Recent advances in ocular-evaluation technology allow for the measurement and quantification of higher-order optical aberrations such as coma, trefoil, and spherical aberration as well as the well-known lower-order optical aberrations of defocus and astigmatism.1 A standard nomenclature has been developed for describing ocular aberrations in terms of a Zernike expansion of the eye's wavefront aberration function.2–4 Any individual Zernike aberration degrades the optical quality of the retinal image and may systematically reduce visual function.5,6 However, when different types of aberrations are combined, they interact in complex ways that make it difficult to predict visual function based on knowledge of the systematic effects of individual aberrations.7 This latter observation raises several clinically relevant questions. Do some combinations of optical aberrations lead to normal or perhaps enhanced visual function? Are particular combinations of optical aberrations beneficial to individuals with visually demanding professions (eg, aircraft pilots or professional baseball players) by enhancing visual function? Can superior visual function be explained by the eye having better-than-normal optical qualities? Can testing combinations of different aberrations systematically yield more accurate and effective refractive therapies for the general population and for individuals with better-than-normal visual profiles? Answers to such questions could guide the prescription of refractive therapies to correct lower-order aberrations (LOAs) and higher-order aberrations (HOAs) in a way that optimizes visual function.
The effect of optical aberrations on visual function depends on the nature of the visual task. For example, there is little correlation between visual acuity for high-contrast, high-luminance letters and retinal-image quality for normal eyes that are fully corrected for spherical and cylindrical refractive errors (L.N. Thibos, et al., “Predicting Visual Performance From Scalar Metrics of Optical Quality,” presented at the meeting of the Optical Society of America, Long Beach, California, USA, October 2001).8–12 This suggests that neural factors account for most of the individual variability in optimally corrected distance visual acuity under standard conditions. However, if visual function is quantified by visual acuity for low-contrast and/or low luminance targets, optical quality plays a major role in the individual variability in functional performance.9–11,13
The effect of optical aberrations on visual function depends on the specific combination of Zernike aberrations in the eye. For example, when spherical aberration is present, a small amount of defocus is needed to optimize visual acuity.14–16 Although this additional defocus increases the total amount of aberration (as quantified by the root-mean-square error), a beneficial interaction between defocus and spherical aberration increases the optical quality of the eye (quantified by the fraction of the pupil area for which wavefront error is small). The result is an increase in acuity because one type of aberration cancels the deleterious effect of a different type of aberration.
Professional baseball players have superior visual function compared with that of the general population.17 Our research over the past 16 years shows that most professional baseball players do not require refractive correction, although a small subset has low spherocylindrical refractive errors. In our experience, although the overwhelming majority of professional baseball players have a visual acuity better than 20/20, many may be limited by uncorrected higher-order optical aberrations. Thus, this visually elite group may benefit from refractive corrections designed to optimize image quality in the presence of higher-order optical aberrations. The first step is to characterize the HOA profiles in professional ballplayers, which was the aim of the present study.
SUBJECTS AND METHODS
Members of the Los Angeles Dodgers and Boston Red Sox professional baseball teams were examined during the 2005 spring training season. As part of the yearly baseball vision screening performed on these players, lower-order and higher-order optical aberrations were measured using 2 devices, the Hartmann-Shack device (LADARWave system, Alcon, Inc.) and the holographic-grating device (Z-Wave aberrometer, Ophthonix, Inc.).
A trained Alcon technician performed the LARDARWave aberrometer measurements following the standard manufacture-recommended technique. This aberrometer uses the Hartmann-Shack method for measuring the effect of the eye's optical system on a wavefront of light reflected from the posterior segment. According to the manufacturer, the system can measure 2nd-order defocus ranging from −15.00 to +15.00 diopters (D) with a maximum measurable cylinder of 8.00 D. The system can measure aberration over a pupil ranging from 2.5 to 10.0 mm and resolves higher-order optical aberrations to the 8th Zernike order.18
The Z-Wave aberrometer is based on a proprietary holographic grating system that does not use Hartmann-Shack technology.19 According to the manufacturer, the system can measure optical aberrations in the Zernike 3rd to 6th order, measures more than 11 300 points over a 6.0 mm pupil, and is applicable to refractive errors between −11.00 D of myopia and +5.00 D of hyperopia.
