Visual Discrimination of Distance by Octopuses

Abstract

If two stimuli are presented to an octopus simultaneously, but at different distances from the animal, the nearer of the two is usually attacked. This preference was used to test the ability of octopuses to discriminate distance. White discs, 37 mm in diameter, were used as stimuli, and two parameters were varied: the distance of the farther stimulus from the animal (D), and the difference between the distances of the farther and nearer stimulus (d). Animals chose the nearer stimulus on 70% of occasions under the most difficult conditions used, where D was 370 mm and d was 50 mm. This percentage increased as D decreased or d increased. Further tests showed that varying the size of the discs, or using white vertical or horizontal rectangles instead of discs as stimuli, did not affect performance.The most likely cue being used by the animals to discriminate distances is accommodation. If this is the case octopuses can detect blurring of points on the retinal image comparable in size to a single retinal receptor, and lens displacements of around 10 μm.