Q: What is functional MR imaging (fMRI) of the brain?
A: MRI is a technique for viewing the brain's structure and functions. Two main forms exist: structural MRI provide detailed pictures of the brain's shape and size. Functional MRI allows researchers to visualize and map the parts of the brain used to perform everyday tasks, such as reading and calculation. Both structural and functional MRI are used for our studies. The MRI machine is, in essence, a big magnet. As you lie in its magnetic field, invisible radio waves are released around you. This will result in harmless radio waves bouncing off the different substances that make up your brain. These radio waves are then detected by a computer, which transforms the data into images of the brain's structure and activity.
Q: Is MRI safe?
A: MRI is a valuable tool used in research and clinical environments with infants, children, and adults. The static magnetic fields used in MRI have no known long-term adverse effects on human or animal tissue. Unlike X-rays or CT scans, MRI does not use ionizing radiation. The MR imaging we conduct for research purposes does not involve any injections.
The major risk of the MR environment is the strong pull of the magnet on any ferromagnetic object. We take every precaution to keep such objects out of the scanner room. All participants, and family members who accompany them, fill out a detailed screening form. Individuals who have metal in their bodies (pacemakers, for example) are ineligible for MRI studies. Jewelry, piercings, and other metal objects must be removed before entering the scanner room.
Q: How does fMRI work?
A: Magnetic resonance imaging (MRI) generates cross-sectional images of the human body by using nuclear magnetic resonance (NMR). The process begins with positioning the imaged body in a strong, uniform magnetic field, which polarizes the nuclear magnetic moments of water protons by forcing their spins into one of two possible orientations. Then an appropriately polarized radio-frequency field, applied at resonant frequency, forces spin transitions between orientations. Those transitions create a signal (which is an NMR phenomenon) that can be detected by a receiving coil. The MRI scanner applies the radio-frequency field as finely crafted pulses, which excite only protons whose resonant frequencies fall within a fairly narrow range. Applying magnetic-field gradients during the radio-frequency pulse creates resonant conditions for only the protons that are located in a thin, predetermined slice of the body. Orientation and thickness of this slice can be selected arbitrarily in the imaged body. The NMR signal encodes positional information across the slice by using a method known as the "spin warp,'' and a two-dimensional Fourier Transform extracts that positional information. The process creates a data matrix in which each element represents an NMR signal from a single, localized volume element, or voxel, within the imaged slice. A two-dimensional display of this matrix's contents creates a human-readable image of the selected slice. Each image element, or pixel, represents the NMR signal strength that was recorded for its corresponding voxel. The MRI image provides unmatched soft-tissue contrast. When compared with other medical-imaging techniques, MRI provides several significant advantages: noninvasiveness, safety (because it uses non-ionizing radiation), and superb soft-tissue contrast, generated by an NMR signal's sensitivity to tissue morphology and pathology.
Q: What is electroencephalography (EEG)?
A: EEG is a technique for recording the electrical activity of the brain. Your brain cells, or neurons, send signals in the brain via electrical impulses. There are big groups of neurons in the brain, and EEG sensors are able to pick up the electrical impulses of these groups of neurons on the surface of your head. EEG is a valuable technique because it gives researchers temporal resolution at a millisecond level, allowing us to understand WHEN brain activity changes more accurately than MRI.
Q: Is EEG safe?
A: Yes! This technique is noninvasive and has minimal risks. There is no danger of electric shock. You sit in a quiet room while wearing a swimming cap-like hat which has sensors attached to it. These sensors pick up the electrical activity generated by your brain. We apply a salt and water-based gel on your scalp in order to create a good connection from the EEG sensors, so your hair may be a little messy after an experiment, but the gel washes out easily in the shower. We also provide you with a towel to clean up your hair before leaving our lab.
Unlike MRI, having metal items such as piercings, braces and other dental devices, etc., does not preclude you from participating in an EEG study.
Q: How does EEG work?
A: In order for neurons to send information to each other, they must be able to transmit signals within themselves and then to adjacent neurons. To do this, neurons utilize ions (electrically charged particles). As ions move in and out of the neurons, this flow of charged particles generates an electrical signal that can either directly affect nearby neurons or trigger chemical changes that then affect neurons. The electrical activity for a single neuron may be very small, but many areas of the brain are organized such that large groups of neurons are spatially oriented in the same way, and when these neurons ‘fire’ or send electrical signals in a synchronous manner, we are able to measure these changes on the surface of the head.
The electrical activity in your brain is ongoing, even when you are at rest. EEG can be used in a clinical setting to monitor sleep, depth of anesthesia, and to identify epileptic seizures (caused by abnormal patterns of electrical activity in the brain).
In research, EEG is most commonly used to study event-related potentials (ERPs). ERPs are EEG signals that are time-locked to a particular stimulus event. After enough stimuli have been presented, it is possible to average the EEG signal taken from a time window after that event. In principle, the averaging will reduce any random noise, and leave only the electrical activity related to a given stimulus or event, hence the term, ‘event-related potential’. Researchers can use ERPs to look at the effects of different types of stimuli and even differences in electrical activity between healthy and clinical populations.