Modern helmet-mounted night vision devices, such as the Thales TopOwlTM helmet, project imagery from intensifiers
mounted on the sides of the helmet onto the helmet visor. This increased effective inter-ocular separation distorts several
cues to depth and distance that are grouped under the term "hyperstereopsis". Stereoscopic depth perception, at near to
moderate distances (several hundred metres), is subject to magnification of binocular disparities. Absolute distance
perception at near distances (a few metres) is affected by increased "differential perspective" as well as an increased
requirement for convergence of the eyes to achieve binocular fixation. These distortions result in visual illusions such as
the "bowl effect" where the ground appears to rise up near the observer. Previous reports have indicated that pilots can
adapt to these distortions after several hours of exposure. The present study was concerned with both the time course and
the mechanisms involved in this adaptation. Three test pilots flew five sorties with a hyperstereo night vision device.
Initially, pilots reported that they were compensating for the effects of hyperstereopsis, but on the third and subsequent
sorties all reported perceptual adaptation, that is, a reduction in illusory perception. Given that this adaptation was the
result of intermittent exposure, and did not produce visual aftereffects, it was not due to the recalibration of the
relationship between binocular cues and depth/distance. A more likely explanation of the observed visual adaptation is
that it results from a discounting of distorted binocular cues in favour of veridical monocular cues, such as familiar size,
motion parallax and linear perspective.