EEG and Evoked Potentials Flashcards
What are brain waves and on what scale are they recorded?
They are voltage fluctuations generated at the surface of the scalp, measured in voltage versus time.
How many electrodes are used to generate a channel, and how many are there generally? What is noise?
Two electrodes: a reference and a recording electrode. Only those signals which are not cancelled out are recorded. Each electrode is regularly spaced about the head to prevent any “straight” lines. The noise will cancel out.
There are 8-40 channels in a standard multichannel recording
What is the scale on which brainwaves are measured?
Microvolts, but they are amplified to near megavolt levels by the amplifier.
What is the significance of Fourier Transforms for EEGs?
They convert the wave data from being a function where voltage varies with time to show how much of the EEG signal on a channel occurs due to a given FREQUENCY
-> very useful for determining the type of waves
What is an artifact?
Disturbances in the EEG caused by technical defects, and are usually transitory. I.e. muscle movements, touching of the head, loss of electrode contact, sweating, etc
How many Hz are associated with gamma, alpha, beta, theta, and delta waves?
gamma: >40 Hz
alpha: 8-13 Hz
beta: 13-40 Hz
theta: 4-8 Hz
delta: <4 Hz
What are gamma waves? What do they do?
The highest frequency waves which are expressed widely in brain areas that are active at the same time. They reflect “binding”, and serve to integrate all of our senses into a single consciousness. It is the rhythm / refresh rate of the brain
What are beta waves? Where are they found?
High frequency, low amplitude waves seen on both sides symmetrically, primarily frontally. Dominant rhythm in patients who are alert or anxious, or have their eyes open
What are alpha waves? Where are they found? How does their amplitude vary with dominance?
Slightly lower frequency than beta. They are best seen in posterior regions of head, and are higher in amplitude on the dominant side. It is the major rhythm in normal, rested adults (past age 13), or if eyes are closed. If thinking or calculating, waves will shift to beta
What are theta waves and at what ages is this normal / abnormal when awake? What part of the brain is theta at rest?
It is considered abnormal in awake adults, but is normal in sleeping adults or awake children <13 years.
Seen normally in hippocampus (not by surface EEG).
What types of brain damage are theta waves seen in?
Will be seen in generalized distribution in metabolic encephalopathies or diffuse disorders, or focally in focal subcortical lesions
What are delta waves? When are they normal?
Highest amplitude, lowest frequency waves. Normal in NREM stages 3-4 of sleep, and correlates with non-moving, low levels of arousal.
Present also normally in newborn infants
When are delta waves abnormal?
Not normally seen in adult EEGs. If they are seen, it’s a sign of disease
How does EEG Brain Topography work?
Larger number of electrodes are configured in a geometric array and spaced about the head. A computer makes a “weathermap” or topography of brain activity, with brighter colors to depict higher amplitudes
What is EEG Brain Topography used for?
Used for detection of disorders of brain conduction. Will not be useful for diseases like depression where the electrical conduction of the brain is not interfered with.
For instance: epilepsy, stroke, AV malformations, psychoactive drug withdrawal
What does a brain wave indicate? What type of potential does it detect?
Synchronized depolarization of the dendrites of pyramidal cells in layers 2-6 of the cerebral cortex which are radially arranged. It detects GRADED potentials (EPSPs and IPSPs), NOT action potentials
What causes the massive synchrony in EPSPs and IPSPs?
Thalamocortical (layer 4) and corticothalamic circuits (layer 6)
What is an ERP or evoked potential?
ERP = event related potential = evoked potential. They are EEG signals that result when the brain is exposed to a specific activity, which could be sensory, cognitive, or motor
How is a sensory evoked potential determined?
Repeatedly expose the subject to a stimuli (500 exposures, 1 sec apart) and record the EEG. Then, average the EEG and all the noise will cancel out and reveal an ERP form
What is the convention for naming deflections in ERPs?
Deflection is called a component
P = positive = downward pointing N = negative = upward pointing
The P or N is followed by the “latency” or time after the stimulus which the peak occurs
What would be the “early” components for a visual stimulus? “late”?
early: <150 ms, related to subcortical processing of the image
late: >150 ms, related to cortical processing
What is the clinical significance of ERPs?
can be used to diagnose subcortical or cortical damage
Why is EEG often preferable to fMRI or PET?
it has the best TIME resolution of any noninvasive brain imaging method, with resolution down to 1 ms.
What are the two types of EEG for diagnosing epilepsy? What are they used for?
Interictal - recording is taken when patient is NOT having seizures. Looking for interictal epileptiform abnormalities -> abnormal brain activity between seizures
Ictal - taken during an aura or a seizure. This is done when seizures do not reply to treatment and doctor wants to confirm diagnosis.
What will an ictal EEG of general epilepsy show? Focal?
General = widespread brain involvement
Focal = seizure starting from specific area but spreading to other areas. Where it starts can be used by the doctor to determine which part of the brain causes the seizure
What is the most common interictal EEG abnormality?
A spike, or a burst of electrical activitiy which stands out. They can be focal, multifocal, or generalized
What diagnoses is EEG useful for?
Sleep disorders (narcolepsy), stroke, head trauma, or encephalopathies
What is an MEG?
A magnetoencephalography, which measures the perpendicular waves of the EM wave.
