We used three consecutive operant conditioning tasks to determine whether the tropical octopus Abdopus aculeatus is able to learn to recognize a symbolic object, in either real or virtual forms. In Experiment 1, we examined whether octopuses can be conditioned to a real object (a white ball) and whether such trained individuals can select the conditioned object when they are presented with an unconditioned object. We show that octopuses learned to respond to and select the conditioned white ball in preference to the unconditioned object. In Experiment 2, we examined whether octopuses can be conditioned to an object that gradually changes from real to virtual (i.e., an image of that object on a computer screen). We presented four types of objects, all variations of a white ball, in a stepwise sequence as a conditioned stimulus: a real white ball, a real image of a white ball without a margin, a real image of a white ball centered within a black margin, and a virtual image of a white ball (a video on a computer screen). Individual octopuses learned to respond to all three real objects, and then a subset of these octopuses responded to the virtual object. In Experiment 3, we examined whether an octopus can learn a virtual image of an object with a specific shape not tested in Experiments 1 and 2. We presented octopuses with an image of a white cross, which was placed at various distances (i.e., close, medium, and far). We found that after having learned these images, octopuses could learn the virtual white cross on a computer screen. Furthermore, when we simultaneously presented octopuses with a conditioned virtual object and an unconditioned virtual object, they selected the former. Through these three experiments, we confirmed that A. aculeatus can learn both real and virtual specific objects.

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Octopuses, a member of molluscan class, posse well-developed nervous system such as lens eyes that is anatomically similar to our own, and the relative proportion of brain to body is equivalent to vertebrates. Octopuses are also characterized with their muscular highly sensitive sensory receptors, namely, suckers on their arms. Due to these biological uniqueness, octopuses have been a target for psychological studies and were reported their advanced abilities for learning and memory, which are achieved via visual and tactile perceptions. All of these findings have come from experiments that tested single sensory perception (i.e., visual or tactile). On the other hand, it is known in vertebrates that they can integrate multiple sensory information, by which they can vividly image their environments. In this review, we will briefly introduce our knowledge for biology of octopuses with special reference to their cognition, and we will shed light on an idea for cross-modal perception in octopuses, which is based on our ongoing projects for visual and tactile learning, and manipulation of arms in tropical octopuses inhabiting the coastal waters of the Ryukyu Archipelago.