Most of an octopus thinks in its arms
About two thirds of an octopus’s neurons are in the arms, not the central brain. Each arm can organise reaching and touch locally.
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A common octopus has on the order of 500 million neurons. The central brain, between the eyes, holds a minority. The axial nerve cords in the eight arms hold the rest, plus ganglia above each sucker. An isolated arm can still reach and grasp in coordinated patterns.
The centre is not irrelevant. It decides when to send an arm into work, and it receives what the arm finds. The division is real: a great deal of the computation of shape, sucker, and bend never goes up to the head. Counting neurons is not counting thoughts. It is a map of where the tissue is.
Caveats
- The two-thirds figure is an estimate from classic counts and varies with the source and the species.
- Local control is not eight separate minds with opinions. Conflict between arms is limited by the central commands.
Sources
- 1Young J. Z.. The Anatomy of the Nervous System of Octopus vulgaris. Clarendon Press, 1971.