Weight Discrimination by Octopus

Blinded octopuses can readily be taught to discriminate between objects touched provided that these differ in 'roughness'; objects with equally irregular surfaces, differing only in the distribution of irregularities, and objects of similar texture but different shape, are not distinguished. It seems that the animals are incapable of learning to recognize specific patterns of stimulation in the tactile sensory field, or to recognize how the arms must bend around objects grasped; they evidently distinguish things that they touch simply from the proportion of sense organs excited in the area of contact (Wells & Wells, 1957). The failure to take into account the relative positions of the sense organs excited seems best explained by supposing that Octopus is incapable of integrating proprioceptive with surface tactile information in learning to recognize objects by touch. This hypothesis is interesting because, if true, it would mean that the tactile learning system of Octopus is peculiarly simple, a mechanism limited to making only a quantitative distinction between surfaces touched. A direct test of the matter is to train octopuses to discriminate between objects of similar texture but different weight; such objects cannot be identified on a basis of surface tactile information alone, but can readily be distinguished by any animal capable of learning to recognize tension or pressure differences within the parts of its own body handling the objects. The experiments reported here show that octopuses cannot be taught to discriminate between objects differing only in weight, although they very readily learn to make textural discriminations under the same conditions.