1. HUMAN CONTRAST ACUITY VARIABILITY Morris R. Lattimore, O.D., Ph.D.
U.S. Army Aeromedical Research Laboratory, Fort Rucker, AL
These data were previously an unpublished subset to a large multi-year applied research study of contact lens wear by Army aviators in the training environment only. The blanket fielding of the M43-A1 protective mask to all Apache aviators created an operational flight issue, because the protective mask design was not compatible with spectacle-wear or insert-correction, leaving contact lens wear as the only possible corrective option at the time (1990). Twenty-four per cent of the active-Army Apache aviator population required spectacles to achieve 20/20 visual acuity. In order to accommodate the Apache community’s flight capabilities (just as Operations Desert Shield and Storm were beginning), the Surgeon General issued medical waivers to the affected aviators, permitting them to fly in both training and combat environments while wearing contact lenses. Waiver subjects, under an informed consent protocol, were thus fit with contact lenses. One of the many performance aspects of investigator interest involved contrast acuity determinations, both under normal target illuminance and under decreased or adverse target illuminance.
The original acuity data, from the initial 223 of the eventual 586 Apache study subjects, have now undergone a response-data-conversion process, which permits continuous variable documentation and parametric statistical analysis, vice the original discrete variable (which restricted past analyses to descriptive and nonparametric statistics). A standard clinical Snellen projector acuity chart (at 106 lux), and a Bailey-Lovie printed 8% low-contrast acuity chart displayed under conditions of dim illuminance (31 lux) were used to assess the quality of vision under the conditions of spectacle- vs. contact lens-wear, additionally assessing the influence of age.
Small Letter Contrast Test
The data plots for both methods of visual “acuity’” utilized within the larger contact lens-related protocol are shown. It is clear by this distinct performance separation that the two “acuity” tests are actually probing two separate mechanisms of human visual functional performance. The Snellen test is an assessment of one’s ability to function in brightly lit environs and resolve fine detail, in essence it is probing a “ceiling function.” The low illuminance, low contrast visual resolution task (reported in LogMAR format) is a determined effort at forcing the cone mechanism to resolve a modicum of detail under poorly defined contrast and lighting conditions, which is actually a threshold-based task of visual function (i.e., a “floor effect”). Some individuals possessed the native capacity to resolve 20/20-sized lettering under the obfuscating conditions of low relative contrast (8%), combined with low illuminance (31 lux), while most others were severely- to only moderately-influenced by those conditions.
Conclusion
Results from supra-threshold Snellen acuity, in and of itself, are unable to predict which individuals are gifted with exceptional threshold contrast acuity under low contrast and low illuminance. The application of a threshold contrast acuity test in the pre-screening and pre-selection of training applicants possessing superior visual performance capabilities under similarly degraded visual conditions could yield increased survivability and combat effectiveness.
In order to improve the threshold contrast acuity method in future studies, it is suggested that the Rabin Small Letter Contrast Test (SLCT), as well as his equivalent Cone Color Test (CCT), which assesses threshold sensitivity in the color contrast arena, be inserted into any future protocols. Implementation of both threshold contrast acuity assessments would significantly improve the visual performance classification system in the evaluation of new military flight applicants.
Introduction
Discussion
“Acuity” measured by both methods used in CL protocol
Scatter plot of CL-wearing low illuminance,
low contrast acuity as a function of age
The Snellen projector acuities displayed essentially a bimodal pattern of 20/16 and 20/20 acuities for each vision-correction condition (which was not surprising, since all Army aviators must maintain 20/20 acuity or better, even with their contact lenses). Low contrast acuities under conditions of low illuminance revealed an unusual pattern, indicating a near-normal distribution, documenting acuity responses varying from 20/25 to 20/125. LogMAR conversion and comparative analyses highlighted the difference between the two target illuminance testing conditions (Snellen Acuity vs. Bailey Lovie Acuity; p < 0.001). The cross-evaluation of all conditions against method of correction (spectacles vs. contact lenses), as well as against subject age (ranging from 19 to 46 years) revealed poor correlation (R= 0.08 for the spectacle condition; and R=-0.12 for the contact lens condition) with the measured indices of Snellen visual performance. Uncorrected astigmatism (the confounding issue associated with soft contact lens fitting) influenced less than 2% of the observed acuity spread.
Results
Methods
Scatter plot of spectacle-wearing low illuminance,
low contrast acuity as a function of age
Recommendation
The views, opinions, and/or findings contained in this report/presentation are those of the author(s) and should not be construed as an official Department of the Army (DA) position, policy, or decision. Citation of trade names in this presentation does not constitute an official DA endorsement or approval of the use of such commercial items.