Contrast adaptation appears independent of the longitudinal chromatic aberration of the human eye

Caroline Kraft; Alexander Leube; Arne Ohlendorf; Siegfried Wahl

doi:10.1364/JOSAA.36.000B77

1. INTRODUCTION

Perceiving contrast is an important feature of spatial vision, and, therefore, the visual system has to be sensitive to luminance modulations in different illuminance and spatial frequency conditions [1]. To provide a constant visual response to contrast with changes of illumination, the visual system uses adaptation to the different environmental conditions [2]. This ability of the human’s visual system to adjust its gain in terms of adaptation to contrast has been investigated in psychophysical experiments extensively [3–10]. These classical studies revealed that contrast adaptation operates on a multiparametric scale and is selective, for instance, to the orientation of the adaptation and test stimulus [11], the spatial frequency of the contrast pattern [9], and the time scale of adaptation [7].

Besides contrast, the visual system continuously adapts to other characteristics of the environment, like global distortions [12] or color [13], and it was further shown that adaptations also occur for induced optical defocus [14], which will cause a change of the spatial frequency distribution of the retinal image. Hence, visual acuity was shown to increase after a period of blurry vision due to induced positive spherical defocus [15–17], which is similar for natural [18,19] and induced astigmatic defocus [15,20,21]. Additionally, to adaptational effects on high frequency visual parameters, like visual acuity, adaptation to contrast further influences lowers spatial frequency bands. An induced optical defocus acts as a low pass filter for the retinal image contrast and, therefore, leads to a bigger change for high and medium spatial frequencies and to a small change for low spatial frequencies [22]. Changes in contrast sensitivity for low spatial frequencies after adaptation to positive defocus were shown previously [14,23–26]. Due to the interaction of defocus and contrast adaptation, contrast adaptation is stated to be a factor that guides the longitudinal eye growth and, therefore, is of high interest in myopia research [14,27–29]. Animal research has shown that the retina itself is able to detect the sign of optical blur by changing the direction of growth depending on the sign of the induced defocus [30]. Therefore, the question of which visual cues are necessary to correctly detect the sign of defocus is of high interest [31–33]. Ohlendorf and Schaeffel (2009) [14] revealed that contrast adaptation occurs while inducing positive defocus, but not for negative defocus. One explanation for the observed differences was suggested to be the longitudinal chromatic aberration (LCA) of the eye [34–37] in combination with the chromaticity of the stimulus. The aim of the current study was to examine the role of chromatic aberration on contrast adaptation. Therefore, it was investigated (1) whether contrast adaptation occurs under monochromatic light conditions, which eliminates the effect of the LCA, and (2) if the strength of contrast adaptation differs for short, medium, and long wavelengths.

2. METHODS

Inclusion criteria for participation were a spherical refractive error of $\pm 6.0 D$ or lower, best corrected visual acuity of at least 0.0 logMAR, and no known ocular diseases. Additionally, participants with a pupil diameter of less than 4.0 mm were excluded from data analysis. The study protocol followed the tenets of the Declaration of Helsinki and was approved by the ethics committee of the Medical Faculty of the University of Tuebingen. Informed consent was obtained from all participants prior to the measurements after receiving both verbal and written explanations of the nature of the study and possible consequences.

A. Apparatus

All test procedures were based on software programmed in Matlab (MATLAB R2016b, MathWorks Inc., Natick, USA) using the Psychophysics Toolbox extensions [38–40]. The stimuli were presented on a LCD display (ViewPixx 3D, VPixx Technologies, Saint-Bruno, Canada) with a gray-level resolution of 16 bits and an angular size of $7 ° \times 5 °$ . The experiment was conducted under polychromatic (poly) and three monochromatic light conditions. To achieve monochromatic light conditions, bandpass filters with central transmission wavelengths of $470 \pm 2 nm$ , $530 \pm 2 nm$ , and $630 \pm 2 nm$ were used (Newport Corporation, Irvine, USA) with a full width at half-maximum of $Δ λ_{FWHM} = 10 nm$ . In order to keep constant experimental settings throughout the filter conditions, the luminance was set to a mesopic level of $1.0 cd / m^{2}$ for all monochromatic filters, as well as for the polychromatic condition. To achieve constant luminance, the monitor settings were adjusted according to the filter’s transmission using a spectroradiometer (DTS140, Instrument Systems, Munich, Germany). A normalized intensity diagram of each light condition is shown in Fig. 1.

