Figure 1. Magnetic field strength versus frequency, for magnetic fields generated by cortical activity and noise sources.

(a) (b)

Figure 2. Flux transformers used to transport flux to the SQUID loop. (a) shows a simple one coil flux transformer, (b) a first order gradiometer.

Figure 3. An MEG measurement using the Aston 19 channel magnetometer. The subject is looking at a visual stimulus projected into the shielded room using a system of mirrors. In this case the liquid helium dewar containing the measurement array is placed over the back of the head, to measure the magnetic field from the occipital cortex.

Figure 4. (a) Typical MEG signals. Each box shows the magnetic field strength recorded in a single channel over a 512 millisecond epoch. The stimulus in this example was a moving pattern, designed to activate an area of the brain implicated in movement perception. The latency of peak magnetic field is 190 milliseconds after stimulus presentation, and the peak field was around 400fT.(b) Contour map of the magnetic field at 190ms for a movement stimulus. The map shows the magnetic field, constructed over the bottom of the dewar. Red/Orange denotes positive field (i.e. field out of the head), blue/purple denotes negative field. The dipolar nature of the map is consistent with the generating current source being a single current dipole

Figure 5. Current dipole fits for 190ms after a movement stimulus, superimposed on MRI representations. In all, the dipole is a red/white square box. In (a) the dipole is shown on slices taken through the head. The sagittal slice is a vertical slice through the head, viewed from the right. The axial slice is a horizontal slice viewed from above the head. In (b) the dipole is shown pasted on to a reconstruction of the cortical surface, viewed from the right.