January 2012
Volume 12, Issue 1
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Article  |   January 2012
Control of accommodation by longitudinal chromatic aberration and blue cones
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Journal of Vision January 2012, Vol.12, 14. doi:10.1167/12.1.14
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      Klaus Graef, Frank Schaeffel; Control of accommodation by longitudinal chromatic aberration and blue cones. Journal of Vision 2012;12(1):14. doi: 10.1167/12.1.14.

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Abstract

Purpose: To better understand the striking overaccommodation that is triggered at wavelengths below 430 nm (below referred to as OAB). Methods: Fourteen students served as subjects, 6 emmetropic and 8 mildly myopic. They fixated a reading target or a Landolt C at 33-cm distance while the wavelength of light illuminating the target was varied. Their accommodation was continuously monitored with the PowerRefractor (Multichannel Systems, Reutlingen, Germany, 1995). Luminances were matched using a candela meter (Minolta LS100) and neutral density filters. The following experiments were done: (1) confirmation of the effect at 10 cd/m2, (2) comparing 10 and 1 cd/m2, (3) foveal stimulation, (4) parafoveal stimulation, (5) testing independent combinations of the wavelength in the center and periphery, (6) testing accommodation tonus without fixation target while the wavelength is varied. Results: (1) OAB was nicely confirmed as initially described by F. J. Rucker and P. B. Kruger (2004a, 2004b) and A. Seidemann and F. Schaeffel (2002). (2) OAB remained stable at target luminances between 10 and 1 cd/m2. (3, 4) OAB was found to be more pronounced when the parafoveal region was stimulated than when mainly the fovea was stimulated. (5) When fovea and parafovea were illuminated by light of different wavelengths, the wavelength in the foveal region had greater impact on OAB. (6) OAB was not elicited in the absence of an accommodation target. Conclusions: OAB appears to be controlled more by the parafovea than by the fovea. The findings are in line with the assumption that OAB is mediated by the short-wavelength-sensitive cones that are absent from the central fovea in most subjects.

