1. Comparison of imaging techniques

2. Magnetoencephalography (MEG)
Passive measurement of minute current dipoles and corresponding magnetic moments
Magnetic field generated by neurons on the order of tens of femtoTeslas
High resolution in both space (2 - 3mm) and time (<1ms)
Patient wears a helmet containing an array of 100+ sensitive magnetic field measurement devices
Measurement devices are called SQUIDs – Superconducting Quantum Interference Devices
Measurements must occur in costly magnetically shielded room

3. Origin of MEG signals Synapse Depolarization
Extracellular (volume) currents
Intracellular currents
Depolarization

5. Superconductivity
Meissner effect
Josephson tunneling
Cooper pair

6. Superconducting Quantum Interference Devices
Superconductor
Current loop
Two Josephson Junctions

7. Apparatus
Prototype noise reducing helmet by Los Alamos National Labs
Lead shell must be kept below 8 Kelvin for superconducting properties
Surface Meissner currents expel external magnetic fields, reducing noise by six orders of magnitude
Screen is used for patient stimulation for functional mapping

8. Magnetic field distribution over the sensors

9. Epilepsy Diagnosis Epileptic seizure scan data and postprocessing
Intracranial EEG

10. Functional Imaging
Functional Imaging utilizes the high temporal resolution to generate real time brain scans
Doctors can use these scans to determine how the brain reacts to various stimuli
Multiple scans allow a brain map to be built, providing a base to begin research linking neural activity to specific classes of stimuli
VSM's DataEditor: An averaged somato-sensory evoked response from a tactile stimulation of the second right digit.
VSM's MRIViewer: Dipole fit results for this scan.

11. Tracking the time course of word recognition with MEG
ms M170
ms M250
ms M350 ms
BEHAVIORAL RESPONSE (lexical decision)
200
[fT]
200
[msec]
Pylkkänen &. Marantz, Trends in Cognitive Sciences , 2003.

12. M170 field pattern
source is bilateral! - depending on the individual subject and your instrument, may not be localizable

13. M250 field pattern
a lot of variance

14. M350 field pattern
Stable

15. Mirror Neurons
Mirror neurons are a type of brain cell that respond equally when we perform an action and when we witness someone else perform the same action.
First discovered in the early 1990s, when a team of Italian researchers found individual neurons in the brains of macaque monkeys that fired both when the monkeys grabbed an object and also when the monkeys watched another primate grab the same object.
We understand actions when we map the visual representation of the observed action onto our motor representation of the same action. An action is understood when its observation causes the motor system of the observer to 'resonate'.

16. Mirror Neurons
Performance
Observation

17. Signal and noise sources in biomagnetic measurements

18. Inverse problem ?