December 2010
Volume 51, Issue 12
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Visual Psychophysics and Physiological Optics  |   December 2010
Pattern-Related Visual Stress, Chromaticity, and Accommodation
Author Affiliations & Notes
  • Peter M. Allen
    From the Vision and Eye Research Unit, Postgraduate Medical Institute, Cambridge, United Kingdom;
    the Department of Vision and Hearing Sciences, Anglia Ruskin University, Cambridge, United Kingdom; and
  • Atif Hussain
    the Department of Vision and Hearing Sciences, Anglia Ruskin University, Cambridge, United Kingdom; and
  • Claire Usherwood
    the Department of Vision and Hearing Sciences, Anglia Ruskin University, Cambridge, United Kingdom; and
  • Arnold J. Wilkins
    the Visual Perception Unit, Department of Psychology, University of Essex, Colchester, United Kingdom.
  • Corresponding author: Peter M. Allen, Department of Vision and Hearing Sciences, Anglia Ruskin University, East Road, Cambridge CB1 1PT, UK; peter.allen@anglia.ac.uk
Investigative Ophthalmology & Visual Science December 2010, Vol.51, 6843-6849. doi:10.1167/iovs.09-5086
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      Peter M. Allen, Atif Hussain, Claire Usherwood, Arnold J. Wilkins; Pattern-Related Visual Stress, Chromaticity, and Accommodation. Invest. Ophthalmol. Vis. Sci. 2010;51(12):6843-6849. doi: 10.1167/iovs.09-5086.

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      © 2017 Association for Research in Vision and Ophthalmology.

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Abstract

Purpose.: To investigate the impact of colored overlays on the accommodative response of individuals, with and without pattern-related visual stress (PRVS), a condition in which individuals manifest symptoms of perceptual distortion and discomfort when viewing a 3-cyc/deg square-wave grating.

Methods.: Under double-masked conditions, 11 individuals who reported PRVS selected an overlay with a color individually chosen to reduce perceptual distortion of text and maximize comfort (PRVS group). Two groups of control subjects individually matched for age, sex, and refractive error were recruited. Control group 1 similarly chose an overlay to maximize comfort. Control group 2 used the same overlays as the paired PRVS participant. The overlay improved reading speed by 10% (P < 0.001), but only in the PRVS group. A remote eccentric photorefractor was used to record accommodative lag while participants viewed a cross on a background. The background was uniform or contained a grating and was either gray or had a chromaticity identical with that of the chosen overlay. There were therefore four backgrounds in all.

Results.: Overall, the accommodative lag was 0.44 D greater in the participants with PRVS. When the background had the chosen chromaticity, the accommodative lag was reduced by an average of 0.16 D (P = 0.03) in the PRVS group, but not in the symptom-free groups: in control group 2 the colored background slightly increased the accommodative lag.

Conclusions.: Accommodative lag was greater in individuals susceptible to pattern-related visual stress and was reduced by a colored background.