Introduction
Accommodation is controlled primarily by optical defocus imposed on the fovea (Kruger & Pola, 1986; Phillips & Stark, 1977; Schaeffel et al., 2006) and by binocular disparity (e.g., Schor, Bridgeman, & Tyler, 1983; Suryakumar, Meyers, Irving, & Bobier, 2007). More subtle influences come from spectral composition of light (e.g., Aggarwala, Nowbotsing, & Kruger, 1995; Kroger & Binder, 2000; Rucker & Kruger, 2004a, 2004b; Seidemann & Schaeffel, 2002). Numerous studies have shown that accommodation, measured as a change in the spherical equivalent refractive error, tends to lag behind the plane of optimal focus during reading (“lag of accommodation,” e.g., Gwiazda, Thorn, Buer, & Held, 1993; Mutti, Mitchell et al., 2006; Seidemann & Schaeffel, 2003), but this lag may be overestimated when higher order aberrations are not considered (Buehren & Collins, 2006; Gambra, Sawides et al., 2009). The lag of accommodation reported in the literature is highly variable among studies (comparisons are provided, for example, in Seidemann & Schaeffel, 2003), but on average, it amounts to about 0.3–0.4 D. Despite the small imperfection of accommodation, the retinal image appears clear to subjects, perhaps because “neural image sharpening” mechanisms are active (Webster, Georgeson, & Webster, 2002). Temporary improvements in focus may also occur due to the fluctuations of the accommodation tonus over time (e.g., Plainis, Ginis, & Pallikaris, 2005). Considering all these factors, accommodation may actually provide near-optimal focus of the retinal image within the range of depth of focus of the eye (with the depth of focus defined as the range of focus over which the Strehl ratio does not drop below 80% of its value at best focus, about 0.3 D in humans at daylight, e.g., Marcos, 2003). 
A complicating factor is longitudinal chromatic aberration. Since refractive indices of optical media vary with the wavelength of light, focal length and refractive state are also variable and the retinal image cannot be focused for all wavelengths at a time. In humans, the foveal long- (L) and mid-wavelength (M) cones provide high spatial acuity as they are densely packed and have “private lines” to the visual cortex. For this reason accommodation focuses the eye in the green–yellow range at 550–570 nm (Kruger, Mathews, Aggarwala, & Sanchez, 1993; Marcos, Burns, Moreno-Barriusop, & Navarro, 1999). As a result, blue cones (peak absorption around 420 nm; Gegenfurtner & Sharpe, 1999) are inevitably exposed to myopic defocus of about 1 diopter. Perhaps because persistent defocus in the blue excludes high spatial resolution, blue cones are absent from the foveal center in most human subjects (Gegenfurtner & Sharpe, 1999). 
Binder and Kroger (2000) found that, in monochromatic light, accommodation follows closely the longitudinal chromatic aberration function. Subjects accommodate less in the blue than in the red, because chromatic aberration adds additional dioptric power in the blue. A striking finding was reported by Rucker and Kruger (2004a, 2004b) and Seidemann and Schaeffel (2002). While both groups confirmed results by Binder and Kroger (2000), they found that accommodation increases again at wavelengths below 430 nm. Apparently, the image on the retina is actively myopically defocused by about half a diopter, and the “lag of accommodation” reverses to a “lead” (“overaccommodation in deep blue,” abbreviated below as “OAB”). The unexpected overaccommodation in the deep blue could be interpreted as follows: Blue cones may “prefer” myopic defocus as they are exposed to it under everyday viewing conditions and they may, therefore, overstimulate accommodation. This hypothesis assumes that light below 430 nm stimulates preferentially blue cones. However, the residual sensitivity of M- and L- cones at short wavelengths could also contribute to the enhanced accommodation although it remains unclear, why. 
To learn more about the physiological mechanisms underlying OAB, we have measured accommodation at different wavelengths to targets with different angular extent to stimulate preferentially either the foveal or parafoveal region. 
Methods
Subjects
Fourteen students with no known ocular pathology, other than low refractive errors, participated in the study (mean age = 27.4 ± 2.8 years, range = 24–32 years). Six subjects were emmetropic (average spherical equivalent (SE) = −0.04 ± 0.06 D) and 8 were moderately myopic (SE = −1.74 ± 0.83 D). All subjects had a corrected visual acuity of at least 20/20. Experiments were performed after their informed consent was obtained in writing. Accommodation measurements with the PowerRefractor were approved by the Ethics Commission of the Medical Faculty of the University of Tübingen. 
Measurements and setup
To minimize head movements, a chin rest was used. Accommodation was recorded at a sampling frequency of 25 Hz with the PowerRefractor (version 1995; MCS, Reutlingen, Germany; Choi, Weiss et al., 2000), an infrared photorefractor that operates at a wavelength of 850 nm and measures refraction in the vertical pupil meridian. Reading targets were placed at 33-cm (3 D) distance from the subjects. The PowerRefractor was positioned at 1-m distance behind the reading target and measured from slightly above the angular position of the target. Accommodation data were collected from both eyes. Since accommodation is strictly coupled in both eyes in primates, data from both eyes were averaged to minimize measurement noise. Accommodation was measured simply as the changes in spherical equivalent under different stimulation conditions. 
There are two ways to calibrate the PowerRefractor. The standard calibration was initially obtained by refracting a large number of subjects with known spectacle prescriptions (Choi et al., 2000). The calibration was such that the best agreement was obtained between PowerRefractor measurements and prescriptions (minimizing least square fit errors). However, because subjects accommodate by an undefined amount when they look at the camera of the PowerRefractor, this calibration leads generally to more hyperopic measurements when accommodation is studied and to an overestimation of the “lag of accommodation.” For this reason, Seidemann and Schaeffel (2003) performed a second calibration when they studied the “lag of accommodation.” They matched the PowerRefractor readings to measurements obtained with a streak retinoscope during accommodation. The calibration showed that accommodation was generally underestimated by about 1 diopter. If their calibration would have been included in the current study, accommodation would always be 1 diopter more. Comparisons of measurements of accommodation at different wavelength as in the current study, however, are still valid even though the absolute amounts of accommodation may be underestimated. 
A conventional slide projector with a 250-W Xenon lamp (Pradovit P300 IR, Leica Camera AG, Solms, Germany) illuminated the reading targets. The room was darkened during the measurements. In Experiment 5, a second projector was necessary (Royal AF Selektiv IR, Zett Geräte, Braunschweig, Germany). Interference filters (10-nm bandwidth; Edmund Optics, Barrington, USA) were used to restrict the spectrum to the desired wavelength band. Luminances of the target were measured with a candela meter (Minolta LS100, Konica Minolta Sensing, Osaka, Japan) and neutral density filters were inserted into the beam to match the luminances achieved with the different interference filters as specified below. Three measurements were performed for each filter and subject. These data were averaged and standard deviations were determined. The sequence of the trials was not randomized because there is no evidence that accommodation can be voluntarily controlled and that it fatigues over time (Wolffsohn, Sheppard, Vakani, & Davies, 2011). The optical configurations used for Experiments 1–6 are shown in Figure 1
Figure 1
 
Schematic illustrations of the reading target presentation and illumination used in the six experiments. (A) Direct illumination of the reading target, used in Experiments 1–3. (B) Illumination of the reading target through a mirror, used for Experiment 4. (C) Dual projector arrangement used for Experiment 5. (D) Diffuse illumination used for Experiment 6.
Figure 1
 
