In the 1940s, Wilder Penfield was working with epileptic patients, treating their epilepsy by excising portions of brain tissue where the seizures started. To target his surgery better, he stimulated the patients' brains while they were conscious, and observed what body parts moved, or where the patients felt a sensation. Based on his experience, he drew an image that would become famous: a little man, grossly distorted, with huge hands and lips — called a homunculus — that mapped which brain regions affected which body regions. Areas close to each other in the brain tended to relate to regions close to each other on the body, roughly preserving the same spatial organization.
This effect is called somatotopy, the correspondence between areas on the body and areas in the brain. While the homunculus that Penfield drew has proved to be an oversimplification, somatotopy seems to be a common organizing principle in the brains of both vertebrates and some invertebrates.
In a recent paper in Current Biology, Letizia Zullo and her colleagues looked for the same sort of organization in the octopus's brain, a ganglion called the supraesophageal mass. Octopi pose a particularly interesting problem for motor control. In contrast to human limbs, which can only bend at joints, octopi can move their arms in an almost unlimited number of ways. How is their nervous system organized to handle this bewildering complexity?
Zullo used microelecrodes to stimulate small areas in the higher motor centers of the octopus brain — not motor neurons that would stimulate muscles directly, but higher areas that control complex motions. They found that stimulation produced a variety of complex but discrete behaviors, including arm extension, crawling, and jetting behaviors. At low voltage, the stimulation produced relatively simple components of these behaviors; at higher voltages, these simple components were combined to produce more complex movements.
But even the simplest motions involved multiple arms or body parts. None of the stimulation sites produced movement in a single arm. Not only that, the researchers also found relatively little spatial organization to the behaviors themselves; the stimulation sites that produced the behaviors, while consistent, were often distributed throughout the brain.
In this sense, the supraesophageal mass of octopus seems to be similar to the integrative areas in the parietal cortex in vertebrates, in which microstimulation produces multijoint movements. But in vertebrates, the parietal cortex connects to the motor cortex, which does have a somatotopic representation — Penfield's homunculus. Octopi, in contrast, seem to lack somatotopy even in regions that control motor output fairly directly.
Zullo and her colleagues hypothesize that the missing ‘octopunculus’ might be related to the octopus's unique body plan, with eight long flexible arms that must be highly coordinated. Even more than other animals, the octopus must integrate its behavior across multiple limbs and senses. Perhaps its unusually integrated nervous system evolved to help it coordinate the practically unlimited number of ways it can move its body.
