Octopus: the animal that keeps itself company

November 8th, 2022

Written by: Omer Zeliger

            In Children of Ruin by Adrian Tchaikovsky, octopuses have evolved to be as intelligent as humans¹. Like us, they can speak with each other and build machines. These octopuses are not human, though; they have a unique way of thinking that shapes their behavior. Each of an octopus’s arms has its own brain, a “submind”, that works separately from the main brain to solve whatever problem the brain assigns it. The book features one octopus whose arms, but not herself, are expert spaceship pilots. When she thinks “turn left”, she only has to send a single signal to her arms. Her arms then figure out which buttons to press and which levers to pull to make that happen, leaving her brain free to figure out where to fly next. As the octopuses in the book say, the brain proposes, the arms dispose.

            Real-life octopuses are wonderfully intelligent animals, capable of solving complex puzzles² and known as masters of disguise³. They, along with their cuttlefish cousins, are remarkably intelligent pranksters⁴ despite their brain structure being radically different from a human’s. The most striking of these differences is how their nervous systems are distributed throughout their whole bodies. Instead of being clustered in the head, the majority of an octopus’s neurons are spread out between its arms⁵. Like in Children of Ruin, this has led to speculation that octopuses’ arms are capable of independent decision making separate from the central brain. Even though octopuses have yet to achieve space travel, research shows this science fiction contains some real science fact.

Octopus Brain Anatomy

            To get an idea of why scientists think octopus arms work independently, let’s take a look at the unique organization of the octopus nervous system. The central octopus brain (figure 1, pink) rests in the middle of the body. Two optic lobes (figure 1, purple), specializing in processing visual information from the eyes, connect to the brain^{5, 6, 7}. These sections only make up one third of the octopus’s nervous system; the rest is spread out between the octopus’s eight arms⁵. Each arm has a thick nerve bundle running along its entire length (figure 1, orange). These bundles are connected to the other arms through thick connections, forming a nerve “ring” encircling the central brain⁷. On the other hand, each arm connects to the brain only through a thin nerve fiber⁵. This paints a picture of arms that communicate plenty of information to each other but rarely speak to the brain.

            Notably, an octopus arm’s robust nervous system is able to generate complex movements on its own without involvement from the central brain. If octopuses were like humans, then stimulating nerve fibers in the arm should cause simple movements like finger twitches⁸. Instead, stimulating nerve fibers in an octopus arm separated from the brain causes the arm to produce complex, complete motions like extending an arm and reaching for something⁹. Scientists call these types of behaviors stereotypic movements, and they are often useful to an animal’s daily life and done the same way by all animals of a species¹⁰. Stereotypic movements are seen in many species, from mating in mice¹¹, to feeding in flies¹⁰. More complex stereotypic movements are typically controlled by the brain, making the octopus’s arms’ ability to generate these movements unique.

            On the other hand, the central octopus brain doesn’t directly control individual muscles in the body¹². This is in stark contrast to the human brain, which has distinct areas dedicated to precise control of individual muscles¹³. You may have seen an image of a “homunculus” (Figure 2), a map of the brain showing which brain region controls which part of the body. Electrically stimulating one of these areas in a human causes a small twitch in that body part¹³. This level of specificity doesn’t seem to exist in the central octopus brain. Instead, stimulating muscle control areas in the central octopus brain leads to a variety of stereotypic movements involving multiple different arms¹². One single stimulation can cause the octopus to crawl along the floor using many of its arms. Another stimulation might make the octopus extend one arm, which involves precisely-timed muscle contractions and relaxations along the arm’s whole length. The stimulations did induce some simpler movements but they were mostly confined to the central body, like eye twitches and head rotations.

Arm Independence

            Does this mean that octopus arms really work independently from the central brain? The anatomical evidence suggests that octopuses have offloaded the parts of their brains that create stereotypic movements to their arms, giving the arms a certain amount of freedom. The brain tells the arms what to do, and the arms figure out how to do it. Currently, scientists believe that the brain passes on three pieces of information to the arms: what motion to do, where to aim it, and how fast to do it⁵. If this is the extent of their ability, then octopus arms are more like remote control cars than fully-functional independent brains. The central brain just needs to press a button to make the car move, and the car itself figures out how to turn on the motor to make that happen. Though impressive, this is a far cry from our fictional space-faring octopus. To live up to her example, a real octopus’s arms would have to be capable of learning difficult tasks and reacting to information from the environment.