Schematic illustrations of the reading target presentation and illumination used in the six experiments. (A) Direct illumination of the reading target, used in Experiments 1–3. (B) Illumination of the reading target through a mirror, used for Experiment 4. (C) Dual projector arrangement used for Experiment 5. (D) Diffuse illumination used for Experiment 6.
The following experiments were performed: 
Experiment 1—Verification of OAB. Five emmetropic (SE = −0.05 ± 0.07 D) and five myopic (SE = −1.91 ± 0.58 D) subjects participated. A text printed on a 4 × 5 cm white paper served as accommodation target. The uppercase letters had a vertical angular subtense of 0.20° and an interline spacing of 0.31°. The luminance of the reading target was adjusted to 10 cd/m2 at all tested wavelength bands, using neutral density filters. Interference filters had peak transmissions at 417, 431, 491, and 615 nm. 
Experiment 2—Luminance dependence of OAB? Five emmetropic (SE = −0.05 ± 0.07 D) and three myopic (SE = −1.94 ± 0.53 D) subjects were measured. In this experiment, a high-contrast Landolt C served as a fixation target (angular subtense 0.2°). The Landolt C was printed in the center of the rectangular white paper of the size as above. Luminance was either 10 cd/m2 or 1 cd/m2. The same filters were used as in Experiment 1. 
Experiment 3—Foveal stimulation of OAB? Subjects were as in Experiment 2. The Landolt C was printed on a round white paper, but its size was reduced to 6 mm in diameter, equivalent to about 1° of visual angle, to stimulate preferentially the foveal region. Luminance and filters were as in Experiment 1. 
Experiment 4—Parafoveal stimulation of OAB? Five emmetropic (SE = −0.05 ± 0.07 D) and four myopic (SE = −1.98 ± 0.72 D) subjects were measured. Stimulation was preferentially in the parafoveal area by using a round white paper accommodation target of 38-mm diameter with a black hole of 6-mm diameter in the center. Luminance of the parafoveal target had to be kept lower as in the previous experiments to make it possible to provide similar luminances at all tested wavelengths and was only 2.5 cd/m2. Interference filters had transmission peaks at 405, 415, 420, 436, 450, 480, 520, 600, and 640 nm. 
Experiment 5—Interactions between foveal and parafoveal regions in OAB? The reason for doing these experiments was that we wanted to find out how the relative weighting is of the foveal versus parafoveal input. Five emmetropic (SE = −0.05 ± 0.07 D) and five myopic (SE = −1.99 ± 0.62 D) subjects participated. Two slide projectors were used to illuminate separately the central 1.0° of the reading target and the peripheral 9.8° around the Landolt C separately. Interference filters had peak transmissions at 405, 420, 436, 450, 520, and 600 nm, and all possible filter combinations were tested for the center and the periphery. During a trial, the wavelength of light in the center was fixed and the wavelength in the periphery was modified. In the subsequent trial, the wavelength of light in the center was altered and then kept fixed again while the wavelength in the periphery was modified. This was repeated until all available filter combinations were tested. Each experimental condition was tested three times, but there was no randomization of the combinations, assuming that accommodation is not under voluntary control in these experiments and not subject to fatigue (Wolffsohn et al., 2011). 
During all these experiments, the subjects were instructed to keep the reading target as clear as possible. 
Experiment 6—Fixation target necessary for OAB? Five emmetropic (SE = −0.05 ± 0.07 D) and three myopic (SE = −2.10 ± 0.63 D) subjects were studied. No accommodation stimuli were provided in this experiment, but rather only the spectral composition of the light varied. Subjects wore trial spectacle frames with an infrared transmitting filter (Kodak 87C) in the left and a severely frosted diffuser glass in their right eyes (equivalent to a Bangerter Foil with a perception of 0.05). Light was projected on the diffuser glass of the right eye. Refractive states were measured in the left eyes through the infrared filter. Brightness was matched at all tested wavelengths based on measurements with a light meter (Light Meter, PCE Instruments, Durham, UK) to 11–15 lux. Interference filters with transmission peaks at 405, 420, 436, 450, 500, and 600 nm were used. 
Statistical analyses
Statistical analyses were performed using Microsoft Office Excel 2007 Analysis Tools (Microsoft, Redmond, USA) and JMP Version 8 software (SAS Institute, Cary, NC). Comparisons of the refractions measured in the same subjects in each experiment were done using paired t-tests. Even though the number of emmetropic and myopic subjects was low, both groups showed similar changes in accommodation in the different experiments and these changes were also significant in most cases for each group if separately tested. Therefore, data are provided separately for each group, as well as pooled. No corrections were made for multiple testing. 
Results
Experiment 1—Verification of OAB
OAB was nicely reproduced, in line with previous observations by Rucker and Kruger (2004a, 2004b) and Seidemann and Schaeffel (2002). While accommodation closely followed the longitudinal chromatic aberration function over the major part of the visible spectrum and decreased continuously from 615 to 431 nm (emmetropic subjects: −0.89 D, p < 0.01; myopic subjects: −0.77 D, p < 0.001; all subjects: p < 0.0001), a sign reversal was found at the short-wavelength end of the spectrum between 431 and 417 nm. Significance levels for these differences are presented below. Results were similar for emmetropic (+0.30 D, p < 0.05, n = 5) and myopic (+0.23 D, p < 0.01, n = 5) subjects. If data from both groups were pooled, the increase in accommodation was highly significant (+0.26 D, p < 0.001, n = 10; Figure 2A). 
Figure 2
 
