What is EEG? Electroencephalography (EEG) is a medical imaging technique that measures brain function by analyzing the scalp electrical activity generated by brain structures. It is a completely noninvasive procedure that can be applied repeatedly in patients, normal adults, and childrenÑwith virtually no risks or limitations. Local current flows are produced when brain cells (neurons) are activated. However, only electrical activity generated by large populations of neurons concurrently active can be recorded on the head surface. The small electrical signals detected by the scalp electrodes are amplified thousands of times, then displayed on paper or stored to computer memory. The scalp electrical activity can also be represented with color maps in 2D and 3D to enhance visualization.
How is EEG used? Clinical applications include the diagnosis of epilepsy (a brain disorder usually not accompanied by any abnormalities detectable with nonfunctional imaging techniques), sleep disorders, stroke, head trauma, encephalopathies, etc. Because the EEG procedure is noninvasive and painless, it is currently being widely used to study the brain organization of cognitive processes such as perception, memory, attention, language, and emotion in normal adults and children. For this purpose, the most useful application of EEG recording is the ERP technique.
What are ERPs? ERPs (event-related potentials) are small voltage fluctuations resulting from evoked neural activity. These electrical changes are extracted from scalp recordings by computer averaging epochs (recording periods) of EEG time-locked to repeated occurrences of sensory, cognitive, or motor events. The spontaneous background EEG fluctuations, which are random relative to when the stimuli occurred, are averaged out, leaving the event-related brain potentials. These electrical signals reflect only that activity which is consistently associated with the stimulus processing in a time-locked way. The ERP thus reflects, with high temporal resolution, the patterns of neuronal activity evoked by a stimulus.
How are ERPs used? Due to their high temporal resolution, ERPs provide unique and important timing information about brain processing. Mental operations, such as those involved in perception, selective attention, language processing, and memory, proceed over time ranges in the order of tens of milliseconds. Most other functional imaging techniques require the integrating evoked brain activity over many seconds and are thus unable to capture the time course (or sequence) of these operations. ERP recordings, however, provide a millisecond-by-millisecond reflection of evoked brain activity. For this reason, ERPs are an ideal methodology for studying the timing aspects of both normal and abnormal cognitive processes. On the other hand, ERP data provide less accurate spatial information than positron emission tomography (PET) or functional magnetic resonance imaging (fMRI), which lack fine temporal resolution. As a result, ERPs represent the natural complement of PET and fMRI to study human cognition. Whereas PET and fMRI can localize regions of activation during a given mental task, ERPs can help in defining the time course of these activations.
In a clinical setting, the temporal resolution of ERPs is useful in identifying at which level along the sensory pathways a lesion is localized. Visual ERPs help the early diagnosis of multiple sclerosis before any structural abnormality is detected. ERPs are also used to monitor comatose patients to evaluate the functionality of vital centers in the brainstem.
ERP Studies at the RIC. Mapping human cognitive function is a major emphasis of ERP studies at the RIC. Research will help explore the organization of cognitive processes such as selective attention, memory, language, and emotion both in normal subjects and in neurological and psychiatric patients. We will be combining ERP studies with PET and MRI imaging to provide a comprehensive picture of brain cognitive functioning. We also plan to use our brain mapping techniques in epileptic patients to more precisely localize seizure activity in preparation for neurosurgery. Other research will combine ERP, PET, and MRI methodologies to study the functional reorganization (plasticity) of the cortex in subjects who are congenitally deaf or blind, and the reorganization that occurs during recovery of cognitive or motor function following a stroke.