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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.

An extended tail or prolonged depolarizing afterpotential (PDA) follows the receptor potential of a locust retinula cell when the stimulating light is in the intensity range that saturates the receptor potential. The amplitude and duration of this afterpotential depend on the intensity and duration of the stimulus. As the afterpotential decays, apparently exponentially, it becomes resolved into bumps, which we call light-induced dark bumps (LID bumps). The intervals between light-induced dark bumps are distributed in a way that is indistinguishable from a random (Poisson) distribution. As previously demonstrated, LID bumps are indistinguishable from bumps directly induced by low intensity light in light-adapted cells, which in turn grade into the slightly larger bumps produced, each by a single photon, in dark-adapted cells. The light-induced dark bumps continue for up to an hour in darkness, slowly becoming like dark-adapted bumps in amplitude and shape. To account for the random occurrence and discrete features of bumps after so long a latency, we propose that intense light generates a significant amount of an intermediate molecule or packet which decays slowly to start the same process that normally generates bumps with a short delay.