The accommodative response is known to vary with many factors, including refractive error, 1 refractive error stability, 2 4 target size, 5 target luminance, 6 target spatial frequency, 7 and method of stimulus presentation. 1,2,4 When a target is viewed at a proximal distance (e.g., when reading), an accommodative lag or underaccommodation relative to the stimulus of up to 0.50 D is expected. 8 The object of regard will remain clear provided that the accommodative error lies within the depth of focus of the eye. The depth of focus depends on a variety of factors including pupil diameter, lighting conditions, and the target viewed. 9 Inappropriate accommodative responses, such as under- or overaccommodation relative to the plane of the object of regard are a frequent correlate of asthenopia. 10  
Differences in accommodative response in persons experiencing visual discomfort from near work have been reported. Simmers et al. 11 found increased accommodative microfluctuations in a small sample of individuals who reported benefit from colored filters, but the accommodative stimulus response function was normal. Chase et al. 12 found a significant positive correlation between symptoms of visual discomfort with near work and accommodative lag (measured objectively using an open-field autorefractor). The prevalence of accommodative insufficiency was much higher than estimated by clinical measures. Tosha et al. 13 reported lag of accommodation to increase over a 90-second measurement period, with the increase being more pronounced in individuals with high visual discomfort. 
Sometimes symptoms of visual discomfort are associated with perceptual distortion, usually of text, in which case they are referred to as visual stress. Colored filters have been shown to alleviate symptoms of visual stress, 14 18 although the mechanisms are poorly understood. 19 The colored filters can take the form of colored sheets placed on the page when reading or colored lenses worn as spectacles. Ciuffreda et al. 20 examined the accommodative response in a group of tinted lens wearers. No significant differences were found in accommodative responses, with and without the colored lenses. 20  
Any form of image degradation (due to contrast, luminance, or spatial frequency composition) has a negative impact on the effectiveness of a target that serves as a stimulus to accommodation. When the stimulus to accommodation is text or gratings and the material is subject to perceptual distortion, as is the case in people who experience PRVS, 21,22 an associated change in accommodation can be anticipated. 
Colored overlays have been shown to improve reading speed in persons who report perceptual distortions of text and gratings that the overlays reduce. Hollis and Allen 23 showed that the increase in speed could be better predicted from the perceptual distortion reported when gratings are observed than from reports of symptoms in extensive questionnaires of the kind used to measure visual discomfort. 24  
Whereas either a Hartinger coincidence optometer 20 or an open field autorefractor 11 13 requiring instrumentation proximal to the participant has been used in previous work, we used an eccentric photorefractor (PowerRefractor, MultiChannel Systems, Reutlingen, Germany) from a distance of 1 m, leaving the proximal field unimpeded. In previous studies, the lack of an internal fixation target or enclosed viewing in the open field autorefractor reduced, but did not remove, the risk of proximal accommodation and thereby an increase in the accommodative response. 25  
Methods
The participants were recruited by advertisement from the student population attending Anglia Ruskin University. All participants gave informed consent after a written and verbal explanation of the procedures involved. All procedures conformed to the tenets of the Declaration of Helsinki and were approved by the Anglia Ruskin University Ethics Committee. 
Two experiments were undertaken. The first involved the use of individually chosen filters in two groups: one (PRVS) with symptoms of visual stress and one (control group 1) with no symptoms. The PRVS group showed improvements in reading speed with the chosen filters, whereas control group 1 did not. In the second experiment, control group 2 (yoked controls), similarly matched to the PRVS for age and optometric status, used the same filters as those chosen by the symptomatic group. The first experiment was undertaken in sessions 1, 2, and 3. The second experiment was undertaken in sessions 4, 5, and 6. 
Experiment 1
Session 1: Screening for PRVS and Control Group 1 Participants.
Eighty-three young adults (51 women and 32 men aged between 18 and 25 years) attended an initial screening session to exclude any participants with migraine and significant optometric and binocular vision anomalies. Symptoms described by persons with PRVS, such as headaches, blurring, and words moving on the page are nonspecific and may also be caused by refractive error or binocular anomalies. The inclusion criteria are shown in Table 1
Table 1.
 
Inclusion Criteria
Table 1.
 
