Proprioception and Visual Discrimination of Orientation in Octopus

Octopuses can be trained to discriminate between figures seen that differ only in orientation. Thus a rectangle shown with its long axis vertical can readily be distinguished from an exactly similar rectangle shown horizontally (Boycott & Young, 1957; Sutherland, 1957). A wide variety of cues are potentially available to octopuses in the training situation used; the figures might, for example, be recognized as distinct because they are parallel or not parallel with the water surface, with the sides of the aquaria, or with the direction of illumination. Since the figures are moved up and down on the ends of rods inserted into the tank, they could also be distinguished as moving along or at right angles to their long axes (see Sutherland & Muntz, 1959) or as parallel or not parallel with the supporting rod. An alternative set of cues is potentially available from within the animal itself, in the form of proprioceptive information about the position of the animal in space, originating in stretch or pressure receptors and/or in the statocysts. It will be shown below that octopuses cannot reliably discriminate between figures differing only in orientation after removal of the statocysts, and that correct performance in such discriminations depends upon correct orientation of the retina, and not upon central integration of visual and positional information.