Project

Signals for ACCOmodative responses in humans (SACCO-ERC-2012-StG-309416)

Goal: To test the hypothesis that the retina detects and accommodation responds to wavefront spherical curvature (optical vergence) itself, and not simply to blur.

Methods: A Shack-Hartmann wavefront sensor measures aberrations and accommodation at 20 Hz, a deformable mirror alters or removes aberrations and feedback from accommodation at 20 Hz, and a Badal optical system alters optical vergence at 20 Hz

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Philip Kruger
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Experiments using moving targets (sinusoidal and sum-of-sine motion) suggest that the longitudinal chromatic aberration (LCA) of the eye provides directional information at edges that specifies ocular focus and drives reflex accommodation (Kruger, Mathews, Aggarwala, Sanchez, 1993). Recently two studies used stationary targets to stimulate accommodation and both concluded that chromatic aberration has no effect on accommodation when the target is stationary (Bobier, Campbell and Hinch, 1992; Kotulak, Morse and Billock, 1995). The issue was addressed in three experiments: First, subjects (8) viewed a stationary target (3.5 cycles per degree square-wave grating) through a 3 mm artificial pupil at three dioptric stimulus levels (0 D, 2.5 D, 5 D) in a Badal stimulus system. There were 3 experimental conditions including Normal LCA, Reversed LCA and Monochromatic (550 nm; 10 nm bandwidth) and the resting position of accommodation was monitored while subjects viewed the grating target through a pinhole pupil. The target remained stationary during randomized 40-second trials, and accommodation was monitored continuously. Most subjects accommodated accurately in the normal condition at all three stimulus distances, a few subjects (3 of 8) had difficulty maintaining focus in monochromatic light at the near (5 D) or far (0) stimulus distances, and most subjects (7 of 8) could not maintain focus at both the near and far distances with LCA reversed. When the target is stationary it must be considerably closer or further away than the (tonic) resting level to demonstrate the effect of LCA. In the second experiment the same subjects (8) viewed the grating target as it moved sinusoidally toward and away from the eye (1.5 to 2.5 D at 0.2 Hz) under the same three experimental conditions. As in previous dynamic experiments, accommodative gain was reduced by approximately 50 % in monochromatic light, and accommodative tracking (gain) was poor with LCA reversed. In the third experiment subjects (12) viewed a stationary sine-wave grating (3.88 c/deg) through a pinhole pupil (0.75 mm). The sine-wave grating was projected at high luminance by a video-projector, and imaged in the Badal stimulus system. The grating target simulated the effects of static defocus and LCA on the contrast of the "red", "green" and "blue" components of the "white" grating for a 3 mm diameter pupil. There were three simulated defocus conditions: myopic focus (1.0 D); hyperopic focus (1.0 D); and a control condition that simulated the effect of defocus without LCA (the three chromatic components of the grating had the same contrasts). The subject viewed a fixation target for 10.24 seconds followed by 10.24 seconds during which one of the defocused sine-wave simulations appeared. Ten of 12 subjects accommodated in the appropriate direction to randomized open-loop simulations of under-and over-accommodation. The results support the hypothesis that the relative contrast of long-middle-and short-wavelength components of the retinal image with the same spatial frequency and spatial phase reliably indicates ocular focus and drives reflex accommodation for both stationary and moving targets.
Philip Kruger
added a research item
Modern methods of measuring the refractive state of the eye include wavefront sensors which make it possible to monitor both static and dynamic changes of the ocular wavefront while the eye observes a target positioned at different distances away from the eye. In addition to monitoring the ocular aberrations, wavefront refraction methods allow measurement of the accommodative response while viewing with the eye's habitual chromatic and monochromatic aberrations present, with these aberrations removed, and with specific aberrations added or removed. A large number of experiments describing the effects of accommodation on aberrations and vice versa are reviewed, pointing out the implications for fundamental questions related to the mechanism of accommodation.