Each instrument was used to record the 12 aberrations listed in Table 1. The mean value and the standard deviation were calculated for each aberration.
Further analysis was performed using a 2-sample t test to examine the differences between the 2 instruments in the mean measurements for each aberration. Although each HOA is supposed to be independent from the other HOAs, the Bonferroni correction was applied to each P value to ensure that any statistical significance would not be contaminated by possible interaction from multiple measures on the same subject. Therefore, P values had to be smaller than 0.004 to be statistically significant (0.05/12 = 0.004). Each eye was treated as an independent sample because of the large sample size. Bilateral symmetry was taken into account by flipping the wavefront aberration map about the vertical axis so right eyes were comparable to left eyes.4
Because all players were tested as part of the normal vision screening they receive during spring training, there were some limitations of the testing environment. The examinations were performed in the players' locker room, and fabric screens were used to minimize visual and auditory distractions. It was not logistically feasible to dilate the players because they were required to return to on-field practice immediately after the testing session. The lighting conditions could not be optimally controlled but were adjusted to a level in a low photopic range in which the pupil could naturally dilate to at least 4.0 mm. Players who normally wore contact lenses for baseball were tested while wearing the contact lenses; the other players wore no optical correction during testing.
The study evaluated 162 players. All 190 eyes (95 subjects) in which wavefront error was measured using the Hartmann-Shack device naturally dilated to at least 4.0 mm. Of the 324 eyes (162 subjects) in which wavefront error was measured using the holographic-grating device, 316 naturally dilated to at least 4.0 mm. Eight eyes (<3%) did not dilate to at least 4.0 mm and were excluded from the analyses.
Table 1 shows the descriptive statistics for each higher-order optical aberration measured using the 2 aberrometry systems. The higher-order optical aberrations were analyzed up to the 6th Zernike order with a pupil diameter of 4.0 mm. The mean values were small because of the relatively small pupil used for analysis. The largest HOA value measured by both instruments was for spherical aberration.
Table 2 shows the differences in wavefront analysis between the 2 aberrometers. There were statistically significant differences between the Hartmann-Shack device and the holographic-grating device in astigmatism C(2,2) (P=.002), trefoil C(3,−3) (P<.001), and secondary astigmatism C(4,2) (P<.001). In each case, values were smaller on the holographic-grating device.
By documenting the HOA profile in a population with superior vision, we hoped to provide some insight as to why they have supernormal visual profiles. From this information, we can learn how to enhance the vision of those who have normal vision. Higher-order optical aberrations become increasing important in determining the quality of the retinal image as the pupil size increases. We had limited control over the pupil sizes in this study because the eyes of the players could not be dilated in the middle of a practice session. Testing with more natural, smaller pupils offered an indication of what effect, if any, HOAs have with a pupil size representative that of professional baseball players during daytime playing conditions.
There were small but significant differences between holographic-grating aberrometry and Hartmann-Shack aberrometry in the players' mean astigmatism C(2,2), trefoil C(3,−3), and secondary astigmatism C(4,2) values. It is not possible to tell whether these significant differences were a result of random variations in the optical quality of the players' eyes, a difference in the aberration structure of the 2 sample populations, or an intrinsic difference in the 2 aberrometers. Despite reaching significance at the corrected P<.004 level, the largest significant difference in the means (0.0147 μm) was for trefoil. A 0.0147 μm difference in means over a 4.0 mm pupil is a 0.031 D equivalent spherical difference. This is approximately 8 times less than the typical 0.25 DS step of a phoropter.