(A) Verification of OAB. Note that spherical equivalents became less myopic from long to short wavelengths but, unexpectedly, more myopic again between 431 and 417 nm, both in emmetropic and myopic subjects. A text target was used to elicit accommodation. The effect was reproduced at (B) 10 cd/m2 and (C) 1 cd/m2. In the latter two experiments, a Landolt C served as accommodation target rather than a text. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Figure 2
 
(A) Verification of OAB. Note that spherical equivalents became less myopic from long to short wavelengths but, unexpectedly, more myopic again between 431 and 417 nm, both in emmetropic and myopic subjects. A text target was used to elicit accommodation. The effect was reproduced at (B) 10 cd/m2 and (C) 1 cd/m2. In the latter two experiments, a Landolt C served as accommodation target rather than a text. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Experiment 2—Luminance dependence of OAB?
Using Landolt C instead of letter targets, the spectral variation of accommodation remained similar. The increase of accommodation at the blue end of the spectrum was less pronounced in the emmetropic group (+0.17 D, p = 0.057, n.s., n = 5) but remained significant for the myopic subjects (+0.33 D, p < 0.01, n = 3). If all subjects were pooled, a higher significance was achieved (+0.29 D, p < 0.01, n = 8; Figure 2B). When luminance was lowered by a factor of 10 (1 cd/m2, Figure 2C), OAB was still detected (emmetropic group: +0.22 D, p < 0.05; myopic group: +0.33 D, p = 0.054; +0.26 D, p < 0.01 for all subjects pooled). 
Experiment 3—Foveal stimulation of OAB?
With an accommodation target with an angular extend of only 1 degree, it was assumed that the stimulation was largely foveal (depending on fixation stability). Strikingly, OAB was no longer induced between 431 and 417 nm. Nevertheless, accommodation still followed the longitudinal chromatic aberration function in the remaining part of the visible spectrum between 615 and 431 nm in the emmetropic group (−0.58 D, p < 0.01, n = 5) but did not achieve significance for the 3 myopic subjects (−0.76 D, p < 0.23, n = 3). If all subjects were pooled, significance levels were improved (−0.65 D, p < 0.01, n = 8; Figure 3A). 
Figure 3
 
(A) Spherical equivalents measured across the visible spectrum when preferentially the fovea was stimulated. Note that the increase in accommodation at the short-wavelength end of the spectrum was only rudimentary. (B) Spherical equivalents when preferentially the parafovea is stimulated. The increase of accommodation below 436 nm is obvious. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Figure 3
 
(A) Spherical equivalents measured across the visible spectrum when preferentially the fovea was stimulated. Note that the increase in accommodation at the short-wavelength end of the spectrum was only rudimentary. (B) Spherical equivalents when preferentially the parafovea is stimulated. The increase of accommodation below 436 nm is obvious. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Experiment 4—Parafoveal stimulation of OAB?
When stimulation was restricted to the parafoveal region by presenting a featureless black circular area in the center of the stimulation field, the increase in accommodation at the blue end of the spectrum below 436 nm was fully preserved or appeared even more pronounced (emmetropic group: +0.36 D, p < 0.05, n = 5; myopic group: +0.53 D, p < 0.05, n = 4; all subjects: +0.44 D, p < 0.001, n = 9; Figure 3B). An interesting side effect was that the curves were shifted toward less myopic refractions when only the parafoveal was stimulated (Figures 3A and 3B). The generally lower accommodation levels observed in the experiment in which preferentially the parafovea was stimulated (Figure 3B) could be due to the lack of a foveal fixation target. It is known that accommodation under normal viewing conditions is mainly driven by foveal input and the lack of this input could reduce accommodation tonus. 
Experiment 5—Interactions between foveal and parafoveal regions in OAB?
Simultaneous central and peripheral simulation with light of different wavelengths produced mixed results. If wavelength was varied in the peripheral 9.8° around the fovea while the fovea was illuminated with a fixed wavelength, the effects of LCA on accommodation were not very clear (Figure 4A). However, if the wavelength varied in the fovea but the 9.8° of the peripheral visual field around the fovea were illuminated with a fixed wavelength (Figure 4B), accommodation followed LCA more clearly and the increase in accommodation below 431 nm was more obvious in most comparisons (Figure 4; Tables 1 and 2). 
Figure 4
 