Philip Kruger
added a research item
Purpose: To test the hypothesis that accommodation responds to the convergence or divergence of light rays that form out-of-focus images and not simply to blur.
Philip Kruger
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Purpose: We tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics. Methods: We performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size. Results: When mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51±0.53 D, and 0.12±0.15, for static and dynamic accommodation, respectively, and were statistically significant (p<0.039). When mydriatic optical effects were not taken into account, the differences in responses before and after PHCl instillation were -0.20±0.51 D, and -0.05±0.14, for static and dynamic accommodation, respectively, and were not statistically significant (p>0.313). Conclusions: The mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation.
Philip Kruger
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Philip Kruger
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Purpose : We propose a new strategy that may permit the visual system to get objective information of the sign of defocus under monocular viewing conditions. It is based on the differences between the retinal images focused in front or behind the peripheral retina produced by the presence of a blood vessel network of arteries and veins in front of the photoreceptor layer. Methods : We performed two types of studies: geometrical and diffractional retinal image simulations using ray tracing software (Optics Studio, Zemax®); and experimental measurements, using a camera with a fine printed irregular reticle of 25 microns, placed a few tenths of a millimeter in front a high resolution CCD. Two types of images were compared: with the image out of focus in front (myopic) or behind (hyperopic) the ideal image plane (photoreceptors or CCD layer). Results : Myopic and hyperopic retinal images of extended objects are different when the simulated blood vessel network or a real reticle is placed in front of the ideal image plane. These differences are not found in the absence of the vessels/reticle. Ray-tracing simulations and experimental images show that myopic images of extended objects formed close to the vessels/reticle plane present larger values of contrast at high spatial frequencies than the hyperopic or in focus images (figure). This effect is more evident when the spatial frequency of objects and vessels are similar. In particular, Fourier analysis of the experimental myopic and hyperopic images defocused approximately by the same amount, shows a different phase transfer function as well as a larger power spectrum magnitude (between 2.4 and 9 times larger at 12 c/deg) in the myopic image compared to the hyperopic one. This phenomenon can be explained by geometrical optics as it is due to different shadow and penumbra shapes originated by the vessels/reticle depending on the sign of defocus. Conclusions : The retinal image formed through a blood vessel network contains information about the sign of defocus that may be used by the visual system as an objective cue for accommodation under monocular vision. The increase of higher spatial frequency content in the myopic image in the retinal periphery (where blood vessels are present) may play a role in the emmetropization process and inhibit myopia progression.
Philip Kruger
added a research item
Eyes of children and young adults change their optical power to focus nearby objects at the retina. But does accommodation function by trial and error to minimize blur and maximize contrast as is generally accepted? Three experiments in monocular and monochromatic vision were performed under two conditions while aberrations were being corrected. In the first condition, feedback was available to the eye from both optical vergence and optical blur. In the second, feedback was only available from target blur. Accommodation was less precise for the second condition, suggesting that it is more than a trial-and-error function. Optical vergence itself seems to be an important cue for accommodation.
Philip Kruger
added 2 research items
Changing size (looming) produces changes in accommodation and vergence. Dynamic responses of vergence and accommodation to sinusoidal looming of a Maltese cross were recorded with an SRI dual-Purkinje-image eyetracker and optometer. The ratio of these two motor responses was compared with the response accommodative convergence/accommodation (AC/A) ratio and convergence accommodation/convergence (CA/A) ratio determined from sinusoidal variations of blur and disparity respectively. The response to changing size was found to be more similar to the AC/A ratio than the CA/C ratio. In addition, when a changing disparity or changing blur stimulus was combined with a changing size stimulus, the response phase lags of accommodation and vergence were decreased. In addition, the CA/C ratio was increased when changing size was added to changing disparity, but the AC/A ratio was unaltered when changing size was added to changing blur. These results indicate that changing size is stimulating accommodation directly and vergence secondarily through an AC/A crosslink.
