Though coordinating eight separate arms might seem a tricky task for an octopus brain, what's really demanding is controlling the arms' flexible, infinitely variable movements. Now researchers have figured out part of their secret.

Unlike us, specific regions of an octopus' motor cortex don't correspond to specific parts of its body. Instead, each region controls different parts at different times. Their motor neural network seems as flexible as their bodies — a phenomenon that expands the range of neurophysiological possibility, and could refine the design of arm-flexing robots.

"We think, because of the complexity of the octopus body and its variability, that it has another way of organizing its control system. That's what we find in this study," said Benny Hochner, a Hebrew University of Jerusalem neurobiologist and author of research published Thursday in Current Biology.

"It's suited to a structure with many more degrees of freedom than our own body, which is constructed around a segmented skeletal structure with few degrees of freedom."

How octopuses control their arms has been a focus of Hochner's work for more than a decade. In earlier studies, he helped show that seemingly complex movements are actually combinations of individually simple motions. Hochner also found that many of the movements are guided peripherally, rather than by the brain, as if each arm had its own spinal cord.

An octopus brain sends a general prompt, and the arm computes the specifics: It's much simpler than running all those calculations in the brain itself. And all this is especially interesting to roboticists who want to build machines with flexible appendages, ideal for rescue bots working in disaster areas or surgical machines weaving through a body.

"The idea is to draw inspiration from biology to answer the question of how to generate movement in a flexible structure, and how to control this with the nervous system," said Hochner.

In the latest study Hochner's team ran electrical currents through wires inserted into in the animals' brains, measured the resulting movements, and then dissected the sacrificed animals to see exactly what the electrodes had stimulated.

They found yet another example of modular, highly efficient design: Each site proved capable of generating different movements, in different arms, with movements becoming more complex as the current increased. In humans, most body parts are controlled at a single, unchanging location.

"The networks are embedded in one another. The system is remodeled according to stimulation. It's more dynamic, rather than strictly organized," said Hochner.

Hochner suspects that other neurological programs, stored elsewhere in the octopuses' bodies — perhaps at the base of each arm — act as gates, blocking signals from the brain or allowing them to pass.

That possibility is especially intriguing to Cecilia Laschi, a biomedical engineer at Italy's Sant'Anna School of Advanced Studies and member of the Octopus Project, a group of researchers building octopus-inspired soft-bodied robots.

"This is very important for robotics. If you build a robot with many degrees of freedom, it becomes very difficult to control." said Laschi, who was not involved in the study. "We know that some movements are controlled peripherally, some parameters are set by the brain, and we will do the same thing in our robots."

But whereas roboticists building humanoid forms can already try to mimic the human brain's layout in their computing, Laschi said that "with the octopus, we're not at that level — yet."

Citation: "Nonsomatotopic Organization of the Higher Motor Centers in Octopus." By Letitzia Zullo, German Sumbre, Claudio Agnisola, Tamar Flash and Binyamin Hochner. Current Biology, Volume 19 Issue 18, September 17, 2009.

Image: Noel Feans/Flickr

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