A proper lookout. Studies of contrast sensitivity

Abstract

Naval navigation and watch keeping in littoral waters is highly dependent on visual function, and all navigators are required to have normal visual acuity (VA) measured by Snellen’s table or equivalent. Although visual functions obviously are important for navigators and watch keepers, there are few studies on visual problems as a cause of marine accidents or navigation performance. Visual function may be evaluated by several means, but it has been a long tradition to use Snellen’s table to describe VA as the main descriptor. Snellen designed his table in 1862 and the method has later been modified into a diversity of test, like the ETDRS table. The improvements enable more accurate testing of VA, but the test still lacks the possibility to test contrast sensitivity (CS). Contrast sensitivity is believed to be a better predictor of visual performance and can be tested on different sizes of objects and for achromatic light or coloured lights in dynamic or static modes. Contrast sensitivity is dependent on the clarity of the optical light way in addition to retinal and neurological function. The CS can be disturbed by corneal changes, as might be seen after corneal surgery or by lens degeneration. Corneal surgery and implantation of intraocular lenses are frequently performed on personnel trying to qualify for work demanding good VA. Few studies have evaluated the work performance and its correlation to VA or CS, and the studies are often non-conclusive. Sailors often stay on long watches and become sleep deprived. Very few studies have studied the visual function after prolonged sleep deprivation and none has looked at the effect of sleep deprivation on CS. The primary research goal of the present study was to obtain more information about the usefulness of various vision tests available for selection of personnel who perform work highly dependent on good visual function. The first study aimed to compare two different CS test methods and to establish reference values for CS in young adults with normal VA under photopic and mesopic light conditions. A total of 180 recruits, age 18-25 years was examined for CS and VA, and the test results were described and compared. In addition, a collated Index of contrast sensitivity (ICS) was computed and described. The agreement between the photopic tests indicated that they might be used interchangeably. There was little agreement between the mesopic and photopic tests. The mesopic test seemed best suited to differentiate between candidates and might therefore possibly be useful for medical selection purposes. In the second paper, sixty cadets at the Royal Norwegian Naval Academy (RNoNA) performed a visual observation task in a ship simulator. Their task performance was recorded according to VA, CS, gender and environmental light. Performance was highly correlated to increased environmental light and to gender. Men seemed to perform better than females, probably due to different approaches to decision making. No significant correlation between performance and CS or VA was found. This apparent absence of proven predictive value of visual parameters for observation tasks in a maritime environment may presumably be ascribed to the normal and uniform visual capacity in all our study participants. The third paper describes the possible influence of prolonged sleep deprivation on CS. During 60-hr sleep deprivation, CS was measured in 11 naval officers every sixth hour. Prolonged sleep deprivation does apparently not cause clinically or occupationally significant changes of CS in otherwise healthy subjects with normal VA.