I need to research this claim more, but interesting stuff.

The brain gets even wilder when we zoom in. Since electrochemical thresholds activate neurons, a single proton could, in principle, be the difference that causes a neuron to fire. If a proton spontaneously jumped out of its atomic bonds, in what physicists call “quantum tunneling,” this could cause a cascade of sudden neuron activity. So even at the tiniest measurable level, the neuron’s physical structure has a non-binary indeterminacy.

The problem is that proton tunneling is much rarer than electron tunneling since the proton is 2000 times heavier than the electron. But if it happens at all in the brain, this means that you have true non-deterministic thoughts sometimes, because proton tunneling is by definition non-deterministic.