Fig. 1. Normalized spectral intensity distribution for the polychromatic (Monitor) and the three monochromatic light conditions. Luminance for all four conditions was set to $1.0 cd / m^{2}$ , and spherical refraction was shifted according to the chromatic aberration of the participant’s eye.

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All experimental measurements were performed for the right eye, and refractive errors were corrected for the test distance of 2 m using trial lenses (Cases BK X/O, OCULUS Optikgeräte GmbH, Wetzlar, Germany) placed at a vertex distance of 8 mm in a trial frame (UB4, OCULUS Optikgeräte GmbH, Wetzlar, Germany) in front of the eye. Shifts in spherical refractive errors due to the chromatic aberration of the eye were checked using subjective best focus adjustments while looking at the lowest line on a visual acuity chart, which was just visible for the participant. The spherical shifts were corrected for each filter separately. Since the pupil size has an effect on human contrast sensitivity [41], all measurements were carried out using an artificial pupil with a diameter of 4 mm, which was placed in front of the participant’s eye. The size of the natural pupil was measured before and after the experiment to ensure a larger size of the natural pupil as the artificial pupil. Prior to the main experiment, the participants were seated in the dark room for at least 20 min to ensure dark adaptation. A training of the contrast sensitivity test was performed for each participant to become familiar with the psychophysical procedure. For every chromatic light condition, the same experimental procedure was executed, and the sequence of measurements was randomized according to the used filters. First, a chromatic adaptation phase of 120 s [6] was facilitated. Prior to the next measurement a chromatic de-adaptation phase of 300 s [42] was performed.

B. Participants

For the single spatial frequency experiment, twelve healthy participants (nine females, three males) with a mean age of $25.5 \pm 2.7$ years were recruited. The mean spherical equivalent refractive error of the participants’ right eye was $- 0.90 \pm 1.20 D$ (range from $- 3.63$ to 0 D), and the best corrected visual acuity of all participants was at least $- 0.1$ logMAR under photopic conditions. For a second experiment, in total, 30 participants (23 females, 7 males) with an average age of $25.0 \pm 4.1$ years and a best corrected visual acuity of $- 0.1$ logMAR in the right eye were recruited. The participants’ corresponding mean spherical equivalent refractive error of the right eye was $- 0.90 \pm 1.68 D$ (range from $- 5.25$ to $+ 3.25 D$ ).

C. Protocol

1. Psychophysical Procedures

To assess the contrast sensitivity in the first experiment, the Tuebingen Contrast Sensitivity Test [43] was modified towards a two-alternative forced choice (2AFC) paradigm with 50 trials per threshold estimation. The 2AFC-paradigm incorporated Gabor patches with a spatial frequency of 4.0 cpd, which were presented with a visual angle of 2°. In order to keep the retinal image size constant for all participants, the image size was adjusted according to the lens magnification factor of the used correction lenses [43]. The participants’ task was to respond to the direction of the Gabor patch, which was either turned 5° to the right or to the left from its original position of 90°. The adaptation stimulus was a similar Gabor patch in a 90° direction with a contrast of $C > 0.99$ . Contrast sensitivity was measured in order to assess the baseline value, followed by an adaptation phase of 90 s. Contrast sensitivity was determined post-adaptation using the same 2AFC-paradigm, as shown in Fig. 2.

Fig. 2. Experimental sequence for single spatial frequency adaptation (Experiment 1).

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To test contrast sensitivity over a wide range of spatial frequencies, a contrast chart based on the investigations of Campbell and Robson (1968) [44] was used (SwipeCSF) [45]. The gray value of the displayed image increased exponentially from left to right, whereas the contrast decreased from bottom to top, following a power function $g (x, y)$ from Eq. (1). Accordingly, spatial frequency of the respective pixel positions $x$ , $y$ is given by the instantaneous frequency $f (x)$ of the power function by Eq. (2):

g (x, y) = \sin (ϕ) y^{3}, ϕ = e^{x},

f (x) = \frac{1}{2 π} \frac{d ϕ}{d x} .