Inclusion Criteria
Visual acuity of at least 6/6 in each eye
Cover test of <5Δ horizontal phoria and <0.5Δ vertical phoria
No slip evidenced on fixation disparity (Mallett unit)
No diplopia reported during the ocular motility test
Near point of convergence (RAF rule) ≤10 cm
Amplitude of accommodation (push-up RAF rule) normal for age (>10 D)
Stereo acuity (Titmus circles) of <80 seconds of arc
Normal red/green color vision (Ishihara)
Astigmatism of <0.75 DC
In addition to the screening tests, all persons attending the initial session had an objective assessment of their refractive error with an autorefractor (AR-600A; Nidek, Gamagori, Japan), 26 and their susceptibility to PRVS was assessed using a pattern glare test as follows: The desk surface was illuminated by the light from a compact fluorescent lamp with a correlated color temperature of 3500°K. At a distance of 0.4 m, the participants were shown a grating with a square-wave luminance profile, Michelson contrast, approximately 0.9; spatial frequency, 3 cyc/deg; circular in outline; and radius 13°. They were asked a series of questions regarding the perceptual distortions that they experienced, each beginning “Looking into the center of the grid that is in front of you, do you see any of the following (please answer each question with either yes or no): pain or discomfort; shadowy shapes among the lines; shimmering of the lines; flickering; red, green, blue, or yellow; blur; bending of the lines; nausea or dizziness; or unease”? Wilkins 21 used this technique to identify whether people are likely to have susceptibility to PRVS. A score of 4 or more indicates that a person may have a sensitivity to pattern glare and experience symptoms. 23  
There were four men and seven women (aged 18–25) with pattern glare scores greater than 3 who were selected to continue to sessions 2 and 3. Eleven control subjects with pattern glare scores less than 3 were also selected, matched for age, sex, and refractive error. (Individuals with scores of 3 were omitted.) They were matched to individuals in the PRVS group for mean spherical equivalent refractive error (spherical power+half cylindrical power), since ametropia has been shown to influence accommodative response. 1 4  
Session 2: Overlay Assessment and Administration of the Rate of Reading Test to the PRVS Group and Control Group 1.
During session 2 and without knowledge of the classifications, the first experimenter undertook additional measurements of reading speed, with and without overlays. All subjects with habitual refractive correction had their vision corrected with spherical contact lenses to within 0.25 D. A commercially available overlay system was used (Intuitive Overlay; ioo Sales, Ltd., London, UK). According to the procedure recommended in the manual, all 22 participants chose from the overlays the color of overlay or combination of two overlays that best improved the clarity and comfort of the text it covered. The Rate of Reading test 27 was administered four times: first with (condition A), then without (condition B), then again without, and finally with overlay(s). The ABBA design was to minimize practice effects. Most of the practice effect occurs from the first to the second administration and the ABBA design therefore biases any mean difference against a benefit. An average rate of reading with and without the overlay was calculated, along with the percentage difference between the two conditions, and the scores are shown in Table 2
Session 3: Measurements of Accommodation in the PRVS and Control Group 1.
A third experimenter conducted the investigations in session 3 without knowledge of the findings obtained in sessions 1 and 2 or the allocation of participants. A slideshow program was constructed to display three targets on the liquid crystal display (LCD) screen of a laptop computer mounted orthogonal to the line of sight at a distance of 0.5 m from the eyes, the minimum at which it was possible to obtain an adequate image of the pupil used by the photorefractor. The targets were (1) a gray field with a central fixation cross with horizontal and vertical lines, each 3 mm long; (2) the same cross superimposed on a horizontal grating with square-wave luminance profile, Michelson contrast approximately 0.9, spatial frequency 1.3 cyc/deg, circular in outline, radius 13°; and (3) a passage of text consisting of randomly ordered common words. The sequence of presentation was an ABBA block nested within an ABBA block, to reduce practice effects. 
To ensure that the chromaticity of the background on the screen matched that of the colored overlays, the colored overlays selected by the participants were placed on white paper illuminated the same as during the screening test. They were observed through one of two circular apertures in an opaque surface in an otherwise dark room. The 302 × 228-mm LCD was viewed through the other aperture, and the hue and saturation of the display were adjusted to match the color appearance of the two apertures. The various chromaticities of the screen background (measured with a TV Color Analyser II; Minolta, Osaka, Japan) are shown in Figure 1
Figure 1.
 
Chromaticities of the screen background. (A) PRVS group and control group 2 (identical); (B) control group 1. The cross in (A) shows the chromaticity of the gray screen. Beside each point is indicated the number of participants who chose the color, if more than one.
Figure 1.
 
