  It is known that bipolar disorder and its lithium treatment involve the
modulation of oxidative stress. Moreover, it has been observed that lithium's
effects are isotope-dependent. Based on these findings, here we propose that
lithium exerts its effects by influencing the recombination dynamics of a
naturally occurring radical pair involving oxygen. We develop a simple model
inspired by the radical-pair mechanism in cryptochrome in the context of avian
magnetoreception and xenon-induced anesthesia. Our model reproduces the
observed isotopic dependence in the lithium treatment of hyperactivity in rats.
It predicts a magnetic-field dependence of the effectiveness of lithium, which
provides one potential experimental test of our hypothesis. Our findings show
that Nature might harness quantum entanglement for the brain's cognitive
processes.