For analysis, the displayed spatial frequencies were corrected for the lens magnification factor [43]. The subjects’ task was to indicate where no strips were visible, which corresponds to the perceived contrast threshold for the displayed spatial frequencies, by swiping a continuous curve using an external mouse. For adaptation, a stimulus with complex spatial frequencies was used incorporating the same spatial frequency distribution as the test stimulus and constant contrast of

C_{low} = 0.20

for low contrast adaptation and

C_{high} > 0.99

for high contrast adaptation. Between the two different amounts of contrast, a de-adaptation stimulus, which was a homogeneous gray screen, was displayed. As presented in Fig. 3, at the beginning of the experimental sequence, the SwipeCSF was performed ten times, followed by the first contrast adaptation phase to either high or low contrast stimulus in randomized order, while participants were instructed to move their gaze over the screen. Subsequently, ten swipes were performed to determine post-adaptation contrast sensitivity. After a contrast de-adaptation phase of 150 s [6], the last two steps were repeated, changing the contrast of the adaptation stimulus.

Fig. 3. Experimental sequence for the complex frequencies adaptation (Experiment 2).

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2. Statistical Analysis

Statistical analysis for the results from Experiment 1 was performed using a statistics software (IBM SPSS Statistics 22, IBM, Armonk, USA) and paired $t$ tests. Contrast sensitivity for pre- and post-adaptation was compared for each chromatic light condition separately. In order to estimate the amount of contrast adaptation (difference between pre- and post-adaptation logarithmic contrast sensitivity, $Δ$ logCS) between the chromatic light conditions, a repeated measurements analysis of variance (ANOVA) was designed. Similar analysis was performed to compare the pre-adaptation contrast sensitivities for different chromatic conditions. The level of significance was set to $α = 0.05$ .

To analyze the contrast sensitivity functions (CSFs) from Experiment 2, before and after adaptation, two different parameters were considered: the area under the curve (AUC), which was calculated as the integral of the CSF for the spatial frequencies between 2.0 and 8.0 cpd, and an analysis of single spatial frequencies. Therefore, values of 2.0, 4.0, and 9.0 cpd were considered. For statistical analysis, common software (R 3.3.4, R Core Team 2017) and the lme4 package [46] were used to calculate linear mixed models [47]. For every chromatic light and spatial frequency condition (AUC; 2.0, 4.0, and 9.0 cpd), a single linear mixed model was designed. The level of significance was set to $α = 0.05$ . Using these linear mixed models, the dependent variables (AUC or logCS), were calculated from fixed (subjects) and random (state of adaptation and swipe number) effects. A second model was created by excluding the variable swipe number. To find the model that explains most of the variation, the Akaike information criterion (AIC) of both models were compared [48], and the models without a swipe number were preferred. Further, the amount of contrast adaptation was compared using a similar linear mixed model, with an additional fixed effect for chromatic light conditions.

3. RESULTS

A. Single Spatial Frequency Adaptation

With a pupil size between 5.1 and 8.1 mm (mean $6.8 \pm 0.8 mm$ ) before the experiment and a range from 4.1 to 7.6 mm (mean $5.9 \pm 1.1 mm$ ) after the experiment, all participants were included in the statistical analysis. The shift in refractive error due to the use of the monochromatic filter was $0.96 \pm 0.32 D$ for the 470 nm filter, $- 0.31 \pm 0.39 D$ for the wavelength of 530 nm, and $+ 0.21 \pm 0.21 D$ for the 630 nm light condition.

When comparing the change in contrast sensitivity before and after adaptation to a high contrast Gabor patch, a statistically significant lower contrast sensitivity for all chromatic light conditions ( $p \leq 0.001$ ) was found, as visualized in Fig. 4. The mean and standard deviation of the amount of contrast adaptation were $0.09 \pm 0.06$ , $0.08 \pm 0.06$ , $0.17 \pm 0.11$ , and $0.18 \pm 0.11$ for polychromatic, 470, 530, and 630 nm chromatic light conditions, respectively. A repeated measurements ANOVA, including Bonferroni post hoc analyses, indicated significant differences in the amount of contrast adaptation for 470 and 630 nm chromatic light conditions ( $p = 0.046$ ), but not for the other combinations. Consistently, comparing pre-adaptation contrast sensitivities for the different chromatic light conditions, only a significant difference between the 470 and 630 nm conditions ( $p = 0.020$ ) was found.

Fig. 4. Mean logarithmic contrast sensitivity before (bright bar) and after (dark bar) adaptation to a high contrast Gabor patch. Error bars represent the inter-subject standard deviation from $N = 12$ participants.

