Cm. Schor et Hl. Task, EFFECTS OF OVERLAY SYMBOLOGY IN NIGHT-VISION GOGGLES ON ACCOMMODATIONAND ATTENTION SHIFT, Aviation, space, and environmental medicine, 67(11), 1996, pp. 1039-1047
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.