(A) When the wavelength of light stimulating the central 1° in the fovea was kept at a constant wavelength (see legend for transmission peaks of the interference filters used to illuminate the periphery) while the periphery was stimulated with light of different wavelengths, the effect of LCA was less conspicuous and accommodation increased only little between 431 and 417 nm. (B) When the 9.8° of the visual field around the fovea were illuminated with light of a fixed wavelength (see legend for the transmission peaks of the interference filters), while the wavelength of the light stimulating the central 1° of the fovea was varied, the effects of LCA showed up more clearly and the increase of accommodation between 431 and 417 nm was more pronounced than in (A). C = center, P = periphery.
Figure 4
 
(A) When the wavelength of light stimulating the central 1° in the fovea was kept at a constant wavelength (see legend for transmission peaks of the interference filters used to illuminate the periphery) while the periphery was stimulated with light of different wavelengths, the effect of LCA was less conspicuous and accommodation increased only little between 431 and 417 nm. (B) When the 9.8° of the visual field around the fovea were illuminated with light of a fixed wavelength (see legend for the transmission peaks of the interference filters), while the wavelength of the light stimulating the central 1° of the fovea was varied, the effects of LCA showed up more clearly and the increase of accommodation between 431 and 417 nm was more pronounced than in (A). C = center, P = periphery.
Table 1
 
Significance levels of the changes in accommodation induced by changes in the wavelength of light in the parafoveal region. The wavelength in the center, stimulating the fovea, was kept constant. Data from which the p-values were derived are shown in Figure 4A.
Table 1
 
Significance levels of the changes in accommodation induced by changes in the wavelength of light in the parafoveal region. The wavelength in the center, stimulating the fovea, was kept constant. Data from which the p-values were derived are shown in Figure 4A.
Filters used for the periphery (nm) Filters used for the center (nm)
405* 420 436* 450 520 600**
405–436 p = 0.17 p = 0.06 p < 0.05 p = 0.08 p < 0.5 p < 0.05
436–600 p < 0.05 p < 0.01 p = 0.20 p = 0.1 p < 0.01 p < 0.05
 

*A 420-nm filter was used for comparison when the 405- or 436-nm filter was already used in the center.

 

**A 520-nm filter was used for the comparison.

Table 2
 
Comparison of the central wavelengths according to fixed, peripheral wavelengths. The graphs underlying these p-values can be seen in Figure 3B.
Table 2
 
Comparison of the central wavelengths according to fixed, peripheral wavelengths. The graphs underlying these p-values can be seen in Figure 3B.
Filters used for the center (nm) Filters used for the periphery (nm)
405* 420 436* 450 520 600**
405–436 p = 0.13 p < 0.001 p < 0.01 p < 0.001 p < 0.001 p < 0.001
436–600 p < 0.01 p < 0.01 p < 0.05 p < 0.01 p < 0.001 p < 0.01
 

*Filter 420 was used for the comparison, when either the 405- or 436-nm filter was already used in the periphery.

 

**Filter 520 was used for the comparison.

A summary of the significance of the effects of mixed foveal and parafoveal stimulation is shown in Tables 1 and 2
Experiment 6—Fixation target necessary for OAB?
Without a reading target, accommodation of the subjects did no longer change with wavelength. Neither had longitudinal chromatic aberration an effect nor was OAB detected (Figure 5). The absence of accommodation stimuli in this setup (described in Figure 1D) was verified by placing various trial lenses in front of one eye in three subjects; no accommodation was induced by the lenses. However, to verify that OAB could still be induced in the setup, a fixation target was provided and OAB was observed as in the previous experiments. 
Figure 5
 
Lack of changes in spherical equivalent refractive errors when the wavelength of light illuminating the eye was varied but no fixation target presented. Error bars represent ±1 standard deviation.
Figure 5
 
