Presentation on theme: "EEG The electroencephalogram (EEG) measures the activity of large numbers (populations) of neurons. First recorded by Hans Berger in 1929. EEG recordings."— Presentation transcript:
EEG The electroencephalogram (EEG) measures the activity of large numbers (populations) of neurons. First recorded by Hans Berger in 1929. EEG recordings are noninvasive, painless, do not interfere much with a human subject’s ability to move or perceive stimuli, are relatively low-cost. Electrodes measure voltage-differences at the scalp in the microvolt (μV) range. Voltage-traces are recorded with millisecond resolution – great advantage over brain imaging (fMRI or PET).
EEG Standard placements of electrodes on the human scalp: A, auricle; C, central; F, frontal; Fp, frontal pole; O, occipital; P, parietal; T, temporal.
MEG Three task conditions: MEG results 1 -- Listening to tones that were delivered with a delay of about 5s. A random time was added to prevent stimulus prediction. The signal is an average over about 80 stimulus presentations. 2 -- Reacting to acoustic stimuli. The same stimulus presentation as in (1) but now the subject was told to press on an air cushion as soon as possible after the tone was heard. 3 -- Synchronizing with a rhythm. Here the tones were presented regularly with a frequency of 1 Hz. The subject was told to press the air cushion in synchrony with the stimulus. 123 Viktor Jirsa (FAU): http://www.ccs.fau.edu/~jirsa/Imaging.html
Cat Scans use x-rays to show structures Really precise maps Hard to determine functions
PET Positron Emission Tomography Requires the injection of a positron-emitting radioactive isotope (tracer) Examples: C-11 Glucose analogs (metabolism) O-15 water (blood flow or volume) C-11 or O-15 carbon monoxide PET tracers must have short half-life, e.g. C-11 (20 min.), O-15 (2 min.). Cyclotron! Positron + electron 2 gamma ray beams. Gamma radiation is detected by ring of detectors, source is plotted in 2-D producing an image slice.
Sensing Techniques-PET Radioactive element decays, gives off positron Positron moves a short distance and gives off two gamma rays in opposite directions
PET - Examples M. Raichle PET images taken at different times, e.g. during learning, can be compared.
PET - Examples PET images are pretty to look at... … and can be combined with other imaging modalities, here MRI.
Functional Magnetic Resonance Imaging Typical MRI Scanner
MRI - fMRI Subjects are placed in a strong external magnetic field. Spin axes of nuclei orient within the field. External RF pulse is applied. Spin axes reorient, then relax. During relaxation time, nuclei send out pulses, which differ depending on the microenvironment (e.g. water/fat ratio). The Physics (sort of)... fMRI – functional MRI Allows fast acquisition of a complete image slice in as little as 20 ms. Several slices are acquired in rapid succession and the data is examined for statistical differences. Hemoglobin is “brighter” than deoxyhemoglobin. Oxygenated blood is “brighter” - active areas are “brighter”. BOLD-fMRI
Sensing Techniques-fMRI Functional Magnetic Nuclear Resonance Imaging Similar to Pet, uses radio frequency information given off by water Gives better time (6 seconds) and spatial (2 mm) resolution than Pet or Cat scans Technology still under revision Slight danger to subjects Expensive ($300 an hour)
PET and fMRI - Similarities and Differences - Different biological signal. Yet, both pick up a signal related to bulk metabolism (not electricity). - fMRI has better temporal (<100 ms) and spatial resolution (1 mm and less) - fMRI does not involve radioactive tracers and subjects can be measured repeatedly, over many trials. - PET images generally represent “idealized averages”. fMRI images are often registered with structural scans to show individual anatomy. - For both, images can be aligned for multiple subjects. - fMRI is widely available, PET is not. - fMRI does not allow localization of neurotransmitters or receptors etc. - For both, it can be tricky to get stimuli to the subject.
Data Analysis Issues Neuroimaging (PET/fMRI): Activation values, spatial resolution, averaging, image alignment and registration. EEG/MEG: Current source localization (inverse problem), time domain data sets, frequency power spectrum, correlation and coherency.
Summary Appropriate technology depends on question ERP has good temporal resolution CAT, Pet, MRI have good to fair spatial resolution, only PET has any functional capture fMRI has reasonably good spatial, reasonably good temporal, but is expensive