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Effects of overlay symbology in night vision goggles on accommodation and attention shift.

BACKGROUND
Binocular night vision goggles that have a monocular symbology overlay provide potential conflicts between perceived symbology distance and optical vergence cues for accommodation.


HYPOTHESIS
Accommodative response was measured in subjects who perceived symbology nearer than the background to determine if accommodation could respond to the optical stimulus and override the perceptual cues.


METHODS
Symbology was presented to the right eye under two conditions (clear and blurred to 20/50 resolution) and the background was presented binocularly with and without added speckle noise that is typical of the NVG at low light levels.


RESULTS
On average, subjects accommodated by less than 0.1 diopters (D) to all 4 of the stimulus combinations even though the perceived proximal depth difference between background and symbology 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 optically and detected by the experimental apparatus. In a final experiment, the duration required to shift attention away from the background to a peripheral symbol, acquire critical information, and return attention to the background was 0.69 s which included saccade latency and scan 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.