for National Geographic News
An octopus may get some mileage out of the excuse "I can't help it, my arm has a mind of its own," as it goes for an extra sea morsel—at least more than can a human who reaches too often into the cookie jar. Neither, however, can lay full blame for their greed on their appendages.
For humans, the brain inside the human skull, the same brain that sees the cookie and wants to eat it, controls the reach into the cookie jar. Octopus arms, on the other hand, really do have a mind of their own, according to research reported in the September 7 issue of Science.
The brain inside the octopus skull sees a tasty sea morsel and decides to eat it, but to get the morsel into its mouth the brain inside the skull sends a message to a mass of nerves inside the octopus arm. That mass of nerves controls the arm movement to snatch the tasty treat.
"In this hierarchical organization, the brain only has to send a command to the arm to do the action—the entire recipe of how to do it is embedded in the arm itself," said Binyamin Hochner of the Institute of Life Sciences at Hebrew University in Jerusalem, Israel, and co-author of the research.
Controlling Freedom
Octopus arms, unlike human arms, are not limited in their range of motion by elbow, wrist, and shoulder joints. To accomplish goals such as reaching for a meal or swimming, however, an octopus must be able to control its eight appendages.
Trying to work out how octopuses control their flexible arms is the goal of Hochner and his colleagues' research at Hebrew University.
The researchers' observations indicate that octopuses reduce the complexity of controlling their arms by keeping their arm movements to set, stereotypical patterns. For example, the reaching movement always consists of a bend that propagates along the arm toward the tip, said Hochner.
Since octopuses always use the same kind of movement to extend their arms, Hochner and his colleagues wondered if the commands that generate the pattern are stored in the arm itself, not in the central brain. Such a mechanism would further reduce the complexity of controlling a flexible arm.
Severed Brain and Functioning Arm
To find out if octopus arms have minds of their own, the researchers cut off the nerves in an octopus arm from the other nerves in its body, including the brain. They then tickled and stimulated the skin on the arm. The arm behaved in an identical fashion to what it would in a healthy octopus.
The implication is that the brain only has to send a single move command to the arm, and the arm will do the rest.
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