Donald Rumsfeld got a pretty hard time from the press after he classified information into ‘known knowns’, ‘known unknowns’ and ‘unknown unknowns’ in 2002 (figure 1). However, other commentators found merit in this pithy and useful summary of a complex idea. From the perspective of the neurologist in the clinic (and hence of Practical Neurology) we think he may have missed an important category—the unknown knowns. An unknown known is when you do not know something but others do—you have a blind spot—even worse if you do not know you do not know it. Addressing this unmet and unrecognised need is a challenge.

Magnetoencephalography is the kind of thing that most neurologists will have heard of but few will be familiar with. What does it do and how does it work? What does …