Study Of Visual Acuity And Reaction Time In Different Contrast Levels At Bright And Dark Background For Military Soldiers

Objective (1) To investigate the variation and relationship of visual acuity and reaction time in different contrast levels at bright and dark background for military soldiers with normal visual acuity. (2) To evaluate the significance of low contrast visual acuity measurement in personnel selection of special branches of the service and conscription physical examination.Methods 54 active-duty Army servicemen (108 eyes) with normal visual acuity were enrolled. logMAR visual acuity and reaction time were measured in four contrast levels (100%, 25%,10%, 5%) at bright and dark background in dark room with multi-functional visual acuity tester (MFVA-100). The measurement values were divided into 8 groups according to the background and contrast level. One-Way ANOVA, Paired-Samples T Test and Linear Regression Analysis were applied to analyzing the datum with SPSS 11.5 statistics software.Results (1) Background luminance being identical, visual acuity of lower contrast group was significantly worse than that of higher contrast group (P< 0.001); with contrast level descending, visual acuity decreased gradually. At the same contrast level, visual acuity of dark background group was significantly worse than that of bright background group (P< 0.001). (2) The visual acuity of contrast 100% group at bright background was not linearly related to that of any contrast level group at dark background (p>0.05). The correlation coefficient (r) was 0.076, 0.001, 0.15 and 0.096. At identical bright background, the visual acuity of contrast 100% group was linearly related to that of any other contrast level group (P< 0.001) with r was 0.665, 0.563 and 0.641 respectively, and the linear regression equation was y=0.165+0.844x, y=0.332+0.775x, y=0.471+0.952x. At dark background, the visual acuity of contrast 100% group was also linearly related to that of any other contrast level (P< 0.001) with r was 0.617, 0.434 and 0.412, and the linear regression equation was y=0.159+0.582x,y=0.4+0.483x,y=0.709+0.688x. (3) The fluctuating differences (log units) of visual acuity of different contrast groups at bright background were 0.45, 0.67, 0.68 and 0.81, and those at dark background were 0.50, 0.57, 0.61 and 1.06. (4) The reaction time (S) of contrast 100%, 25%, 10%, 5% at bright background was: 2.05±0.47, 2.38±0.52, 2.62±0.49 and 2.7±0.58; at dark background, it was 2.13±0.47, 2.45±0.55, 2.72±0.58 and 2.96±0.67 respectively.Conclusion The health adult's visual acuity at bright background was better than at dark background. With luminance weakening and contrast descending, visual acuity decreased, individual variation increased, and the reaction time of stimulant recognition extended gradually. As the evidence showed no linear correlation of visual acuity between bright and dark background, it can be conjectured that there may be distinct mechanisms about sensation and perception, conduction, and disposal of various characteristic stimuli in human visual system. MFVA-100 offers a new sensitive, convenient adjunctive measure of contrast sensitivity which may be useful for identifying the unique visual abilities required for the military.