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B. Complex Spatial Frequency Adaptation

Adaptation to a complex spatial frequency stimulus containing low and high frequencies was additionally investigated. The CSF before and after adaptation to such a stimulus was measured using a novel SwipeCSF test [45]. In a first analysis, a linear mixed model was designed that included the variable swipe number. As the swipe number had no significant influence on the post-adaptation contrast sensitivity ( $p > 0.6$ ), there was no change in contrast sensitivity due to increasing time delay between the adaptation and swipe number. One participant had to be excluded from the analysis, since the pupil size measured after the experiment run was 3.6 mm. The pupil size of the 30 participants that were included in the analysis ranged between 4.6 and 7.2 mm (mean $5.8 \pm 0.7 mm$ ) before and between 4.1 and 6.4 mm (mean $5.2 \pm 0.7 mm$ ) after the experiment. The shifts in the spherical refraction were $0.83 \pm 0.38 D$ , $0.19 \pm 0.35 D$ , and $+ 0.24 \pm 0.25 D$ for the three light conditions of 470, 530, and 630 nm, respectively.

The mean and standard deviation of logCS pre- and post-adaptation to the high and low contrast stimulus are presented in Fig. 5. Evaluation of AUC is shown in Table 1. For all tested spatial frequency and chromatic light conditions, the pre-adaptation contrast sensitivity was higher compared to the post-adaptation contrast sensitivity for both the low or high contrast stimulus. This difference showed statistical significance for AUC and the analyzed spatial frequencies of 2.0, 4.0, and 9.0 cpd for all chromatic light conditions while participants adapted to a high contrast stimulus ( $p \leq 0.012$ ). Analyzing the single spatial frequencies, no statistically significant effect was found for the wavelength of 530 nm and the low contrast adaptation ( $p \geq 0.122$ ). Comparing the amount of contrast adaptation (pre–post sensitivities), significant differences were found for all single spatial frequency conditions between polychromatic and 470 nm ( $p \leq 0.001$ ), polychromatic and 630 nm ( $p \leq 0.018$ ), and 530 and 470 nm ( $p \leq 0.003$ ) and 530 and 630 nm ( $p \leq 0.01$ ) chromatic light conditions.

Fig. 5. LogCS pre- (bright bars) and post-adaptation to low (medium bars) and high (dark bars) contrast, for the complex frequencies adaptation experiment and all four chromatic light conditions. For the spatial frequencies (a) 2.0 cpd, (b) 4.0 cpd, and (c) 9.0 cpd. Error bars represent one standard deviation, and asterisks mark the differences’ levels of significance.

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Table 1. AUC Pre- and Post-Low and High Contrast Adaptation for the Complex Frequencies Adaptation Experiment and All Four Spectral Light Conditions

View Table

4. DISCUSSION

The conducted experiment resulted in three main outcomes: (1) Adaptation to high contrast leads to a decrease in contrast sensitivity under polychromatic and monochromatic light conditions for a single spatial frequency stimulus. (2) The amount of contrast adaptation is different for long ( $630 \pm 2 nm$ ) and short ( $470 \pm 2 nm$ ) wavelengths. (3) Small amounts of contrast adaptation occur for a complex frequency adaptation stimulus. There were shifts in the refractive error of around 1.17 D (single frequency adaptation experiment) and 1.07 D (complex frequency adaptation experiment) between 470 and 630 nm, due to the chromatic aberration of the eye’s optic. These results are comparable to previous findings [35–37], since the Indiana chromatic eye model predicts a change in refractive error of $Δ R_{x} = 1.16 D$ for the wavelength range of 470–630 nm.

A. Single Spatial Frequency Adaptation

While comparing the amount of contrast adaptation that was found in the current study to previously published data [11,49], the observed effects were lower. In comparison to the current designed paradigm, previous measurements employed the same orientation between adaptation and test stimulus. According to Blakemore and Nachmias (1971) [11] differences in the orientation of the stimuli downsizes the amount of contrast adaptation, which is reported to be reduced to half by a discrepancy in orientation of 8°. A difference of 5° in orientation between the adaptation and test stimulus was designed to overcome the disadvantages of a simple adjustment method by employing a 2AFC-paradigm. The amount of contrast adaptation slightly increases as a function of adaptation time [6,50], and the current study adaptation times were shorter than in the previously published studies [11,49]. Therefore, a slightly smaller amount of contrast adaptation in comparison to previous research can be expected.