Chromaticities of the screen background. (A) PRVS group and control group 2 (identical); (B) control group 1. The cross in (A) shows the chromaticity of the gray screen. Beside each point is indicated the number of participants who chose the color, if more than one.
The accommodative response was measured by the PowerRefractor (MultiChannel Systems) photorefractor at a distance of 1 m, while each of the targets in the presentation was observed on the laptop screen. The PowerRefractor is an eccentric photorefractor that captures reflected infrared light from the participant's eye. The photorefractor was used in the monocular mode, whereby the refractive error was measured dynamically in the vertical meridian of the eye at a frequency of 25 Hz, so that a reading of refractive error and pupil size was taken once every 0.04 seconds, for 10 seconds. Allen et al. 26 showed that the validity and repeatability of the PowerRefractor is high, with no significant difference being found between measurements obtained with the PowerRefractor and subjective refraction. The 95% limits of agreement in monocular mode ranged from −0.32 to +0.62 D. 
The use of an adjustable chin and forehead rest allowed optimum positioning of the right eye in line with the center of the photorefractor's head, thereby reducing artifacts and parallax error due to head movement. The vision of all subjects wearing habitual refractive correction was corrected with spherical contact lenses to within 0.25 D. To ensure that the vision of all participants was optimally corrected, any small residual refractive errors were corrected where necessary with trial lenses, the maximum additional trial lens used being 0.25 D. This was necessary in only three participants (one from the PRVS group and two from control group 1). 
Because of large variations in calibrations among participants, 28,29 the PowerRefractor was calibrated for each participant individually. For calibration, the left eye fixated a 6/9 letter placed at 6 m. The right eye was occluded with an infrared transmitting filter (Wratten 87c; Eastman, Kodak, Rochester, NY). Trial lenses (+4.00 to −1.00 DS) were placed in front of the filter, which occluded visible light from the right eye. Measured refraction was compared to the refraction expected from the trial lenses, with allowances made for a vertex distance of 12 mm. The correction factor was taken from the slope and intercept of the linear regression trendline and used to calibrate the photorefractor's measurements from that participant. Before starting calibration of the photorefractor, the participant was dark adapted for 4 to 5 minutes, to allow dissipation of any transient changes in the tonic position of accommodation due to previous near work. 30 After calibration, all viewing was binocular, although measurements were taken from the right eye. 
The convergence necessary to fixate the cross was approximately 7°. This is within the tolerance of the PowerRefractor (∼0.50-D change in apparent accommodation with gaze 25° eccentric to the optical axes). 31  
The order of slide presentation was the same for all the participants. The participants initially viewed a cross (A) and then a grating (B) on a gray background for 10 seconds in an ABBA design. Then, they were asked to read a passage of text for 45 seconds with a gray (A) and colored (B) background in an ABBA design. The color was similar to that individually chosen during session 2 (overlay assessment and administration of the Rate of Reading Test to the PRVS group and control group 1). Next, the participants were required to look at a cross (A) and grating pattern (B) with their chosen colored background, again in an ABBA design; and finally, the first four presentations (cross and grating on a gray background) were repeated. The participants were asked to concentrate on the central fixation point (a cross superimposed on the uniform background or on the grating, luminance 76 cd.m−2) for a duration of 10 seconds. With the prose targets, the subjects were asked to read the displayed text for durations of 45 seconds. Brief rest periods were taken after each measurement. 
Experiment 2
The purpose of experiment 2 was to provide “yoked” control participants who used background colors identical with those used by the PRVS group in experiment 1. 
As in experiment 1, the participants (control group 2) were recruited from the student population attending Anglia Ruskin University (4 men and 7 women, aged 18 to 24). As before, the participants had pattern glare scores <3 and were chosen to match the PRVS group with regard to sex and age. The same inclusion criteria were adopted (Table 1). 
Experiment 2 was conducted in three sessions (sessions 4, 5, and 6, corresponding respectively to sessions 1, 2, and 3 in experiment 1). The sessions were identical with those in experiment 1, apart from the exclusion of the conditions in session 3 (measurements of accommodation) in which the participants were required to read. 
Masking.
The participants and the experimenters who undertook the reading rate measurements were unaware of the group allocations; the participants were first- and second-year students who were unaware of the purpose of the pattern glare test. 
Data Integrity.
Because of eye movement, the data obtained during reading in experiment 1 session 3 (measurement of accommodation) were technically poor, and were rejected. The remaining recordings allowed the comparison of two target types (cross and grating) on two backgrounds (gray and colored) for each of the three groups (PRVS group and the two control groups). 
Results
Table 2 summarizes the clinical data and the results of the screening used to group the participants and also includes the rate of reading. 
Table 2.
 
Mean Scores for the Characteristics Used to Group the Participants (Pattern Glare Score) and the Reading Rates, with and without an Overlay
Table 2.
 
Mean Scores for the Characteristics Used to Group the Participants (Pattern Glare Score) and the Reading Rates, with and without an Overlay
Mean Pattern Glare Score Mean Age (y) Mean Spherical Equivalent Refractive Error (D) Mean Reading Rate without Overlay Mean Reading Rate with Overlay Difference in Rate (%)
PRVS Group 4.91 20.6 −1.28 152 167 9.9*
(0.94) (18–25) (2.29) (32.7) (37.9)
(4–7) (+0.63 to −7.02)
Control Group 1 1.00 20.6 −1.47 171 172 0.5
(0.82) (18–24) (1.49) (18.3) (19.8)
(0–2) (+0.50 to −4.26)
Control Group 2 0.82 20.8 −1.22 166 165 −0.1
(0.75) (18–24) (2.05) (13.4) (10.3)
(0–2) (+0.75 to −6.75)
The mean results (the average response over the 10-second measurement period) in each condition of experiment 1 are presented in Figure 2. Any periods of data loss (e.g., when a participant blinked) have been removed, together with the associated artifact. 26 From the figure, it can clearly be seen that the lag of accommodation was greater in the group with PRVS than in control group 1. As can also be seen, the effect of color was to reduce the lag of accommodation for the PRVS group and to increase it marginally in control group 1. These effects were confirmed in an analysis of variance, with color and stimulus as within-subject factors and participant group as a between-subject factor. The analysis revealed a significant effect of group (F (1,20) = 9.04, P = 0.007, η2 = 0.017), but not of color, and a significant color by group interaction term (F (1,20) = 6.86, P = 0.016, η2 = 0.117). 
Figure 2.
 
