Correspondence between William Grey Walter, of the Burden Neurological Institute, and and Flight Lieutenant Reginald ('Reggie') G. Bickford, of the Electroencephalography Department at the Military Hospital (Head Injuries), St Hugh's College, Oxford, and later, the Mayo Clinic, Rochester, MN. Discussions include the development of the frequency analyser and E.E.G. research conducted at the Mayo Clinic, including work on sub-cortical electrodes and the flicker effect.