presented against a background
luminance of 200–300 cd/m2 with Michelson contrast of
0.33 and unlimited observation time, the observer’s optometric
status was unexceptional. Inter-observer variations are common
in learning research, but four participants were deemed sufficient
here when their individual results showed such high concordance.
The study was approved by the institution’s human subject review
board and complied with the Helsinki declaration.