Mean lag of accommodation when viewing cross and grating targets on gray or colored backgrounds for participants, with and without visual stress. Error bars, SD.
Figure 2.
 
Mean lag of accommodation when viewing cross and grating targets on gray or colored backgrounds for participants, with and without visual stress. Error bars, SD.
Separate analyses of variance for the two groups of participants revealed a significant main effect of color for the PRVS group (F (1,10) = 5.86, P = 0.036, η2 = 0.136) and no significant main effect for control group 1. There were no other significant effects or interactions, apart from an effect of stimulus, present in all analyses. For example, the overall analysis of variance revealed a significant effect of stimulus such that for both groups of participants, the accommodation was weaker with the cross as stimulus than with the grating (F (1,20) = 6.02, P = 0.02, η2 = 0.077). 
To assess accommodative microfluctuations, we calculated root mean square (RMS) deviation of the accommodative response, according to Anderson et al. 32 :   We found no significant correlation between the RMS and the mean accommodative response, and so we analyzed the variation separately. The RMS deviation was significantly greater in control group 1 than in the PRVS group, irrespective of the color of the background (0.592 vs. 0.359 D, P = 0.01). The difference remained when the signal was detrended and band-pass filtered in the frequency range of 0.2 to 0.6 Hz (P = 0.03), a range suggested by the work of Simmers et al. 11  
The chromaticities of the colored screen shown in Figure 1 were used to calculate the hue angles of the screen relative to the gray. The hue angle was used to obtain the dominant wavelength of the stimulus: the monochromatic light that, when additively mixed in suitable proportions with the reference white light, matches the color of the stimulus. The Spearman rank correlation across participants between the dominant wavelength and (1) refractive error and (2) mean accommodative error when viewing the colored screen were −0.25 and −0.27, respectively, both non-significant. 
The PowerRefractor measurements included concurrent measurements of pupil diameter. On average, the pupil diameter in the PRVS group (4.8 mm; minimum, 3.8 mm) was slightly greater than that in control group 1 (4.4 mm; minimum, 3.5 mm), although analysis of variance failed to reveal any significant main effects or interactions. 
The association between the refractive error measurement by autorefractor in sessions 1 and 4 (initial screening) and the average lag of accommodation measured by photorefractor when the uniform gray background was viewed are shown in Figure 3. The correlations were PRVS group: r = 0.49, P = 0.06; control group 1: r = 0.65, P = 0.01; and control group 2: r = 0.24, P = 0.23. 
Figure 3.
 
Lag of accommodation as a function of refractive error.
Figure 3.
 
Lag of accommodation as a function of refractive error.
The results of experiment 2 in which the yoked control group (control group 2) participated (Fig. 4) showed a larger lag of accommodation on a colored background than on gray. A repeated-measures analysis of variance with color and target as main effects revealed a main effect of color (F (1,10) = 6.10, P = 0.033, η2 = 0.149) and no other significant effects. 
Figure 4.
 
Mean lag of accommodation in control (yoked) group 2 participants viewing cross and grating targets on gray or colored backgrounds. Error bars, SD.
Figure 4.
 
