TY - JOUR
AU - (Ret.), Morris R. Lattimore, MS USA
AB - ABSTRACT Background: Army vision standards have varied little from Aviation's nominal birth. On the basis of classic Snellen acuity, we simply cannot predict threshold skill levels of any one individual(s). A growing number of Army Flight Surgeons, clinicians, and vision scientists have argued for the inclusion of contrast acuity metrics within flight physical standards. Methods: Previous monitoring of operational contact lens utility in 223 Apache pilots, visual acuity data were gathered under two conditions: high illuminance; low illuminance combined with low contrast. Spectacle, contact lens, and aging influences were evaluated. Results: The high-contrast Snellen acuities clustered at 20/15 and 20/20. Low-contrast acuities stretched from 20/25 to 20/125. LogMAR analysis highlighted statistical significance between the two acuity sets (p < 0.001) to an unanticipated data spread. The known underlying mechanisms possibly related to this effect are poorly documented; all such variables collectively explain <30% of the known variation in low-illuminance vision. Discussion: Some pilots possessed the capacity to resolve 20/25 lettering under obfuscating conditions; others were adversely influenced by those same conditions. Snellen acuity involves target recognition; contrast acuity detects threshold differences; both aspects can be important. Conclusion: Prescreening under both vision assessment conditions will help identify and select superior vision performers. The validity and predictability of documenting this effect is targeted within planned future research efforts. INTRODUCTION Established Army Aviation vision standards have varied little from the initial days of the U.S. Army Aviation's nominal birth, which was after the U.S. Army Air Corps' transition to the U.S. Air Force in the late 1940s. In 1991, approximately 23% of all Army aviators wore a spectacle correction1 in order to comply with aviation's high-contrast Snellen acuity standard of 20/20 in accordance with Army Regulation 40-5012; at that time, the U.S. Air Force figure was slightly higher at 27%,3 while a more recent USAF document reports 41% of its active duty pilots require corrective lenses to perform flight duties.4 Historically, military aviation research has varied little beyond the utilization of those long-established, standardized clinical tests, with no directed goal toward an improved understanding of natural threshold-level physiological limitations; particularly as these tests relate to the specialized military equipment, fielded in response to traditional or operational combat challenges. Current Army flight standards for vision require only basic high-contrast assessments for near and distant Snellen visual acuity, stereo acuity, confrontation fields, color vision, and phoria or vergence testing.2 The advantage of high-contrast testing is that it easily permits the determination of population norms, as well as the determination of performance standards (with very small standard errors). Beyond the classic Snellen acuity record, we simply do not know what the near-threshold skill levels (in low contrast under dim retinal illuminance) of any 1 individual, or group of individuals might be. For this reason a growing number of Army Flight Surgeons, as well as clinicians and vision scientists have argued for the inclusion of contrast sensitivity/contrast acuity metrics within flight physical standards. Second, passing a flight physical when well rested is not the same as when fatigued. For example, any individual with a well-compensated phoria may easily fuse binocular images when well rested, but suffer from double vision when stressed and/or fatigued. New operational challenges have been identified primarily as a function of the environmental conditions encountered, and their secondary effect on visual performance requirements. A degraded visual environment (DVE) has been variously described as being under fog, brownout, or whiteout conditions, such that the overall level of illumination is decremented, matched by decreased target contrast. Brownout conditions, also referred to as a DVE have cost the Army numerous rated aviator lives, as well as over $1 billion in rotary-wing aircraft damage, resulting from approximately 800 class A accidents over the 8-year period of 2002 to 2009 (a class A accident involves the possible loss of one or more lives, with aircraft destruction or damage exceeding $2 Million).5 The potential fielding of a variety of technological solutions in response to the DVE threat is the Aviation Program Executive Officer's (PEO's) number one priority. Although these U.S. Army Research, Development and Engineering Command-developed technological countermeasures to DVE have the goal of making landing, navigating, and fighting easier, the countermeasures themselves may task certain individual's sensory limitations in ways that are not addressed under current physical examination standards. Human visual resolution performance, initially characterized within the academic community in 19726 had more recently been highlighted in 2009 by way of the publication of the Training and Doctrine Command's Human Dimension White Paper.7 New imaging and cockpit technologies, developed to compensate for engineering-developed DVE limitations, have the expressed goal of enhancing situational awareness under adverse conditions. However, they can potentially serve to increase workload, and may require changes to medical surveillance, as well as require changes in entrance or accession vision standards (e.g., visual performance-: under low-contrast conditions; or -after