Lack of changes in spherical equivalent refractive errors when the wavelength of light illuminating the eye was varied but no fixation target presented. Error bars represent ±1 standard deviation.
Discussion
This study has confirmed the striking overaccommodation in deep blue light (OAB) below 430 nm that was initially described by Rucker and Kruger (2004a, 2004b) and Seidemann and Schaeffel (2002). The effect remains stable at target luminances between 10 and 1 cd/m2. OAB was found to be more pronounced when the parafoveal region was stimulated than when mainly the fovea was stimulated. When fovea and parafovea were illuminated by light of different wavelengths, the wavelength in the foveal region had a greater impact of accommodation than the light in the periphery. The results of this experiment favor the view that the parafovea drives OAB and not the fovea. Likewise, in the absence of peripheral stimulation, OAB appears to be controlled more by the parafovea than by the fovea, in line with the assumption that OAB is mediated by the short-wavelength-sensitive cones, the S-cones, which are absent from the central fovea in most subjects. 
The absence of blue cones in the fovea can be demonstrated by observing a computer screen that is homogeneously illuminated with the blue channel only but with the brightness of the blue channel sinusoidally modulated at low frequency (1 to 5 Hz). A related psychophysical procedure, using an optical bench and filters, was described by Stromeyer et al. (1991). If the screen is observed with a cutoff filter that transmits light only below 450 nm, the blue cone-free area in the fovea shows up as a star-shaped black area of about 1° diameter, most likely reflecting the blue scotoma in the fovea. An alternative interpretation is likely, namely, that it results from the macular pigment. Fundus images show that the distribution of the macular pigment is wider than the black area seen in the psychophysical test. Testing students attending a lecture showed that 17 of 21 saw their blue scotoma at the fixation point, suggesting that they may have no blue cones in the foveal center (Schaeffel, unpublished observation, 2010). 
Why should S-cones overstimulate accommodation? One would expect that accommodation in monochromatic light would generate the best focused image at all wavelengths to provide as much spatial information as possible. In deep blue monochromatic light, the hyperopic defocus imposed on L- and M-cones should represent no problem because these cones are stimulated only at the tail of their pigment absorption functions. However, the reality is different: Blue cones are actively myopically defocused by overaccommodation, while the L- and M-cones continue to receive images in good focus. A reason for this behavior might be that accommodation is not just driven by luminance contrast but rather by chromaticity contrast (Rucker & Kruger, 2004a, 2004b; Wang, Kruger, Li, Lin, & Stark, 2011). In a model of accommodation that includes chromaticity contrast as a stimulus, ratios of contrasts measured separately by the three cone types drive accommodation. Thus, L-cone contrast > M-cone contrast specifies myopic defocus and reduces accommodation, while M-cone contrast > L-cone contrast specifies hyperopic defocus and increases accommodation for near. Two chromatic opponent mechanisms may be involved: (L–M) that compares L-cone and M-cone contrasts and [S–(L + M)] that compares S-cone contrast and luminance (L + M) contrast. In summary, contrast measured by L-cones (L-cone contrast) drives accommodation for far, while contrast measured by M-cones (M-cone contrast) drives accommodation for near. Similarly, S-cone contrast, especially in the absence of L- and M-cone contrasts, drives accommodation for near. Thus, the deepest blue condition (417 nm) may inadvertently mimic underaccommodation and drive the eye to overaccommodate (OAB). If the eye would use color contrast to guide accommodation and it is illuminated with monochromatic blue light, then the contrast for blue cones will be higher than that for red cones. The eye might be expected to interpret this as being underaccommodated because it would only experience this sort of situation in white light if the image was hyperopically defocused. When the eye is hyperopically defocused, blue light is focused closer to the retina than red light (because of LCA), as a result the contrast for blue cones would be higher than for red cones. 
OAB may also provide yet another explanation for “night myopia” (e.g., Tousey, Koomen, & Scolnik, 1953). In the dusk, the spectral composition of light moves into the short-wavelength range (Vetter, Juda, Lang, Wojysiak, & Roenneberg, 2011), which may therefore trigger OAB and night myopia. 
Any technical explanations for the measurements of OAB?
Although the PowerRefractor was calibrated to take changes of pupil size into account when measuring refraction (Choi et al., 2000), the effects in the current study were small and change in pupil size need attention. There was striking change in pupil size with wavelength, even though the luminances were matched with a candela meter (Figure 6). Pupil size declined with decreasing wavelength, reaching a minimum between 430 and 450 nm. At even shorter wavelengths, it increased again. This spectral response curve followed the absorption curve of melanopsin, the photopigment of the intrinsic light-sensitive ganglion cells, and it is well known that they influence light-driven pupil responses. At the end, the refractions and pupil sizes were not correlated. Pupil size was, therefore, no confounding factor for the accommodation measurements. 
Figure 6
 
Pupil sizes of the subjects (A) when fixation targets were presented (data from Experiment 4) and (B) when no fixation target was presented (data from Experiment 6). Note that the minimal pupil sizes were recorded between 430 and 450 nm, even though the brightness at the different wavelengths was matched, using a candela meter and neutral density filters. Error bars represent ±1 standard deviation.
Figure 6
 
