Tactile Discrimination of Surface Curvature and Shape by the Octopus

Abstract

ABSTRACT Experiments are described in which octopuses were trained to discriminate by touch between pairs of Perspex cylinders of different diameter. The proportion of errors made in experiments with thirteen different pairs of cylinders shows that octopuses distinguish cylinders on a basis of the difference in their surface curvature. Curvature is detected from the degree of distortion of individual suckers. The bend of the arm or arms grasping an object can be shown to be irrelevant by using composite cylinders built up from narrower rods. These are treated as being of the diameter of their components. Having been trained to take the larger and reject the smaller of two cylinders, octopuses tested with rough and smooth objects of the same size reject the rough and accept the smooth. Apparently the sensory input produced by contact with an object having a rough surface is similar to that produced by bending the suckers round a smooth curve of narrow radius. The discrimination of cubes and spheres, which appears to be based on sucker distortion at the corners of the cube, is upset by cutting grooves into the surfaces of the two objects. These findings are discussed in relation to the anatomy of the sense organs in the suckers. The development of two parallel mechanosensory systems, one related exclusively to the local adjustment of muscle tension, the other more concerned with the animal’s relations to its external environment and hence involved in learned responses, is common to the organization of cephalopods and vertebrates.