refractive surgery; -exclusionary concussive history; -inability to utilize spatial hearing). Existing occupational vision standards fail to reflect the unique visual demands of these newly developed technologies, as well as their modern combat-employment environment. Given those realities, the U.S. Army Aeromedical Activity Director and the U.S. Army Consultant for Aviation Medicine have both requested research into the possible establishment of new aeromedical standards (aligned toward documenting aircrew proficiency and fitness to operate these novel technological systems under diminished environmental conditions). Therefore, the intent of this article is to review background threshold-associated visual function issues as they relate to demonstrated contrast acuity variability under varying conditions of illumination; the goal of which is the development of an evidence-based medical reference for justifying the establishment of new rotary-wing aeromedical vision performance standards. Consequently, the impetus for the preparation of this manuscript stems from the already existing summed scientific evidentiary support for the inclusion of contrast acuity metrics in flight physical requirements. METHODS AND MATERIALS A much earlier study on the practical usefulness of contact lens wear by Army aviators had recorded standard Snellen acuity and contrast-based acuity as a part of the subject follow-up process. That specific USAARL Technical Report8 provided no comparative analysis, since the primary focus of the study, completed in 1992, was the operational flight utilization of contact lenses under deployed combat conditions. However, AR 40-501 has established medically based vision performance standards for accession, as well as for retention in all aspects of Army service. Usage of de-identified data from that original contact lens protocol was approved by the Institutional Review Board of the U.S. Army Medical Research and Materiel Command for use in this expanded data analysis and report. Each subject had been provided written and verbal informed consent before their original study participation. Examination initially occurred daily then quarterly exams occurred thereafter over the next 3 years. The initial data evaluation involved the utilization of two separate means of assessing subjective visual acuity documented. Acuity testing used a standard Snellen projector, shown onto a highly reflective screen (at 106 lux), and a Bailey–Lovie low-contrast (8%) chart. Acuities were determined before, and following every contact lens research evaluation. Additionally, Bailey–Lovie low-contrast acuity was measured at the same 20-foot distance and under conditions of moderate to low illuminance, at 31 lux. Visual acuities were measured in 223 subjects numerous times over 3 years. A logarithmic conversion of the Snellen data, using a minimum angle of resolution (LogMAR) equivalent, gained the benefit of parametric continuous-variable analyses, including the application of inferential statistical analysis (as opposed to the use of nonparametric discrete variable determinations, which are awkward and of limited usefulness). RESULTS Figure 1 provides a generalized visual comparison of the acuities of all 223 CONUS subjects, constituting over 2,000 clinical exams. Frequency distribution analyses of the LogMAR converted acuity responses highlighted the vast difference between the two disparate visual acuity tests (high-illuminance–high-contrast acuity versus low-illuminance–low-contrast acuity; p < 0.0001). Examining the Figure 1 data, it can be seen that any aviator could possess normal Snellen acuity (20/20 or slightly better), yet potentially exhibit anywhere on a continuum from exceptional to poor-quality low-contrast/low-illuminance acuity. Reverting to the original study, high-contrast Snellen visual acuities through contact lenses were not significantly different from high-contrast Snellen visual acuities through the subject's normally worn corrective spectacles. However, the acuity data in Figure 1, which is entirely through contact lenses under both conditions, reveals a significantly different distribution spread, with the low-contrast, low-illuminance data possessing an expanded data spread. FIGURE 1 View largeDownload slide Acuity response frequency. FIGURE 1 View largeDownload slide Acuity response frequency. DISCUSSION In consideration of the two singular assessments reviewed in this manuscript, a variety of potential sources for error were in operation to varying degrees. Comprehensive higher-order aberrations were evident in all conditions at equal degrees of involvement. Since each subject served as their own control, this issue presented no differential effects. The presence of high-illuminance/high-contrast acuity conditions equated to miotic or small pupil sizes, potentially affecting subject responses under one condition. Under the latter testing condition noncycloplegic influences, such as reducing retinal illuminance via the use of partial filters caused pupillary dilation, thereby indirectly inducing an increase in spherical aberration, a differential confounding factor for the low-contrast condition. Furthermore, defocus or an optical blur in the absence of a dilated pupil, was operant on an equal basis, because both acuity testing methods were performed under the refractive condition of “best visual acuity” or BVA. Thus, other issues regarding the disparate data distribution evidenced for low-contrast/low-illuminance acuities in Figure 1 must be considered. Extreme data spread in the low-illuminance/low-contrast condition were concluded to not be due to direct spectacle, contact lens, or