The significant difference in the amount of contrast adaptation for the 470 and 630 nm chromatic light conditions could be related with the observed significant differences in pre-adaptation contrast sensitivity. A similar discrepancy for visual acuity between 450 and 630 nm has been reported recently [51]. A possible reason for this difference could be accounted for by the lack of shortwave-sensitive cones in the fovea in contrast to long and medium wave-sensitive cones [52]. However, the applied luminance setting of $1.0 cd / m^{2}$ involves rod-stimulation [53] that would affect the 470 and 530 nm condition more than the 630 nm measurement. Furthermore, the rationale behind the current research question was whether contrast adaptation also occurs without the presence of the chromatic aberration of the eye. Therefore, bandpass filters were used, which narrow the spectrum ( $Δ λ_{FWHM} = 10 nm$ ) so that the chromatic aberration is minor. One major limitation of this approach is that the results cannot be translated to single cone sensitivities [54,55], and the relative contribution of the S, M, and L cones to the resulting contrast adaptation was not investigated.

An expected cue for the contrast adaptation regarding the process of emmetropization is LCA [14,56,57]. Ohlendorf and Schaeffel (2009) [14] found that contrast adaptation occurred only when spherical defocus was induced with positive lenses but not with negative lenses. This effect could be caused by a discrepancy in the image blur between positive and negative defocus, since there is an asymmetric difference in LCA between short and long wavelengths [14]. Since contrast adaptation also appears under monochromatic light conditions, as shown in the current study, the chromatic aberration does not drive the amount of contrast adaptation alone. In case of an induced defocus, the reduction of contrast is small for low spatial frequencies [28]. It can be acknowledged that contrast adaptation to middle and high spatial frequencies is suggested to be more important under non-defocus conditions [22]. To test whether the observed contrast adaptation under monochromatic light conditions occurs further for higher spatial frequencies, a second experiment, using a complex frequency adaptation stimulus, was performed. Additionally it was investigated whether a smaller amount of contrast adaptation for an adaptation stimulus with lower contrast, as was reported for polychromatic light contrast adaptation [4–6], can be found for monochromatic light conditions.

B. Complex Spatial Frequency Adaptation

Applying the SwipeCSF, the results of the current study showed a significant effect of contrast adaptation for most of the analyzed chromatic light and spatial frequency conditions. Taking the small amounts of contrast adaptation into account, the results miss clinical relevance.

The amount of contrast adaptation was shown to be reduced for an adaptation stimulus with lower contrast under polychromatic light conditions [4–6]. Considering the small amount of contrast adaptation using high contrast complex frequencies adaptation, and an even smaller amount for the adaptation to a low contrast stimulus, in the current study, no significant differences were found. Therefore, this effect could not be detected statistically for poly- and monochromatic light conditions using the complex frequencies adaptation stimulus. Besides with the amount of the adaptation contrast, time is an influencing factor on contrast adaptation. The time needed to perform ten repetitions of the wwipe test was less ( $t_{swipe} = 1.35 \pm 0.57 \min$ ) compared to a standard psychophysical CSF test ( $t_{TueCST} = 12.30 \pm 2.45 \min$ ) [45]. An analysis of whether there is a significant effect of time on contrast sensitivity measured post-adaptation over the range of repeated swipes revealed no differences ( $p > 0.05$ ), and it can be concluded that the adaptation phase was adequate. Due to the finding that the amount of contrast adaptation increases as a function of adaptation time [6,50], the duration of adaptation was chosen as a compromise between a high amount of contrast adaptation and exceeding fatigue of the participants. Because the adaptation time cannot explain the small amount of contrast adaptation, a possible reason for the current findings can be the multitude of spatial frequencies in the stimulus.

It was reported that the amount of contrast adaptation decreases as the difference in spatial frequency between adaptation and test grating increases [58,59] and that the effect can reverse if the difference between the spatial frequencies is higher than one octave [9,60,61]. Therefore, an indirect interaction between the frequency channels can be assumed [9,61]. This interaction between frequency channels could cause the observed small amounts of contrast adaptation. Further, for the complex frequency stimulus, it is not possible to anticipate which spatial frequencies the visual system had primarily adapted and how this contrast adaptation was influenced by simultaneous contrast adaptation to adjacent spatial frequency channels. However, Webster and Miyahara (1997) [62] found that contrast adaptation occurs while using multi-spatial frequency natural images as adaptation stimuli. The average amplitude spectra of natural images are characterized by a $1 / f$ falloff [63]. For the used complex frequency stimulus, the amplitude spectra show a similar characteristic, since the instantaneous frequency $f (x)$ is defined by Eq. (2). However, for the presence of higher spatial frequencies, this is limited by the pixel resolution and the viewing distance of the used screen.

Apart from interactions between frequency channels, the amount of contrast adaptation could also be reduced due to eye movements, as the adaptation phase was not gaze-contingent. Since the amount of contrast adaptation depends on the eccentricity in which the adaptation stimulus appears [6,10], this could explain the smaller amount of contrast adaptation. Nevertheless, eye movements were necessary during the adaptation phase to avoid after images.