Voluntary and reflex accommodation were measured monocularly in five normal subjects using a dynamic infrared optometer. Comparison of the peak velocity/amplitude relation (i.e., "main sequence") showed complete overlapping of the response distributions, suggesting similarity of motoneuronal controller signals for voluntary and reflex accommodation. Voluntary accommodation may represent a preprogrammed maneuver used in a variety of real-life predictable situations to optimize performance. Clinically, it may represent an important component in the training of accommodation dynamics.
Philip Kruger
added 4 research items
We have investigated the spatiotemporal transfer function of human "reflex" accommodation. An accommodative mechanism that is sensitive to an intermediate temporal rate of retinal image contrast change is proposed as the basis of the fine focus control hypothesis. To test the proposed mechanism accommodative responses were monitored by a dynamic infrared optometer while the subject focused on sinusoidal gratings (0.98-10.5 c/deg) which were moving sinusoidally at temporal frequencies in the range of 0.05-0.80 Hz over a 0.50 or 2.00 D peak-to-peak amplitude. The accommodative responses were best at 3 and 5 c/deg at both amplitudes of target motion. This result does not support the proposed mechanism or the fine focus control hypothesis for "reflex" accommodation. Fitting the data with first-order response functions showed little evidence of prediction. In addition, a second experiment found that the profile of the accommodative gain function is not altered by instruction at spatial frequencies above 5 c/deg in this type of dynamic accommodation experiment. The use of sinusoidally moving accommodative blur targets, particularly with careful instruction, seems to discourage voluntary accommodation in investigations of "reflex" control mechanisms of accommodation.
When subjects view an edge in white light, a colour fringe, produced by longitudinal chromatic aberration (LCA) of the eye, is formed at the edge. The colour fringe changes with changes in focus, and serves as a complex colour-coded cue for reflex accommodation. Fincham found that 60% of his subjects failed to accommodate appropriately when the colour fringe was removed with an achromatizing lens or by the use of monochromatic light. Our experiment sought to determine the spatial frequencies at which LCA is most effective. We monitored accommodation in 10 subjects while they viewed sinusoidally moving sine-wave gratings (1-3 D at 0.2 Hz; 1-10.5 c/deg) in a Badal optometer. The targets were 'white' gratings with LCA normal, doubled, neutralized or reversed. Doubling the aberration has minimal effect, removing the aberration reduces gain and increases phase-lag, and reversing the aberration severely disrupts accommodation. Sensitivity to these chromatic cues exists at all spatial frequencies tested, but is most prominent between 3 and 5 c/deg. These results support the view that the system monitors focus by comparing contrast in red-green and perhaps blue-yellow colour-opponent mechanisms.
Previous studies have suggested that targets illuminated by monochromatic (narrow-band) light are less effective in stimulating the eye to change its focus than are black-white (broadband) targets. The present study investigates the influence of target spectral bandwidth on the dynamic accommodation response in eight subjects. The fixation target was a 3.5-cycle/deg square-wave grating illuminated by midspectral light of various bandwidths [10, 40, and 80 nm and white (CIE Illuminant B)]. The target was moved sinusoidally toward and away from the eye, and accommodation responses were recorded and Fourier analyzed. Accommodative gain increases, and phase lag decreases, with increasing spectral bandwidth. Thus the eye focuses more accurately on targets of wider spectral bandwidth. The visual system appears to have the ability to analyze polychromatic blur to determine the state of focus of the eye for the purpose of guiding the accommodation response.