Mean lag of accommodation in control (yoked) group 2 participants viewing cross and grating targets on gray or colored backgrounds. Error bars, SD.
The repetition of the gray background at the beginning and end of sessions 3 and 6 (measurement of accommodation) permitted an assessment of the effects of any fatigue. There was no statistically significant difference between the first and second presentations in any of the groups (P > 0.05). 
Discussion
The accommodative lag was clearly greater in the PRVS group, and, for this group, the colored background reduced the accommodative lag, although it did not reach the same level as in either control group. However, it is striking that in control group 2, there was a significant effect of color, and it was in the opposite direction from that observed in the PRVS group. Indeed in both control groups, the lag of accommodation was larger, with the colored background. In the PRVS group, the lag of accommodation was smaller with the colored background. The reversal in the direction of the effect of color for the PRVS and control groups cannot be attributed to ceiling and floor effects (i.e., to the lower overall lag of accommodation seen in the control groups). 
There was an effect of target stimulus, similar for both control group 1 and PRVS groups (but not seen in control group 2): the accommodation response was slightly greater for the grating than for the cross. The difference was only 0.1 D and therefore was not clinically significant; both stimuli elicited an adequate accommodative response. 
Previous studies used either a Hartinger coincidence optometer 20 or an open-field autorefractor. 11 13 Although the open-field autorefractor allowed targets in real space to be used, it necessitated objects in the field of view close to the eyes and nearer than the target. Proximal accommodation may therefore still have been evoked. 25,33,34 Chase et al. 12 using a Grand Seiko WAM-5500 autorefractor (Seiko, Tokyo, Japan) showed a greater lag of accommodation in individuals with high visual discomfort scores, but only after prolonged recording. There was no difference in accommodative lag between individuals with PRVS and control subjects in the study by Simmers et al., 11 but the sample size was small and the measurement duration was short. However, it is possible to discern in their data a small difference in the same direction as that obtained in the present study. Ciuffreda et al. 20 found no difference in accommodative response with or without colored lenses. The present study differed from previous studies in that the refractive power was measured remotely with an instrument at a distance of 1 m with no proximal stimuli. 
Measurements of accommodative response have been shown to be influenced by the spatial frequency of the target in both static 7,35 and dynamic measurements. 36,37 Simmers et al. 11 used a Maltese cross as a target and Chase et al. 12 a five-pointed star, both of which would have provided energy at low spatial frequencies. In the present study, we compared two stimuli—a small cross and a grating—and showed a slightly greater accommodative lag for the former. The cross was evidently a sufficient stimulus for accommodation, given that the accommodative response was within normal limits, but may nevertheless have provided a slightly weaker stimulus to accommodation compared with the gratings. The gratings provided contrast energy in one meridian only, but this was the meridian in which the PowerRefractor measured accommodation. 
Pupil diameters <2.0 mm have been found to increase depth of focus, but in the present study, pupil diameters were in a range (3.5–6.6 mm) that produces fairly stable blur sensitivity. 38 The lack of a significant difference in pupil size between groups and the marginally larger pupil size in the PRVS group combine to indicate that the accommodative findings are independent of pupil size. 
The color of the background was not related to the size of the accommodative lag or to the refractive error. It did not appear that the color of the background acted to reduce the effects of chromatic aberration, because there was no association between the dominant wavelength and the magnitude of the refractive error or accommodative lag. 
There are a number of potential mechanisms by which color may have improved the accommodative response (reduced the lag of accommodation) in the PRVS group. First, Chase et al. 39 measured the subjective speed matches between L-, M-, and S-cone–isolating stimuli in good and poor readers and suggested that differences in L/M- cone ratios in the retina may contribute to reading difficulties. As the L/M ratio influences accommodation, 40 then changing the L/M excitation with color will change the accommodative response. Second, if the text is found to be uncomfortable for the reader (PRVS group) because of cortical overactivation, 21 then blur would reduce such activation by contrast reduction. If color reduces overactivation, then a reduced lag of accommodation may result. 
Irrespective of the color of the background, the variability in accommodation (accommodative microfluctuation) was greater for control group 1 than the PRVS group, which showed the greater accommodative response. This is unsurprising as Day et al. 41 have shown that a greater accommodative response results in a larger variability in the response. The present RMS values are high, but within the range shown by Anderson et al. 32 which was 0.1 to 0.7 D for a 2-D response amplitude, even in older participants. 
These findings with respect to accommodative microfluctuations add to the inconsistencies in the literature. Tosha et al. 13 used monocular viewing and showed a larger variability in accommodation at close viewing distances, but no differences between groups with high and low visual discomfort scores. Simmers et al. 11 showed a greater variability of accommodation in a small group with PRVS and a reduction in the variability with colored filters. 
Plainis et al. 42 suggested that lag of accommodation may be influenced by the change in spherical aberration that occurs during accommodation. Indeed, it has recently been shown that inducing negative spherical aberration in myopes can increase the accommodative response and reduce any lag of accommodation present. 43 In several studies, 44 49 the changes in both spherical aberration and other higher-order aberrations during accommodation have been examined, but with variable results. In general, spherical aberration tends to change with increasing accommodative effort, from an initially slightly positive value toward a negative value. The various relationships between image quality, higher order aberrations, and accommodation are still unsettled, and it remains possible that manipulation of aberrations affects accommodation and thereby PRVS. 
The spatial frequency of the target viewed during accommodation measurements was 1.3 cyc/deg and lower than that at which the pattern glare was measured in sessions 2 and 5. The spatial frequency of the target grating was low relative to that optimal for the induction of illusions. The spatial frequency of the target grating was a compromise between the requirements to provoke illusions and those necessary to avoid extreme discomfort. We wished to reduce the blinks and gaze aversion associated with extreme discomfort, because they would have interfered with the recording. Using a 1.3-cyc/deg grating rather than the more aversive 3-cyc/deg grating leaves open the possibility that accommodation might have been even more adversely affected in PRVS subjects had a 3-cyc/deg target been used. 
A major strength of the present study is that it was double masked. The instructions to participants are known to influence the accommodative response, 50 but could not have affected the findings because both the experimenters and participants were unaware of the allocation of groups or the relevance of the measurements undertaken. 
In all previous studies cited herein, the participants viewed the stimuli monocularly with the nonviewing eye occluded with a patch. Another strength of the present study is that the participants viewed the stimuli under normal binocular reading conditions. Seidel et al. 51 showed that binocular viewing resulted in accommodative responses that were more accurate (showed less lag of accommodation) than those obtained under monocular viewing. 
Chase et al., 12 who used the Conlon Visual Discomfort questionnaire, found accommodative lag to be correlated strongly with symptoms of headache, blur, and diplopia, but not with distortions of text. The participants in the present study were selected on the basis of pattern glare scores, which have been shown to predict the improvement in reading speed with colored filters 23 better than symptom questionnaires. 24  
The differences in accommodative lag observed in the present study were within the range for which associated blur is tolerated. Within this range, central mechanisms that are independent of optical factors may predominate. The chromaticity of illumination individually chosen to reduce perceptual distortion has been shown to improve reading speed. If the chromaticity of illumination differs from the optimal chromaticity by a separation of ∼0.07 in the CIE UCS diagram, the color offers no improvement. 52 It will be interesting in future work to determine whether the accommodative changes found in this study have similar chromatic specificity, and, if so, whether the reduction in accommodative lag is long lasting. 
Footnotes
 Disclosure: P.M. Allen, None; A. Hussain, None; C. Usherwood, None; A.J. Wilkins, P
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Appendix
Table A1.
 
