The New
Aniseikonia Test (
NAT), a hand-held direct-comparison test using red/green anaglyphs, has several potential advantages as a screener. We compared the validity of the
NAT to that of the Space Eikonometer in three experiments: (1)
aniseikonia was induced by calibrated size
lenses in a double-blind study of 15 normal subjects; (2) habitual
aniseikonia was measured with both instruments in four patients; and (3) eight of the normal subjects were retested with a computer-video simulation of the
NAT. The
NAT underestimated induced
aniseikonia by
a factor of 3 in the normal subjects and underestimated habitual
aniseikonia in four patients. The Space Eikonometer correctly measured the magnitude of induced
aniseikonia in the normal subjects. The simulation test did not show underestimation in the eight normal subjects. We could not attribute the
NAT's underestimation of
aniseikonia to the red/green anaglyph method, printing error, psychophysical method, or the direct-comparison test format. We speculate that the
NAT induces a different sensory fusion response to
aniseikonia than do the other tests, and that this altered sensory fusion response diminishes measured
aniseikonia. We conclude that the
NAT is not a valid measure of
aniseikonia.