Non-commutativity in the brain

In non-commutative algebra, order makes a difference to multiplication, so that a x b ≠ b x a. This feature is necessary for computing rotary motion, because order makes a difference to the combined effect of two rotations. It has therefore been proposed that there are non-commutative operators in the brain circuits that deal with rotations, including motor circuits that steer the eyes, head and limbs, and sensory circuits that handle spatial information. This idea is controversial: studies of eye and head control have revealed behaviours that are consistent with non- commutativity in the brain, but none that clearly rules out all commutative models. Here we demonstrate noncommutative computation in the vestibulo- ocular reflex. We show that subjects rotated in darkness can hold their gaze points stable in space, correctly computing different final eye-position commands when put through the same two rotations in different orders, in a way that is unattainable by any commutative system.