Two thirds of octopi neurons lie outside the head and instead in the tentacles. The theory is that the cognitive load away from the head allows for and is a product of eight highly flexible and intricate limbs.
But how much intelligence is delocalised away from the head?
A resent study investigated the behaviour of severed octopi limbs and found that these limbs remain responsive to perceived danger long after being severed and the octopus owner has been euthanised. I feel I must address the ethics of this study and say that the animals were used in other unrelated previous studies and would have been euthanised regardless.
“After the animals were euthanized, their arms were removed and kept in chilled seawater for up to an
hour until they were ready for experimentation. Some arms were suspended vertically, and others were laid out horizontally. When pinched, suspended arms recoiled from the unpleasant stimulus by shortening and curling in a corkscrew shape within one second. (After this, the arms slowly relaxed and returned to their previous length.) Tap water and acid applied to the arms evoked a similar response. Horizontal arms also moved away from the undesirable stimuli, many bending in a sort of contrived joint toward the top.
The authors of this study believe this is due to the tentacles containing nociceptors, neurons that respond to danger. In humans and most other vertebrates these cells are controlled by the spinal column, and a severed limb will not respond to dangerous stimuli.
Some experts are saying this is evidence of ‘distributed intelligence’, a theory that some animals with a large proportion of neurons not located in their head have thought processes distributed throughout the body. This theory is not limited to invertebrates, and other conglomerations of neurons have been postulated as ‘second brains’. This has been postulated to have occurred in some sauropod species (from fossil evidence) and even a collection of neurons in the human stomach has been called a ‘second brain’.
I guess it’s all still at the theory stage still, albeit with some great evidence. One thing is for sure though, there is an increasing grey area when dealing with grey matter.