Pupil sizes of the subjects (A) when fixation targets were presented (data from Experiment 4) and (B) when no fixation target was presented (data from Experiment 6). Note that the minimal pupil sizes were recorded between 430 and 450 nm, even though the brightness at the different wavelengths was matched, using a candela meter and neutral density filters. Error bars represent ±1 standard deviation.
Possible chromatic effects on myopia development?
Recently, several studies in children have demonstrated that development of myopia is modulated by the time spent outdoors (e.g., Mutti, Hayes et al., 2007). Furthermore, it was found that chickens raised in bright light (15,000 lux) in the laboratory develop less myopia with diffusers or negative lenses than chicks that received the same treatment under regular laboratory illumination (500 lux; Ashby & Schaeffel, 2010). These observations suggest that light intensity has a major impact on myopia development. 
It is especially tempting to hypothesize that the spectral composition of light can affect the development of refractive errors. Seidemann and Schaeffel (2002) raised chicks in red light and found that they develop about 1 diopter more myopia than those raised in blue light, in agreement with expectations from longitudinal chromatic aberration. The change in refractive state was preceded by a change in accommodation of similar magnitude. These findings are in conflict with the “lag of accommodation theory” that assumes that hyperopic defocus as induced by insufficient accommodation drives a compensatory increase in axial eye growth and myopia. Unexpectedly, when chicks accommodated more in the red end of the spectrum, they also became more myopic (and not less), and in the blue at 450 nm, they accommodated about a diopter less and became also 1 diopter less myopic (and not more). A conclusion from these observations was (Seidemann & Schaeffel, 2002) that accommodation and local retinal defocus detection must somehow interact during emmetropization, although it remains unclear how. Kroger and Binder (2000) proposed therefore that children should be exposed to blue light, or read from blue paper, to become less myopic. While there may be some merit in this idea for light above 430 nm, the results of the present study suggest that light below 430 nm should be avoided. However, experimental proof is not yet available in the literature. A few related experiments have been done. In experiments where animals were raised in monochromatic light, emmetropization compensated for the chromatic defocus (fish: Kroger & Wagner, 1996; chicken: Seidemann & Schaeffel, 2002). In UV light (385 or 390 nm), lens compensation was found to be compromised in chicks at 1.5 “chicken lux” (Rohrer, Schaeffel, & Zrenner, 1992), or at 20 lux, lenses induced responses, but the detection of the sign of imposed defocus was no longer possible (Hammond et al., 2011). At 365 nm, all chicks became myopic no matter which lens they were wearing. At 390 nm and 68 lux, however, Hammond and Wildsoet (2010) found largely normal compensation of spectacle lenses by emmetropization. However, there have been no experiments to test emmetropization in blue light between 400 and 430 nm. It can be expected that, at least in primates, such light could, in fact, induce more myopia than white light, even if it is bright. 
Acknowledgments
This work was supported by the European Union Marie Curie Research Training Network MYEUROPIA Grant MRTN-CT-2006-034021 and the Bernstein Center for Computational Neuroscience Tübingen, Project Number FKZ 01GQ1002, A2. 
Commercial relationships: none. 
Corresponding author: Frank Schaeffel. 
Email: frank.schaeffel@uni-tuebingen.de. 
Address: Section of Neurobiology of the Eye, Ophthalmic Research Institute, Calwerstrasse 7/1, 72076 Tübingen, Germany. 
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Figure 1
 
Schematic illustrations of the reading target presentation and illumination used in the six experiments. (A) Direct illumination of the reading target, used in Experiments 1–3. (B) Illumination of the reading target through a mirror, used for Experiment 4. (C) Dual projector arrangement used for Experiment 5. (D) Diffuse illumination used for Experiment 6.
Figure 1
 
Schematic illustrations of the reading target presentation and illumination used in the six experiments. (A) Direct illumination of the reading target, used in Experiments 1–3. (B) Illumination of the reading target through a mirror, used for Experiment 4. (C) Dual projector arrangement used for Experiment 5. (D) Diffuse illumination used for Experiment 6.
Figure 2
 
(A) Verification of OAB. Note that spherical equivalents became less myopic from long to short wavelengths but, unexpectedly, more myopic again between 431 and 417 nm, both in emmetropic and myopic subjects. A text target was used to elicit accommodation. The effect was reproduced at (B) 10 cd/m2 and (C) 1 cd/m2. In the latter two experiments, a Landolt C served as accommodation target rather than a text. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Figure 2
 
(A) Verification of OAB. Note that spherical equivalents became less myopic from long to short wavelengths but, unexpectedly, more myopic again between 431 and 417 nm, both in emmetropic and myopic subjects. A text target was used to elicit accommodation. The effect was reproduced at (B) 10 cd/m2 and (C) 1 cd/m2. In the latter two experiments, a Landolt C served as accommodation target rather than a text. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Figure 3
 