The Texas Investigation of Normal and Cataract Optics (TINCO) study20 described wavefront errors in normal adult eyes with a variety of pupil diameters. The study reported a subset of 38 eyes of 38 individuals between the ages of 20 years and 40 years. In the present study, we used the TINCO study population as an age-matched control group to determine whether professional baseball players have a different higher-order wavefront aberration profile than a population of persons who are not professional athletes. Even though the data in the TINCO study were collected under tightly controlled laboratory conditions, we believe there is value in comparing baseball players with those who are not professional athletes. Those in the control group were age matched to the players, and the measured aberrations in both studies were recorded with a 4.0 mm pupil. Toward these ends, Table 2 compares the measurements of the 2 aberrometers with those in the TINCO study.
A 2-sample t test showed small but statistically significant differences between the baseball players' data and the norms reported in the TINCO study. The Hartmann-Shack aberrometer found a difference in the mean value for trefoil C(3,−3) versus the control TINCO population. The mean difference over a 4.0 mm pupil was 0.039 μm, which is an equivalent defocus error of 0.071 D and 3.5 times smaller than the typical 0.25 D step of a standard phoropter.
Aberrations do not function independently and can combine to increase or decrease visual performance.16 Single-value metrics of overall retinal-image quality have been developed to address interactions between aberrations.21 Some metrics predict close to 40% of the variance in difficult visual acuity tasks (mesopic low-contrast [11%] contrast acuity). To assess whether the retinal-image quality resulting from the HOAs of professional baseball players is better than that in the normal control group, we used the top 3 single-value retinal-image quality metrics for predicting low-contrast acuity in individuals with 20/17 or better high-contrast acuity to calculate the retinal-image quality in professional baseball players and in a control group10 (Table 3). There were no significant differences in the means of each metric between professional baseball players and the control group (Figure 1).
Our data, which we believe are the first to describe HOAs in this visually elite population, are important in several regards. First, in light of the higher-than-normal visual function in this group, we previously learned that there are fewer LOAs (eg, myopia, high levels of hyperopia or astigmatism) when the player is playing than in the normal population.17 In the present study, we found that this population has essentially the same HOA profile as an age-matched population. The significant differences in measurements between the 2 aberrometers and the control population appear to be random because only 1 of the 12 possible aberrations, trefoil, showed statistical significance. If this elite visual population had more HOAs than the general population, it would be important to know what they are and whether they interact to produce higher image quality and therefore better acuity. This does not appear to be the case.
The absence of clinically significant higher-order optical aberrations in the professional baseball population further highlights the importance of performing careful refraction in this group. In light of these data, when a baseball player presents with poor visual function (worse than the average 20/12.5 in this population), careful correction of all spherical and astigmatic refractive errors takes on greater importance. Because we found that professional baseball players have no clinically significant higher-order optical aberrations, we believe their visual system is limited by LOAs. A simple correction of a small spherical and/or astigmatic refractive error, which may not be applicable to the general population, may have a significant impact on the visual function and success of a professional baseball player. To the extent possible, given the exceptional visual acuity demands, contact lenses and refractive surgery could be considered treatment options where applicable. Careful preoperative measurement of LOAs is critical when planning a refractive procedure. Although the population of baseball players had a mean Zernike astigmatism near zero, individual eyes typically have significant amounts of Zernike astigmatism that could be limit visual performance. In this study, we concentrated solely on visual acuity as it relates to sports performance. Professional baseball players and other exceptional athletes have other visual attributes that set their visual systems apart from those in the general population. Their visual acuity must be combined with their superior stereopsis and contrast sensitivity to tell the complete story of why their visual function is exceptional.17
Future issues regarding the role of higher-order optical aberration in professional baseball players are many. For example, does varying the pupil size produce appreciably different results? Certainly, baseball players are subject many different lighting conditions and the bright light of a day game may cause the pupil to constrict and thus significantly affect the optical aberrations in the eye compared with a night game, during which the pupil would be expected to be larger. Also, which optical aberrations, if any, may be beneficial to baseball performance, providing an advantage in identifying the spin pattern of the pitched ball or the pitcher's finger position immediately before the ball is released. Finally, does a combination of optical aberrations that might provide this benefit exist?
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