The Mean Spherical Equivalent Refractive Error, Pattern Glare Score, Overlay Color Chosen and Mean Lag of Accommodation for All Participants
Table A1.
 
The Mean Spherical Equivalent Refractive Error, Pattern Glare Score, Overlay Color Chosen and Mean Lag of Accommodation for All Participants
MSE Pattern Glare Score Chosen Color Mean Lag of Accommodation
PRVS
    1 −0.06 5 Rose 0.68
    2 −1.75 5 Lime −0.05
    3 −1.73 5 Orange 0.87
    4 −0.52 5 Mint and mint 0.24
    5 −7.02 4 Pink 0.18
    6 0.63 4 Orange 1.17
    7 −0.37 5 Rose and orange 0.82
    8 −3.00 4 Orange 0.62
    9 0.12 6 Aqua and mint 0.57
    10 0.25 7 Orange 1.02
    11 −0.61 4 Blue 1.05
Control Group 1
    1 −3.43 1 Blue 0.23
    2 −1.44 2 Pink 0.21
    3 −4.26 2 Rose −0.34
    4 −1.63 1 Mint −0.02
    5 −0.82 0 Mint 0.07
    6 −2.95 1 Aqua 0.16
    7 0.50 0 Orange 0.51
    8 −0.50 0 None chosen 0.01
    9 −1.36 2 Aqua 0.43
    10 −0.37 1 Blue 0.18
    11 0.12 1 Blue 0.34
Control Group 2 (Yoked control)
    1 −0.04 1 Rose 0.24
    2 −1.67 0 Lime 0.22
    3 −1.72 1 Orange 0.17
    4 −0.87 2 Mint and Mint 0.10
    5 −6.75 1 Pink 0.34
    6 0.75 0 Orange 0.30
    7 −0.75 1 Rose and orange 0.14
    8 −2.25 1 Orange 0.11
    9 0.12 0 Aqua & Mint 0.17
    10 0.25 0 Orange 0.27
    11 −0.50 2 Blue 0.12
Figure 1.
 
Chromaticities of the screen background. (A) PRVS group and control group 2 (identical); (B) control group 1. The cross in (A) shows the chromaticity of the gray screen. Beside each point is indicated the number of participants who chose the color, if more than one.
Figure 1.
 
Chromaticities of the screen background. (A) PRVS group and control group 2 (identical); (B) control group 1. The cross in (A) shows the chromaticity of the gray screen. Beside each point is indicated the number of participants who chose the color, if more than one.
Figure 2.
 