            The jury’s still out on whether octopus arms can learn to solve new problems, and new research is held back by the octopus’s own biology. First and foremost, octopuses don’t have eyes on their arms. The octopus’s visual system connects directly to the central brain, so to access visual information the arms must consult the brain. Unsurprisingly, octopuses use their brains when performing visual tasks¹⁴. Any research that wants to investigate whether arms can make decisions independently from the brain cannot require vision. Luckily the arms can both feel⁷ and taste¹⁵, which scientists use to design experiments that only need senses already available to the arms.

            With this in mind, one team of researchers developed a maze where an octopus can find a piece of food by reaching a single arm in and following the texture of the maze walls¹⁶. They aimed to prove that octopus arms could learn to associate texture with food by training some arms, but not others, to solve the maze. If the untrained arms failed the maze, that would show that learning can happen in a single arm without involving other areas. The scientists found the opposite: even untrained arms could solve the maze, meaning that the knowledge was shared between different areas of the nervous system. The experiment failed to prove that arms can learn independently from the brain. However, it doesn’t quite prove that they can’t, either.

            Here the experiment ran into another hurdle of octopus biology. We know that octopus arms can communicate with each other through connections that bypass the brain (see figure 1, orange). The research doesn’t rule out that the arms communicated the maze’s solution to each other through these connections. To test for certain whether arms have the ability to learn, scientists would have to completely isolate an arm from the brain. They could then run similar experiments asking whether arms can associate texture with food. This style of experiment could test other types of learning as well, such as associating a texture with danger and avoiding it, or learning a series of motions it can perform to get delicious food as a reward.

The Perks of Arm Independence

            It’s clear that octopus arms won’t be flying spaceships any time soon. Regardless, the science shows that octopus arms have an unusual amount of independence, especially in generating stereotypic movements. This raises the question, what purpose does this kind of independence serve? What would drive the octopus to evolve such a complex nervous system? While we don’t know for sure, scientists believe that two factors may be responsible: processing power⁶ and reaction speed¹⁷.

            Look back at the homunculus and pay attention to which body parts are biggest. The tongue takes up a much larger chunk than you would expect just based on its size. Your tongue has to be able to move nimbly to let you talk and chew, and can move in almost infinite combinations since it has no bones to limit its movement. It’s no wonder the brain needs extra space to control it. Octopus arms work on the same principle taken up a notch; they’re longer than a human tongue, have to be nimble enough to grab things, and have dozens of suckers to control^{5, 6, 7}. Octopuses need to wrangle eight of them. Rather than waste valuable real estate in the brain on its arms, the octopus transfers precise muscle control from the central brain to the arms. As a result, the central brain can dedicate more brainpower to other vital processes, like vision and learning⁶. Meanwhile, the arms’ nervous systems can take as much space as they need without crowding the central brain⁷.

            The other major benefit is speed. It takes time for information to travel along neurons, so having the arm’s control center in the arm itself gives the octopus an advantage in reacting to new information as quickly as possible¹⁷. If an animal wants to react to a danger in the environment as quickly as possible, it can use reflexes to react without consulting its brain^{18, 19}. However, reflexes are simple and always react the same way, by pulling a limb back whenever it feels pain. For more complicated decisions, other animals would have to send the information back to the brain for processing, wasting precious time. Octopuses have sophisticated processing centers in their arms already, eliminating the need to send that information all the way to the brain before reacting.

            We as humans could learn some lessons from the octopus. Roboticists are looking at how the separation between general commands from the brain and precise control by the arms can improve computer programs²⁰. They believe that building programs from multiple components that each have a specialized role will let each part focus on doing its job the best it can. In octopuses, the brain focuses on deciding what to do and the arms specialize in figuring out how to do it. The communication between the octopus’s eight arms is also of interest, since figuring out how semi-independent arms coordinate with each other is very relevant to designing “flocks” of smaller robots that work together to achieve a shared goal¹⁷. We’ll certainly take inspiration from these fascinatingly-designed creatures to advance robotics in the future. And to any space capable octopuses out there reading this – you’re super cool, and the parts about cutting off octopus arms were definitely just jokes.

References

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Cover photo by ErikTanghe via Pixabay.