Anotomical images of the brain
You are all familiar with CT, MRI, and other devices that radiologists use to take pictures of the structure or anatomy of the brain.There are some relatively new techniques that are now being used to get pictures of brain activity in addition to brain anatomy.
fMRI facility and fMRI video
Functional magnetic resonance imaging (fMRI) is a novel, non-invasive method for measuring activity in the human brain, and for investigating the relationship between brain activity and behavior. The technique is similar to conventional MRI, which is used to acquire structural images of the brain, but can generate images of brain activity in addition to brain anatomy. Works by measuring blood flow. The brain controls the flow of blood to brain regions where there is more neural activity. Oxy- and dexy-hemoglobin have different magnetic properties, and this can be picked up by the MR scanner.
In fact, we've known about the control of blood flow in the brain for many years...
Mosso's recording device
Angelo Mosso, late 19th century physiologist fascinated with pulsations in the brain (e.g., easily observable in newborn infants) that keep pace with the heartbeat. He worked with patients who had suffered head injuries that left them with permanent defects in the skull over the frontal lobes of the brain. Noticed that pulsations increased in magnitude during periods of mental activity.
Mosso's measurements
Mosso's recordings taken of the forearm (A) and the head (C) show stronger brain pulsations after the events marked by the arrows have stimulated brain activity.
Relationship between brain function and blood flow was further characterized in 1890 by Roy and Sherrington (Cambridge University). Based on experiments with animals, they suggested that there exists and automatic mechanism that regulates the blood supply in response to brain activity.
Walter K recordings
Extraordinary example: patient Walter K, observed in 1920's. Congenitally abnormal blood vessels serving his visual cortex, turbulent flow, created a brief rushing sound with each heartbeat. Sound was audible to the patient and to his physicians. Dr. John Fulton (famous neurosurgeon) recorded these sounds. The patient would open his eyes and start to read a newspaper. A few seconds later there was a noticeable increase in the intensity of the sound. Recordings taken of Walter K's skull show little activity while his eyes are closed (top line), but much more activity while he is reading (bottom 3 lines).
Optical imaging apparatus
Recently, blood flow response has been measured carefully in animal studies. Open hole in skull, point video camera at it, collect images that reflect the relative amount of oxygenated ("red") versus deoxygenated ("blue") blood.
Illustration of blood flow response
There is actually an overcompensation in the blood flow response. When the brain is active, oversupply of oxygenated blood is delivered. Can use that blood flow response as an indirect measure of the underlying neural activity.
PET scanner
PET = positron emission tomography. Patient lies in the scanner after being injected with radioactive-labeled water. Because more blood goes to active brain regions, there is more radioactivity in those brain regions. The PET scanner detects and localizes these pockets of higher radioactivity.
Illustration of PET detectors
Radioactity releases energy in the form of two photons (particles of light) that leave the in exactly opposite directions. The PET scanner is set up to detect the coincidental arrival of pairs of photons. The location of the radioactivity is determined by which pair of detectors are simultaneously active.
Subtraction methodology
Test condition: flickering visual stimulus. Control condition: blank screen. Subtract the two PET images. Gives a picture of where in the brain there was a greater response to the test condtion.
Often, PET studies average results across several/many subjects. This is a problem because individual human brains can be quite different from one another. Would like to repeat measurement in same brain, and average to reduce noise. Can't do that because of too much exposure to radioactivity.
Comparing PET and fMRI: Both fMRI and PET depend on regional control of blood flow in the brain. But current fMRI methods go way beyond the conventional PET subtraction methodology. Critically, fMRI is non-invasive - no radioactive stuff injected in your blood stream. So one can measure fMRI in the same subject as many times as you like. This is important when doing science: repeatability of the measurements, averaging out the noise in the measurements, can do proper control expts. But mainly, important for experimental design. Redo a study a slightly different way based on some preliminary results. This is really the way science is done. Scientists don't think up "the right'' expt and go into work the next day and do it. Rather, they have a concept for what experiment they want to do. They try it. It typically doesn't work so well. Perhaps the data is too noisy (or not repeatable) to be conclusive. Perhaps they realize that there is a critical confound. Perhaps decide to use a different stimulus, etc. Then try it again.
Spatial and temporal resolution of fMRI is limited by blood flow. FMRI response depends on average activity of neurons in a little chunk of brain, perhaps a couple mm on a side (recall that there are about 50,000 neurons per cubic mm), and averaged over time. Temporal resolution is better with fMRI than PET because you get a single PET image from an entire (e.g., 30 sec) scan whereas with fMRI you collect a time-series of images. Spatial resolution with fMRI is typically better than PET, because we can do experiments on individual subjects. Averaging data from PET experiments across subjects tends to blur the results because there are individual differences in the shapes of our brains.
EEG sensors
EEG (electroencephalograph) is also being used to study brain function. At each electrode, the electrical activity is recorded at fixed intervals following a stimulus presentation - say, every 4 milliseconds. The electrical values at each interval are taken from many trials and averaged together so that electrical activity not caused by the stimulus averages to zero and the resultant signal shows only the activity produced by the stimulus.
ERP data
The result is called an event-related potentials (ERP). Usually 10 to 100 presentations of the stimulus suffice to produce a reliable measurement that reflects characteristics of both the individual brain and the particular stimulus.
Main advantage over PET and fMRI is temporal resolution. With PET and fMRI, you get one image of activity for several seconds of stimulus presentation. With EEG, you get millisecond time resolution. One HUGE disadvantage is localization. Don't really know exactly where these electrical signals are coming from.