Mean lag of accommodation when viewing cross and grating targets on gray or colored backgrounds for participants, with and without visual stress. Error bars, SD.
Figure 2.
 
Mean lag of accommodation when viewing cross and grating targets on gray or colored backgrounds for participants, with and without visual stress. Error bars, SD.
Figure 3.
 
Lag of accommodation as a function of refractive error.
Figure 3.
 
Lag of accommodation as a function of refractive error.
Figure 4.
 
Mean lag of accommodation in control (yoked) group 2 participants viewing cross and grating targets on gray or colored backgrounds. Error bars, SD.
Figure 4.
 
Mean lag of accommodation in control (yoked) group 2 participants viewing cross and grating targets on gray or colored backgrounds. Error bars, SD.
Table 1.
 
Inclusion Criteria
Table 1.
 
Inclusion Criteria
Visual acuity of at least 6/6 in each eye
Cover test of <5Δ horizontal phoria and <0.5Δ vertical phoria
No slip evidenced on fixation disparity (Mallett unit)
No diplopia reported during the ocular motility test
Near point of convergence (RAF rule) ≤10 cm
Amplitude of accommodation (push-up RAF rule) normal for age (>10 D)
Stereo acuity (Titmus circles) of <80 seconds of arc
Normal red/green color vision (Ishihara)
Astigmatism of <0.75 DC
Table 2.
 
Mean Scores for the Characteristics Used to Group the Participants (Pattern Glare Score) and the Reading Rates, with and without an Overlay
Table 2.
 
Mean Scores for the Characteristics Used to Group the Participants (Pattern Glare Score) and the Reading Rates, with and without an Overlay
Mean Pattern Glare Score Mean Age (y) Mean Spherical Equivalent Refractive Error (D) Mean Reading Rate without Overlay Mean Reading Rate with Overlay Difference in Rate (%)
PRVS Group 4.91 20.6 −1.28 152 167 9.9*
(0.94) (18–25) (2.29) (32.7) (37.9)
(4–7) (+0.63 to −7.02)
Control Group 1 1.00 20.6 −1.47 171 172 0.5
(0.82) (18–24) (1.49) (18.3) (19.8)
(0–2) (+0.50 to −4.26)
Control Group 2 0.82 20.8 −1.22 166 165 −0.1
(0.75) (18–24) (2.05) (13.4) (10.3)
(0–2) (+0.75 to −6.75)
Table A1.
 
The Mean Spherical Equivalent Refractive Error, Pattern Glare Score, Overlay Color Chosen and Mean Lag of Accommodation for All Participants
Table A1.
 
The Mean Spherical Equivalent Refractive Error, Pattern Glare Score, Overlay Color Chosen and Mean Lag of Accommodation for All Participants
MSE Pattern Glare Score Chosen Color Mean Lag of Accommodation
PRVS
    1 −0.06 5 Rose 0.68
    2 −1.75 5 Lime −0.05
    3 −1.73 5 Orange 0.87
    4 −0.52 5 Mint and mint 0.24
    5 −7.02 4 Pink 0.18
    6 0.63 4 Orange 1.17
    7 −0.37 5 Rose and orange 0.82
    8 −3.00 4 Orange 0.62
    9 0.12 6 Aqua and mint 0.57
    10 0.25 7 Orange 1.02
    11 −0.61 4 Blue 1.05
Control Group 1
    1 −3.43 1 Blue 0.23
    2 −1.44 2 Pink 0.21
    3 −4.26 2 Rose −0.34
    4 −1.63 1 Mint −0.02
    5 −0.82 0 Mint 0.07
    6 −2.95 1 Aqua 0.16
    7 0.50 0 Orange 0.51
    8 −0.50 0 None chosen 0.01
    9 −1.36 2 Aqua 0.43
    10 −0.37 1 Blue 0.18
    11 0.12 1 Blue 0.34
Control Group 2 (Yoked control)
    1 −0.04 1 Rose 0.24
    2 −1.67 0 Lime 0.22
    3 −1.72 1 Orange 0.17
    4 −0.87 2 Mint and Mint 0.10
    5 −6.75 1 Pink 0.34
    6 0.75 0 Orange 0.30
    7 −0.75 1 Rose and orange 0.14
    8 −2.25 1 Orange 0.11
    9 0.12 0 Aqua & Mint 0.17
    10 0.25 0 Orange 0.27
    11 −0.50 2 Blue 0.12
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