Tactile discrimination of shape by octopus

Experiments are described showing that Octopus can be trained to distinguish by touch between a cube and a sphere, a surprising result since previous work has always indicated that octopuses cannot take proprioceptive information into account in learning. It can be shown, however, that the cube/sphere discrimination is made by detecting the distortion imposed on the rims of individual suckers in contact with the corners of the cube. Thus it is shown (1) that a cube with the corners even slightly rounded is less readily distinguishable from a sphere than a cube with sharp corners and, more important, (2) that a narrow rod is readily accepted in place of the cube by trained animals. Indeed, the rod seems to be a “better” cube than the original, and errors of discrimination decrease when it replaces the cube, presumably because it lacks flat surfaces liable to be confused with the surface of the sphere by a system depending on information from sense organs arranged in circles on the rims of the suckers.