C. Mesopic Luminance Conditions

The design of the current study involved constant luminance settings for all chromatic light conditions, which was limited to $1.0 cd / m^{2}$ due to the narrow bandwidth of the filters. Since spatial vision in humans is mediated by rods and cones, and their sensitivity depends on the luminance level, the findings from the current experiment are limited to the mesopic range of vision. However, considering that the luminance was set as constant throughout the filter conditions, any effect of luminance on the results for the different wavelengths can be excluded. To further increase the luminance level, a polychromatic adaptive optics system, which consists of a supercontinuum laser source in combination with dual acousto-optic tunable filters [37], could enable a photopic luminance level for different monochromatic spectral bands. Future studies using such technical setups could examine the luminance dependency of monochromatic contrast adaptation.

D. Contrast Adaptation and Eye Growth

The mechanism of contrast adaptation was shown to provide the visual system an estimate of the amount of retinal blur [29] in humans and was further demonstrated to act as a time-averaging mechanism that is supposed to drive the emmetropization in chickens [28]. Furthermore, contrast adaptation seems to play a major role in distinguishing the sign of defocus, since it was found that contrast adaptation occurs for imposed positive lenses but not for negative lenses [14]. Perceptual differences between negative and positive induced spherical defocus were revealed psychophysically for visual acuity [64] and contrast sensitivity [65]. Since, in the absence of higher order aberrations, the optical blur circles should exhibit the same size, question arises as to where this perceptual difference originates.

When the measurements of visual acuity under induced spherical defocus were repeated in monochromatic light conditions, the asymmetry vanished [51]. The authors suggested that the LCA of the eye is used as a directional cue to identify the sign of defocus. As the results from the current study reveal, contrast adaptation to low and medium spatial frequencies occurs under poly- and monochromatic conditions as well, indicating the important role of spatial frequency channels in the understanding of how the visual system detects the sign of defocus. The use of the complex frequency stimulus was not feasible to assess contrast adaptation for higher spatial frequencies, since repeatability is lower compared to low and medium spatial frequencies [45].

5. CONCLUSION

Adaptation to spatially modulated contrast patterns was found to arise under monochromatic light conditions for single and complex frequency adaptation stimuli and take place over a range of low and medium spatial frequency channels. Although interaction between frequency channels reduced the amount of contrast adaptation using a complex frequency stimulus, it can be concluded that, in comparison to polychromatic conditions, contrast adaptation appears independent of the LCA but is weaker for the short wavelength band when compared with the medium and long wavelengths.

Funding

Bundesministerium für Bildung und Forschung (BMBF) (ZUK 63).

Acknowledgment

This work was done in an industry-on-campus-cooperation between the University Tuebingen and Carl Zeiss Vision International GmbH. A. Leube, A. Ohlendorf, and S. Wahl are employees of Carl Zeiss Vision International GmbH. C. Kraft declares no potential conflict of interest.

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State of Adaptation	Area Under the Curve from 2.0 to 9.0 cpd
State of Adaptation	poly	470 nm	530 nm	630 nm
Pre-adaptation	$9.89 \pm 1.33$	$9.84 \pm 1.53$	$9.82 \pm 1.51$	$10.30 \pm 1.61$
Post-adaptation to $C_{low}$	$9.58 \pm 1.33$	$9.14 \pm 1.50$	$9.75 \pm 1.55$	$9.91 \pm 1.66$
post-adaptation to $C_{high}$	$9.69 \pm 1.26$	$9.26 \pm 1.44$	$9.54 \pm 1.55$	$9.75 \pm 1.58$

Contrast adaptation appears independent of the longitudinal chromatic aberration of the human eye

Abstract

1. INTRODUCTION

2. METHODS

A. Apparatus

B. Participants

C. Protocol

1. Psychophysical Procedures

2. Statistical Analysis

3. RESULTS

A. Single Spatial Frequency Adaptation

B. Complex Spatial Frequency Adaptation

4. DISCUSSION

A. Single Spatial Frequency Adaptation

B. Complex Spatial Frequency Adaptation

C. Mesopic Luminance Conditions

D. Contrast Adaptation and Eye Growth

5. CONCLUSION

Funding

Acknowledgment

REFERENCES

Cited By

Figures (5)

Tables (1)

Equations (2)

Journal of the Optical Society of America A