Philip Kruger
added a research item
Purpose: To determine if human accommodation uses the eye’s own monochromatic aberrations to track dynamic accommodative stimuli. Methods: Wavefront aberrations were measured while subjects monocularly viewed a monochromatic Maltese cross moving sinusoidally around 2D of accommodative demand with 1D amplitude at 0.2 Hz. The amplitude and phase (delay) of the accommodation response were compared to the actual vergence of the stimulus to obtain gain and temporal phase, calculated from wavefront aberrations recorded over time during experimental trials. The tested conditions were as follows: Correction of all the subject’s aberrations except defocus (C); Correction of all the subject’s aberrations except defocus and habitual second-order astigmatism (AS); Correction of all the subject’s aberrations except defocus and odd higher-order aberrations (HOAs); Correction of all the subject’s aberrations except defocus and even HOAs (E); Natural aberrations of the subject’s eye, i.e., the adaptive-optics system only corrected the optical system’s aberrations (N); Correction of all the subject’s aberrations except defocus and fourth-order spherical aberration (SA). The correction was performed at 20 Hz and each condition was repeated six times in randomised order. Results: Average gain (+/-2 standard errors of the mean) varied little across conditions; between 0.55 +/- 0.06 (SA), and 0.62 +/- 0.06 (AS). Average phase (+/-2 standard errors of the mean) also varied little; between 0.41 +/- 0.02 s (E), and 0.47 +/- 0.02 s (O). After Bonferroni correction, no statistically significant differences in gain or phase were found in the presence of specific monochromatic aberrations or in their absence. Conclusions: These results show that the eye’s monochromatic aberrations are not necessary for accommodation to track dynamic accommodative stimuli.
Philip Kruger
added a research item
Ocular accommodation is the autofocus mechanism of the healthy young eye that allows objects at different distances to be seen clearly. Even in the absence of binocular, monocular, and chromatic cues, most eyes continue to be able to accommodate. How? The fundamental mechanism driving accommodation under these stringent conditions is still unclear. There are three main theories. The first theory is that the visual system is able to use the small differences in retinal images formed when the object is focused behind or in front of the retina. These differences are due to imperfections that exist in the optics of all eyes, such as monochromatic aberrations and irregularly shaped pupils. The second, and most widely accepted theory is that accommodation works as a trial-and-error system, with the goal to minimize retinal image blur, and thus maximize retinal contrast. The third theory is that the eye is able to extract the necessary information to accommodate in the right direction directly from light vergence. The aim of this thesis was to put the aforementioned theories to the test. Three experiments were devised that required the use of a custom-built adaptive optics system. Accommodation was recorded with a Shack-Hartmann wavefront sensor, while participants viewed a monochromatic Maltese cross monocularly through a Badal optical system and while their aberrations were being corrected with the adaptive-optics system. This thesis presents evidence against the first theory (in agreement with previous works) and in support of the second and third theory. Even though the human eye can accommodate using a trial-and-error function to minimize blur and maximize contrast, it is considerably more precise when light vergence is present. Thus, the results obtained here support the less accepted theory that the human eye is able to detect and use light vergence to accommodate.
Philip Kruger
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The aim of this study was to determine whether dynamic accommodation responds to isolated blur cues without feedback, and without changes in the distance of the object. Nine healthy subjects aged 21 to 40 years were recruited. Four different aberration patterns were used as stimuli to induce blur: (1) the eye's natural, uncorrected, optical aberrations, (2) all aberrations corrected, (3) spherical aberration only, and (4) astigmatism only. The stimulus was a video animation based on computer-generated images of a monochromatic Maltese cross. Each individual video was generated for each subject off-line, after measuring individual aberrations at different accommodation levels. The video simulated sinusoidal changes in defocus at 0.2 Hz. Dynamic images were observed through a 0.8 mm pinhole placed at a plane conjugated with the eye's pupil, thus effectively removing potential feedback stemming from accommodation changes. Accommodation responses were measured with a Hartmann-Shack aberrometer for the four different aberration patterns. The results showed that seven out of nine subjects did not respond to any stimuli, whereas the response of the other two subjects was erratic and they seemed to be searching rather than following the stimulus. Therefore, a significant reduction in average accommodative gain (from 0.52 to 0.11) was obtained when the dioptric demand cue was removed. No statistically significant differences were found between experimental conditions used. We conclude that aberration related blur does not drive the accommodation response in the absence of feedback from accommodation.
