique in which spectra are obtained against bright chromatic fields, was used, for instance, by Wald (1968) to isolate cone spectra from normal and color blind subjects. This methodology is simple but may be complicated in that differing colors
Boettuer, E. A. & Wolter, J. R. (1962). Transmission of the ocular media. Investigative Ophthalmology and Visual Science, 1, 776783. Ebrey, T. G. & Honig, B. (1977). New wavelength dependent visual pigment nomograms. Vision Research, 17, 147-l 5 1. Griswold, M. S. & Stark, W. S. (1992). Scotopic spectral sensitivity of phakic and aphakic observers extending into the near ultraviolet. Vision Research,
32, 1739-I 743.
Schnapf, J. L., Kraft, T. W., Nnnn, B. J. & Baylor, D. A. (1988). Spectral sensitivity of primate photoreceptors. Visuul Neuroscience, I, 255-261.
Stark, W. S. (1987). Photopic sensitivities to ultraviolet and visible wavelengths and the effects of the macular pigments in human aphakic observers. Current Eye Research, 6, 631638. Stark, W. S. & Tan, K. E. W. P. (1982). Ultraviolet light: Photosensitivity and other effects on the visual system. Photochemistry and Photobiology,
36, 371-380.
Stark, W. S., Wagner, R. & Martin, C. (1993). Ultraviolet sensitivity of three cone types in aphakic observers determined by chromatic adaptation. Investigative Ophthalmology and Visual Science, 34, 751. Stiles, W. S. (1959). Colour vision: The approach through increment threshold sensitivity. Proceedings of the National Academy of Sciences, U.S.A, 75, 100-114. Tan, K. E. W. P. (1971). Vision in the ultraviolet Ph. D., Utrecht. In University of Missouri Library (call No. QP48l.Tl6). Wald, G. (1968). Molecular basis of visual excitation. Science, 162, 230-239.
Acknowledgenzenrs-Supported by NIH grant ROl EY07192 and by an undergraduate fellowship to Carolyn Martin Gillespie from a Howard Hughes Medical Institute grant to The University of Missouri&Columbia where the data were collected. We thank M. Kirks for assistance with data collection and for serving as a control (phakic) subject.