EFFECTS OF OVERLAY SYMBOLOGY IN NIGHT-VISION GOGGLES ON ACCOMMODATIONAND ATTENTION SHIFT

Background: Binocular night vision goggles that have a monocular symbo logy overlay provide potential conflicts between perceived symbology d istance and optical vergence cues for accommodation. Hypothesis: Accom modative response was measured in subjects who perceived symbology nea rer than the background to determine if accommodation could respond to the optical stimulus and override the perceptual cues. Methods: Symbo logy was presented to the right eye under two conditions (clear and bl urred to 20/50 resolution) and the background was presented binocularl y with and without added speckle noise that is typical of the NVG at l ow light levels. Results: On average, subjects accommodated by less th an 0.1 diopters (D) to all 4 of the stimulus combinations even though the perceived proximal depth difference between background and symbolo gy was 10 times greater. No perceptible blur resulted from this small change in accommodation because it is well below the depth of focus of the human eye. A control experiment illustrated that an accommodative response, at least as small as 0.25 D, could easily be stimulated opt ically and detected by the experimental apparatus. In a final experime nt, the duration required to shift attention away from the background to a peripheral symbol, acquire critical information, and return atten tion to the background was 0.69 s which included saccade latency and s can time of the symbol. Conclusions: These results indicate that small changes in accommodation that occur when pilots shift attention from background to symbology do not produce perceived blur of the symbology or the background and thereby do not impair performance.