Philip Kruger
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Philip Kruger
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Philip Kruger
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The objective of the current study was to compare accommodation to targets illuminated with monochromatic light from different regions of the visible spectrum with accommodation to white-light targets. One of 10 marrow-band interference filters (430, 450, 470, 500, 530, 550, 570, 590, 630, and 670 nm) was used to produce monochromatic light from a tungsten-halogen source to illuminate a Maltese cross-target in Maxwellian view. Luminance of each monochromatic light was matched by minimum border photometry against a standard white light (3000 K) that was maintained at 200 cd/m2. Chromatic difference of focus of the eye was minimized for all monochromatic targets by the use of an achromatizing lens. A white-light target also was used, and the subject's eye was achromatized or the eye had normal chromatic aberration. The target was moved sinusoidally toward and away from the eye at a temporal frequency of 0.2 Hz over a 1 D amplitude (peak to peak). Accommodation was monitored continuously by an infrared recording optometer, and responses were Fourier analyzed to obtain gain and phase lag at the temporal frequency of stimulation. Accommodative gain was highest and phase lag was smallest when the target was illuminated by white light in the presence of normal chromatic aberration. The achromatized white-light gain of accommodation was statistically similar to the gain for monochromatic targets, indicating that the presence of chromatic aberration facilitates accommodation. Significant intersubject variability was present in the accommodative tracking ability to monochromatic targets. Accommodation to monochromatic targets is not as accurate as accommodation to a white-light target, and this effect is related to the presence of ocular longitudinal chromatic aberration for the white-light target.
The eye's longitudinal chromatic aberration (LCA) is known to drive 'reflex' accommodation to moving objects, but the evidence is not as clear for stationary objects. The present study examined whether accommodation can be driven by static simulations of the effects of defocus and LCA. Accommodation was recorded continuously while each of 12 subjects viewed images (through a 0.75 mm pinhole) that simulated the appearances of blurred sine wave gratings (3.9 c.p.d.). In two experimental conditions, an eye with normal LCA was assumed and defocus of +1 D or -1 D was simulated. In a control condition, an eye with neutralised LCA was assumed and target defocus of 1 D was simulated. Subjects' accommodation responses were consistent with the hypothesis that LCA provides a stimulus to accommodation. Chromatic aberration drives accommodation to both moving and stationary objects, and thus is an important stimulus for accommodation in everyday situations. The study findings are discussed in relation to colour vision, visual display terminals and emmetropization.
Philip Kruger
added 2 research items
Both long- and middle-wavelength sensitive cones mediate the reflex accommodation signal but the contribution from the short-wavelength sensitive cones is unknown. A short-wavelength sensitive cone contribution could extend the range of the signed defocus signal from chromatic aberration. The aim was to determine whether isolated short-wavelength sensitive cones mediate reflex accommodation independently of long- and middle-wavelength sensitive cones. Accommodation was monitored continuously (eight subjects) to a sine-wave grating (3 cpd; 0.53 contrast) moving with a sum of sines motion in a Badal optometer. Two illumination conditions were used: a 'blue' condition that isolated short-wavelength sensitive cones, and a 'white' control condition that stimulated all three cone types. Of the eight subjects, two responded equally in the 'white' and 'blue' condition, four gave reduced responses in the 'blue' condition and two failed to respond in both conditions. The mean response in the 'blue' condition was reduced by 50% compared to the 'white' condition. Further analysis indicated that four of the eight subjects gave responses that were considerably greater than noise (S.D.>1.82) when short-wavelength sensitive cones were isolated. Some subjects can accommodate using only S-cones.
