Application of a SQUID for monitoring magnetic response of the human brain

Noise characteristics are reported for a SQUID system which is sufficiently sensitive to detect visually evoked magnetic fields of the human brain without shielding. Discrimination against the much larger ambient background fields is obtained through use of a flux transporter with detection coils in the form of a second order gradiometer. The continuous spectrum of noise, coherent noise at isolated frequencies, and transient noise features are described. Examples of the spatial variation of the evoked neuromagnetic field are given with emphasis on those aspects which are important considerations in the design of effective detection systems.