14 DAY TRIAL // The octopus is a surprising animal in many ways - it is an invertebrate so you would think it's rather primitive, but in fact it is quite intelligent. Its brain is involved in one of the most developed vision system in the aquatic world and in case of some species it controls the animal's color texture allowing it to mimic with incredible accuracy things like the rocks on the bottom of the ocean. But, perhaps most importantly, the brain controls its eight highly flexible tentacles equipped with suckers on their entire length.
Due to the fact that no bones are involved the octopus has an almost infinite flexibility. But high flexibility also means that a large "computing power" is required to control the motion. Now a team of researchers has figured out how octopuses go easy on their brains.
To study how the creatures perform their movements, a team led by neurobiologist Binyamin Hochner of the Hebrew University in Jerusalem filmed them picking up scraps of food floating in their tanks. The team placed electrodes at different locations along the octopuses' arms to measure the neural activity along the tentacle.
When performing point-to-point movements, such as taking a piece of food from the ocean floor to their mouth, the octopus adopts a strikingly humanlike approach: it forms three bends in their tentacles, which act as joints. The tentacle that picks up the food looks like an arm: one joint forms in the middle, the "elbow", one at the base, the "shoulder", and one near the tip, the "wrist". Scientists think octopuses do this precisely to avoid overwhelming their nervous systems, which would have trouble coordinating a large number of bends and rotations.
By measuring the neuronal activity along the tentacles, researchers found out how the joints are formed. They found that, when the octopus reaches out, two signals travel along the arm: one from the base of the tentacle and one from the sucker that took the food, near the tip of the tentacle (although the octopus's suckers allow it to hold an object anywhere along its arm). The two impulses propagate toward each other and the elbow forms where they meet, near the middle of the tentacle. Then, the "wrist" forms behind where the suckers grasp the food, and the "shoulder" forms at the arm's base.
Most of the motion is directed by the "shoulder". The "elbow" is used to bring the food closer to the mouth and a final rotation around the "wrist" places the food inside the mouth.
According to Hochner, "because the octopus nervous system only has to compute the movements of three joints, the problem is much simpler than it would be if the creature actually harnessed its unlimited flexibility". Hochner's team goal is to use this knowledge in order to device a highly flexible robotic arm.
This research is also interesting from an evolutionary perspective, as it shows why the vertebrate arms have formed with three joints - and not with two or four. It was a compromise between maximizing mobility and minimizing the motor-control complexity. As the octopus shows, even when the animal has the actual possibility of flexing its arm in as many places as it wants it only resorts to three joints. (Recent research has shown how a genetic mutation can transform muscle tissue into bone tissue - when this happens in humans, fortunately only very rarely, a large number of the person's muscles turn into a "second skeleton" preventing any movement.)
Photo credit: Binyamin Hochner