Previous studies have demonstrated that accommodation will respond to sine gratings in which the relative modulations of red, green and blue image components have been altered to simulate the effects of defocus and longitudinal chromatic aberration. The present study aimed to determine the tolerance of the accommodative system to relative phase shifts in those components induced by chromatic misalignment. It was found that accommodation can tolerate moderate amounts of chromatic misalignment (6'), but responds adversely when misalignments are large. Applications to visual display terminals and spectacle lens and instrument design are discussed.
Philip Kruger
added a research item
PURPOSE. To determine whether changes in wavefront spherical curvature (optical vergence) are a directional cue for accommodation. METHODS. Nine subjects participated in this experiment. The accommodation response to a monochromatic target was measured continuously with a custom-made adaptive optics system while astigmatism and higher-order aberrations were corrected in real time. There were two experimental open-loop conditions: vergence-driven condition, where the deformable mirror provided sinusoidal changes in defocus at the retina between 􏰀+1 and -1 diopters (D) at 0.2 Hz; and blur-driven condition, in which the level of defocus at the retina was always 0 D, but a sinusoidal defocus blur between 􏰀+1 and -1 D at 0.2 Hz was simulated in the target. Right before the beginning of each trial, the target was moved to an accommodative demand of 2 D. RESULTS. Eight out of nine subjects showed sinusoidal responses for the vergence-driven condition but not for the blur-driven condition. Their average (6SD) gain for the vergence- driven condition was 0.50 (60.28). For the blur-driven condition, average gain was much smaller at 0.07 (60.03). The ninth subject showed little to no response for both conditions, with average gain <0.08. Vergence-driven condition gain was significantly different from blur- driven condition gain (P 1⁄4 0.004). CONCLUSIONS. Accommodation responds to optical vergence, even without feedback, and not to changes in defocus blur alone. These results suggest the presence of a retinal mechanism that provides a directional cue for accommodation from optical vergence.
Philip Kruger
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Accommodation Responds to Optical Vergence and Not Defocus Blur Alone.
Antonio J. Del Águila-Carrasco, Iván Marín-Franch, Paula Bernal-Molina, José J. Esteve-Taboada, Philip B. Kruger, Robert Montés-Micó, Norberto López-Gil.
Investigative Ophthalmology and Visual Science, 2017;58(3):1758-1763.
doi: 10.1167/iovs.16-21280.
View and download the paper from the IOVS website at:
 
Philip Kruger
added 6 research items
The aim of the experiment was to test for a contribution from short-wavelength sensitive cones to the static and step accommodation response, to compare responses from short and long- plus middle-wavelength sensitive cone types, and to examine the contribution of a signal from longitudinal chromatic aberration to the accommodation response. Accommodation was monitored continuously (eight subjects) to a square-wave grating (2.2 c/d; 0.57 contrast) in a Badal optometer. The grating stepped (1.00 D) randomly towards or away from the eye from a starting position of 2.00 D. Five illumination conditions were used to isolate cone responses, and combine them with or without longitudinal chromatic aberration. Accuracy of the response before the step, step amplitude, latencies and time-constants, were compared between conditions using single factor ANOVA and t-test comparisons. Both S-cones and LM-cones mediated static and step accommodation responses. S-cone contrast drives "static" accommodation for near, but the S-cone response is too slow to influence step dynamics when LM-cones participate.
The aim was to identify the cone contributions and pathways for reflex accommodation. Twelve illumination conditions were used to test specified locations in cone-contrast space. Accommodation was monitored continuously in a Badal optometer while the grating stimulus (2.2 c/d sine-wave; 0.27 modulation) moved sinusoidally (0.195 Hz) towards and away from the eye from a mean position of 2.00 D (+/-1.00 D). Mean accommodation level and dynamic gain and phase at 0.195 Hz were calculated. Mean accommodation level varied significantly when the long- and middle-wavelength cone contrast ratio was altered in both the luminance and chromatic quadrants of cone-contrast space. This experiment indicates that L- and M-cones contribute to luminance and chromatic signals that produce the accommodation response, most likely through magno-cellular and parvo-cellular pathways, respectively. The L:M cone weighting to the luminance pathway that mediates accommodation is 1.63:1. The amplitude and direction of the response depends on changes in chromatic contrast and luminance contrast signals that result from longitudinal chromatic aberration and defocus of the image.