(A) Spherical equivalents measured across the visible spectrum when preferentially the fovea was stimulated. Note that the increase in accommodation at the short-wavelength end of the spectrum was only rudimentary. (B) Spherical equivalents when preferentially the parafovea is stimulated. The increase of accommodation below 436 nm is obvious. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Figure 3
 
(A) Spherical equivalents measured across the visible spectrum when preferentially the fovea was stimulated. Note that the increase in accommodation at the short-wavelength end of the spectrum was only rudimentary. (B) Spherical equivalents when preferentially the parafovea is stimulated. The increase of accommodation below 436 nm is obvious. Data labeled with “white light” represent spherical equivalents measured in broadband white light. Error bars denote ±1 standard deviation.
Figure 4
 
(A) When the wavelength of light stimulating the central 1° in the fovea was kept at a constant wavelength (see legend for transmission peaks of the interference filters used to illuminate the periphery) while the periphery was stimulated with light of different wavelengths, the effect of LCA was less conspicuous and accommodation increased only little between 431 and 417 nm. (B) When the 9.8° of the visual field around the fovea were illuminated with light of a fixed wavelength (see legend for the transmission peaks of the interference filters), while the wavelength of the light stimulating the central 1° of the fovea was varied, the effects of LCA showed up more clearly and the increase of accommodation between 431 and 417 nm was more pronounced than in (A). C = center, P = periphery.
Figure 4
 
(A) When the wavelength of light stimulating the central 1° in the fovea was kept at a constant wavelength (see legend for transmission peaks of the interference filters used to illuminate the periphery) while the periphery was stimulated with light of different wavelengths, the effect of LCA was less conspicuous and accommodation increased only little between 431 and 417 nm. (B) When the 9.8° of the visual field around the fovea were illuminated with light of a fixed wavelength (see legend for the transmission peaks of the interference filters), while the wavelength of the light stimulating the central 1° of the fovea was varied, the effects of LCA showed up more clearly and the increase of accommodation between 431 and 417 nm was more pronounced than in (A). C = center, P = periphery.
Figure 5
 
Lack of changes in spherical equivalent refractive errors when the wavelength of light illuminating the eye was varied but no fixation target presented. Error bars represent ±1 standard deviation.
Figure 5
 
Lack of changes in spherical equivalent refractive errors when the wavelength of light illuminating the eye was varied but no fixation target presented. Error bars represent ±1 standard deviation.
Figure 6
 
Pupil sizes of the subjects (A) when fixation targets were presented (data from Experiment 4) and (B) when no fixation target was presented (data from Experiment 6). Note that the minimal pupil sizes were recorded between 430 and 450 nm, even though the brightness at the different wavelengths was matched, using a candela meter and neutral density filters. Error bars represent ±1 standard deviation.
Figure 6
 
Pupil sizes of the subjects (A) when fixation targets were presented (data from Experiment 4) and (B) when no fixation target was presented (data from Experiment 6). Note that the minimal pupil sizes were recorded between 430 and 450 nm, even though the brightness at the different wavelengths was matched, using a candela meter and neutral density filters. Error bars represent ±1 standard deviation.
Table 1
 
Significance levels of the changes in accommodation induced by changes in the wavelength of light in the parafoveal region. The wavelength in the center, stimulating the fovea, was kept constant. Data from which the p-values were derived are shown in Figure 4A.
Table 1
 
Significance levels of the changes in accommodation induced by changes in the wavelength of light in the parafoveal region. The wavelength in the center, stimulating the fovea, was kept constant. Data from which the p-values were derived are shown in Figure 4A.
Filters used for the periphery (nm) Filters used for the center (nm)
405* 420 436* 450 520 600**
405–436 p = 0.17 p = 0.06 p < 0.05 p = 0.08 p < 0.5 p < 0.05
436–600 p < 0.05 p < 0.01 p = 0.20 p = 0.1 p < 0.01 p < 0.05
 

*A 420-nm filter was used for comparison when the 405- or 436-nm filter was already used in the center.

 

**A 520-nm filter was used for the comparison.

Table 2
 
Comparison of the central wavelengths according to fixed, peripheral wavelengths. The graphs underlying these p-values can be seen in Figure 3B.
Table 2
 
Comparison of the central wavelengths according to fixed, peripheral wavelengths. The graphs underlying these p-values can be seen in Figure 3B.
Filters used for the center (nm) Filters used for the periphery (nm)
405* 420 436* 450 520 600**
405–436 p = 0.13 p < 0.001 p < 0.01 p < 0.001 p < 0.001 p < 0.001
436–600 p < 0.01 p < 0.01 p < 0.05 p < 0.01 p < 0.001 p < 0.01
 

*Filter 420 was used for the comparison, when either the 405- or 436-nm filter was already used in the periphery.

 

**Filter 520 was used for the comparison.

© 2012 ARVO
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