Comparison of imaging techniques

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

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

Superconductivity Meissner effect Josephson tunneling Cooper pair

Superconducting Quantum Interference Devices Superconductor Current loop Two Josephson Junctions

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

Magnetic field distribution over the sensors

Epilepsy Diagnosis Epileptic seizure scan data and postprocessing Intracranial EEG

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.

Tracking the time course of word recognition with MEG 150-200ms M170 200-300ms M250 300-400ms M350 400-500ms BEHAVIORAL RESPONSE (lexical decision) 200 [fT] 200 -100 0 100 200 300 400 500 600 700 [msec] Pylkkänen &. Marantz, Trends in Cognitive Sciences , 2003.

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

M250 field pattern a lot of variance

M350 field pattern Stable

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'.

Mirror Neurons Performance Observation

Signal and noise sources in biomagnetic measurements

Inverse problem ?