The accommodation response is sensitive to the chromatic properties of the stimulus, a sensitivity presumed to be related to making use of the longitudinal chromatic aberration of the eye to decode the sign of the defocus. Thus, the relative sensitivity to the long- (L) and middle-wavelength (M) cones may influence accommodation and may also be related to an individual's refractive error. Accommodation was measured continuously while subjects viewed a sine wave grating (2.2c/d) that had different cone contrast ratios. Seven conditions tested loci that form a circle with equal vector length (0.27) at 0, 22.5, 45, 67.5, 90, 120, 145 deg. An eighth condition produced an empty field stimulus (CIE (x,y) co-ordinates (0.4554, 0.3835)). Each of the gratings moved at 0.2 Hz sinusoidally between 1.00 D and 3.00 D for 40s, while the effects of longitudinal chromatic aberration were neutralized with an achromatizing lens. Both the mean level of accommodation and the gain of the accommodative response, to sinusoidal movements of the stimulus, depended on the relative L and M cone sensitivity: Individuals more sensitive to L-cone stimulation showed a higher level of accommodation (p=0.01; F=12.05; ANOVA) and dynamic gain was higher for gratings with relatively more L-cone contrast. Refractive error showed a similar correlation: More myopic individuals showed a higher mean level of accommodation (p<0.01; F=11.42; ANOVA) and showed higher gain for gratings with relatively more L-cone than M-cone contrast (p=0.01; F=10.83 ANOVA). If luminance contrast is maximized by accommodation, long wavelengths will be imaged behind the photoreceptors. Individuals in whom luminance is dominated by L-cones may maximize luminance contrast both by accommodating more, as shown here, and by increased ocular elongation, resulting in myopia, possibly explaining the correlations reported here among relative L/M-cone sensitivity, refractive error and accommodation.
Philip Kruger
added 2 research items
High order aberrations have been suggested to play a role in determining the direction of accommodation. We have explored the effect of retinal blur induced by high order aberrations on dynamic accommodation by measuring the accommodative response to sinusoidal variations in accommodative demand (1-3D). The targets were blurred with 0.3 and 1mum (for a 3-mm pupil) of defocus, coma, trefoil and spherical aberration. Accommodative gain decreased significantly when 1-mum of aberration was induced. We found a strong correlation between the relative accommodative gain (and phase lag) and the contrast degradation imposed on the target at relevant spatial frequencies.
Longitudinal chromatic aberration (LCA) provides a cue to accommodation with small pupils. However, large pupils increase monochromatic aberrations, which may obscure chromatic blur. In this study, we examined the effect of pupil size and LCA on accommodation. Accommodation was recorded by infrared optometer while observers (nine normal trichromats) viewed a sinusoidally moving Maltese cross target in a Badal stimulus system. There were two illumination conditions: white (3000 K; 20 cd/m) and monochromatic (550 nm with 10 nm bandwidth; 20 cd/m) and two artificial pupil conditions (3 and 5.7 mm). Separately, static measurements of wavefront aberration were made with the eye accommodating to targets between 0 and 4 D (COAS, Wavefront Sciences). Large individual differences in accommodation to wavefront vergence and to LCA are a hallmark of accommodation. LCA continues to provide a signal at large pupil sizes despite higher levels of monochromatic aberrations. Monochromatic aberrations may defend against chromatic blur at high spatial frequencies, but accommodation responds best to optical vergence and to LCA at 3 c/deg where blur from higher order aberrations is less.
Philip Kruger
added a project goal
To test the hypothesis that the retina detects and accommodation responds to wavefront spherical curvature (optical vergence) itself, and not simply to blur.