aging influences. The data spread could be the result of poor overall uncorrected astigmia control, since the contact lenses were spherical in nature. The contact lens-wearing condition did correlate inversely in a marginally significant fashion (R = 0.12). Therefore, some of the low-illuminance/ low-contrast acuity distribution spread is exaggerated due to uncorrected astigmatism in the contact lens condition. A correlation of 0.12 essentially meant that no more than 2% of the observed low-illuminance/low-contrast acuity data spread had been due to uncorrected astigmia in soft contact lens wear (i.e., 1.44%). Similarly, Allard et al9 had documented age-based effects on acuity as a potential source of data contamination; we saw no systematic age related spread in acuities. However, Allard's subjects were in their 60s and 70s (the decremented effect had been hypothesized as perceptual “noise,” due to advanced age); our subjects in this limited dataset were 19 years to 46 years of age, explaining why we saw no age-based acuity changes. Since the diameter of the pupil of the human eye changes as a function of retinal illuminance, many studies seek to control pupil size by using an artificial pupil of a constant diameter in order to keep the degree of retinal illuminance constant (thus controlling one of the variables potentially influencing rod vs. cone functioning). However, while an artificial pupil is an excellent means of variable control, it induces an artificial condition that is not normally encountered by the visual system, potentially artificially influencing the experimental outcome. Therefore, control of the pupil size should not be relied on for use under DVE conditions. In recent years, a number of investigators have sought to develop sensitive means of assessing visual resolution via practical clinical-based testing.10,11 Although contrast sensitivity testing has proven itself as a penetrating visual performance diagnostic, such testing has been primarily isolated to the research realm due to its cumbersomeness to administer and apply. Consequently, a number of practical clinical offshoots have evolved (e.g., the Bailey–Lovie Computerized Low-Contrast Test, and the computer-based and tablet-based Rabin small letter contrast test [SLCT]).12,13 The latter example represents the current USAF standardized contrast acuity test, or have become proven, established tools capable of easy application within the realm of an aviation-based eye care clinical screening program. Their demonstrated benefits are 3-fold: as a measurement of the integrity of both the central and peripheral visual processing centers; as an indicator of detail resolution (pertinent to facial recognition or highly specific tasks); and, as an indicator of general figure/ground function (pertinent to movement within a complex environment). Contrast sensitivity testing has been shown to be superior at predicting a pilot's performance in detecting small, low-contrast targets in simulators as well as in the field,14,15 which is of potential importance to current military aviation DVE research efforts. Full scope contrast sensitivity testing under cycloplegic conditions had been proposed as a critical visual assessment task integral to the Army's class 1 flight physical. During a class 1 flight physical, a topically applied 1% cyclopentolate solution will artificially induce paralysis of the ciliary muscle of the eye. The topical cycloplegic pharmaceutical primarily inhibits accommodation; a subsequent secondary effect is then seen as pupillary dilation, which can induce some spherical aberration or secondary blur.16,–19 Initial SLCT research has shown the SLCT's sensitivity to be more discriminating than traditional visual acuity testing. It is also more responsive to small amounts of blur,20 to subtle changes in the luminance of the stimulus,21 to vision with 2 eyes compared to one eye,22 and for identifying visual differences among pilot trainees.23,–25 The goal of reviewing the preceding research data had been to examine the practical importance of this type of visual performance tool as a means of quantifying the degree of subtle visual performance differences that high-contrast/high-illuminance Snellen acuity fails to detect. However, all the justifications summarized in the preceding text highlight the importance of testing DVE sensitivities as a normal function of military physical examination. Computer display-based contrast threshold systems are available for the organization of faster subject screening assessments, which are now included in visual performance planning by this investigator. As part of the “Force 2025 and Beyond” Initiative,26 the Army has begun to reprioritize its Science and Technology (S&T) needs. Key to that S&T reprioritization is an increased emphasis on human performance optimization, defined as the process of applying knowledge, skills, and emerging technologies to improve and preserve the capabilities of Department of Defense personnel to execute essential tasks. External comparisons of the two acuity determination methods emphasized statistically significant differences that have potential for future use in identifying superior visual performers. If this research potential is realized, then the establishment of visual performance standards, in the military in general, or in military aviation specifically, will need to be modified to include the full-scope application of these visual performance assessments. The current vision sensitivity recording of only the upper-level or ceiling effect documented by high-contrast Snellen acuity will necessarily be expanded on, in order to document the floor- or