Visual resolution of gratings by the compound eye of the bee Apis mellifera.

Early measurements of the resolution of horizontal versus vertical gratings were confirmed, with a limit near a period of 2.5 degrees, and the resolution is similar when vertical or horizontal gratings are tested separately against grey. Bees were next trained to discriminate from a distance between gratings at 45 degrees versus 135 degrees, with no green contrast, on targets presented in a vertical plane at a fixed distance. As expected, they fail to learn; however, with green contrast but no modulation difference the resolution limit is near 3.5 degrees. With vertical and horizontal gratings with no green contrast they discriminate but do not learn an orientation cue. In order to eliminate the orientation cue altogether, new bees were then trained with alternating vertical and horizontal gratings versus grey, or with a black and white checkerboard versus grey. Tests of these trained bees with horizontal or with vertical gratings separately against grey again show a resolution down to a period near 2.5 degrees. These results, taken together, show that when edge orientation alone is the cue, the limit of resolution is near 3.5 degrees, but when receptor modulation is the cue, the limit is near 2.5 degrees.