threshold-level of absolute visual sensitivity. Given the Training and Doctrine Command's emphasis on the identification, development, and optimal integration of human capabilities, this manuscript had been prepared in an effort to stimulate such an expanded assessment of Army physical examination, visual performance standards (color standard modernization should not be overlooked either). Long-term approaches toward expanded examination regarding the role of color sensitivity, binocularity, and stereopsis, as well as cognitive visual processing (e.g., reading comprehension, short- and long-term memory, and eidetic memory) would complete the overall physical, and neurological functional analyses required for a complete understanding of one's ophthalmic health. A wide variety of additional factors with the potential to influence visual resolution (corneal distortions, lenticular alignment, aspect relationship errors, fusional, and stereoscopic errors, as well as numerous anatomical optical system variations)27,28 can cumulatively contribute to reduced image clarity, as well. However, neural processing applications could partially balance those confounding effects from anatomical variation. This neurological adaptational ability has previously been identified as a critical factor related to visual recovery from refractive surgery.29 The conceptual framework for providing a global assessment of threshold-linked visual performance is dependent, to varying degrees of influence, on two primary factors: Optical factors (i.e., pupil size and shape; corneal shape, lenticular shape, and overall ocular shape changes over time) have been identified as a likely predominant influence responsible for affecting contrast acuity,30 and/or: Neural, adaptational factors as the predominant influence responsible for affecting contrast acuity (i.e., when image presentation is under low contrast, and retinal illuminance is decremented).30 In general, recent investigators, after reviewing their own data, as well as that of others, have concluded that factors other than refraction are of primary influence in visual acuity resolution under mesopic, low-contrast/low-luminance conditions.31,32 The mesopic range of illuminance, roughly described as a mid-level illuminance reduction, is characterized by concurrent rod and cone function, which could theoretically occur in-concert, but not necessarily as a constant, due to theories of photoreceptor competition. Basic and applied research into this quasi-joint realm of conflicting photoreceptor functions is critical to understanding DVE sensitivities. Further, decreases in visual resolution occuring at low-contrast and decreased light levels appear to be subordinate or secondary to internal central nervous system-based (cns) neural factors, and not from optical blur or spherical aberration secondary to increased pupil size.33,34 The same investigators also recognized that microfluctuations within the accommodative system, occurring once again under a setting of low-contrast and decreased illuminance, will contribute to decreased visual resolution.35 Similarly, eye movement variability, which also increases in the dark, directly contributes to increased fixational instability and decreased visual resolution.36 The payoff in providing a medical evidence-based array of understanding threshold-level visual function influences would be a physical performance standard, against which selection of ideal candidates for specialized duty could be based. This end result would be an optimized force, capable of making use of every level of technology that is, and will be available. CONCLUSION Thus, identifying those individuals with superior contrast acuity resolution capabilities is perhaps the most effective solution in achieving an effective combat operations capability under DVE conditions (in both ground and air operations). The increased response range under low-contrast, low-illuminance conditions highlights the need for further investigation. At issue are other well-established sources for potential variation in low-illuminance conditions. Theories regarding possible benefit from increased macular pigment deposition, which varies considerably across our population, serve to increase the signal-to-noise ratio due to stray light absorbance. Iris color variation is suggested to reflect varied stray light absorbance, as well. As earlier alluded to, there are a myriad of optical and neuronal activation issues potentially in operation. Lastly, mid-level lighting intensity (mesopic lighting), when both rods and cones are both activated, can be variably be influenced via photoreceptor competition issues, pitting one type against the other. Ideally, only a comprehensive visual resolution research program, in collaboration with all the Services programs, will provide a thorough understanding of the key pertinent processes governing contrast acuity. Only then can the selection and accession of superior aviation candidates be guaranteed. ACKNOWLEDGMENTS The author has indicated no financial relationship relevant to the manufacturers of the tests, equipment, or contents of this article to disclose. This work was supported by the U.S. Army Aeromedical Research Laboratory, Fort Rucker, AL 36362. REFERENCES 1. Schrimsher RH, Lattimore MR Prevalence of spectacle wear among U.S. Army aviators. 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TI - Human Contrast Acuity Variability
JO - Military Medicine
DO - 10.7205/MILMED-D-16-00208
DA - 2017-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/human-contrast-acuity-variability-j134Msi32K
SP - 234
EP - 238
VL - 182
IS - suppl_1
DP - DeepDyve
ER -