Transcript ELECTROENCEPHALOGRAM_(EEG).

ELECTROENCEPHALOGRAM
(EEG)
ELECTROENCEPHALOGRAM (EEG)
• Term introduced by Hans Berger
• Definition: record of potential fluctuations or
electrical activity of brain
• Electrodes
– Scalp
– Cortical
– Depth
EEG
• Complex structure
• Superposition of volume of volume-conductor
fields
• Variety of active neuronal generators
• Neuronal tissues non uniform
Background Information
1. Anatomy and function of the brain
2. Ultrastructure of cerebral cortex
3. Potential fields of single neurons leading to
cortical potentials
4. Typical clinical EEG waveforms
1. Anatomy & Function of Brain
• CNS – spinal cord in vertebral column and
brain in skull
• Brain & spinal cord– three meninges and CSF
• Brain
– Cerebrum
– Brain stem
– Cerebellum
Brain stem
• Short extension of spinal cord
1. Connecting link of spinal cord and cerebral
cortex
2. Center of integration of several visceral functions
•
Eg., HR and RR
3. Integration center for various reflexes
• Most superior part – diencephelon
– Its chief component and largest structure thalamus
Thalamus
• Major relay station and Integration center for
all general and special sensory systems
• Sending information to their respective
cortical reception areas
• Serves as gateway to cerebrum
Cerebellum
• Coordinator in the voluntary muscle system
• Acts in conjunction with brainstem and
cerebral cortex
• Maintain balance and harmonious muscle
movements
• Dominant position in CNS
• Conscious function
CNS
• Ascending (sensory) nerve
– Spinal cord or brain stem to various areas of brain
– Variety of sensors
• general: temperature, pressure, pain, fine touch,
• special senses: vision, audition, equilibrium, taste, and
olfaction
2. Ultrastucture of Cerebral
Cortex
Bioelectric Potentials from the Brain
• Bipolar electrodes records resultant field
potentials of a large conducting medium
– Medium consists of array of acting elements
• Conducted axon potentials in axon
– Contribute little to surface cortical records
– Occur asynchronously in time
– Run in many direction relative to the surface
– Net influence at the surface is negligible
• If cell bodies and dendrites randomly are
arranged in cortical matrix  net influence of
synaptic current will be zero”closed field”
• Any potentials recorded at surface must be from
orderly and symmetrically arranged cells
• Pyramidal cells :
– oriented vertically
– Potential change in one part relative to other create
“open” potentials field
– Potential measurable at cortical surface
Resting Rhythms of Brain
• Electrical recording  continuous oscillating
electrical activity
• Intensity and pattern: determined by the
overall excitation of brain
• Result from function in the brainstem reticular
activation system (RAS)
Pacemaker of Brain
• Various regions of cortex, though capable of
exhibiting rhythmicity, require trigger input to
excite rhythmicity
• reticular activation system (RAS) provides this
pacemaker function
The Clinical EEG
EEG
• Intensity
– surface of brain : 10 mV
– Scalp : 100 V
• Frequency : 0.5 to 100 Hz
• Wave groups: alpha, beta, theta, delta
Types of EEG Recording
• Routine
– analog, digital
– with computerized analysis &
brain electrical activity mapping
• Long-term Monitoring
Two types of recording
• Bipolar – both the
electrodes are at active
site
• Unipolar – one
electrode is active and
the other is indifferent
kept at ear lobe.
(A) Bipolar and (B) unipolar measurements
Electrode
contd
EEG Electrodes
• Each electrode site is labeled with a letter and a
number.
• The letter refers to the area of brain underlying the
electrode
e.g. F - Frontal lobe and T - Temporal lobe.
• Even numbers denote the right side of the head and
• Odd numbers the left side of the head.
EEG
• The amplitude , phase and frequency of EEG
depend on electrode placement.
• The placement is based on Frontal, Parietal,
temporal and occipital areas .
• One of the most popular schemes is the 10-20
EEG Placement System established by the
International Federation of EEG socities.
Contd…
• In this setup, the head is mapped by four
standard points.
• The nasion , the inion and the left and right
pre auricular points.
• Nineteen electrodes plus one for grounding
the subject are used.
Contd…
• Electrodes are placed on the scalp by
measuring the nasion-inion distance and
marking the points on the head 10% 20% 20%
20% and 10%.
Montage
• Different sets of electrode arrangement on
the scalp by 10 – 20 system is known as
montage.
• 21 electrodes are attached to give 8 or 16
channels recording.
Routine EEG Techniques
• 20-min or longer sampling of brain activity
• Written out or recorded directly on magnetic
tape or digitally by computer
• Disc electrodes are applied according to 10-20
system
• Montages: bipolar, referential, changeable
with digital recording
10-20 System Of Electrode
Placement
International 10-20 System of
Electrode Placement
• Established in 1958
• Electrodes are spaced at 10% or 20% of distances
between specified anatomic landmarks
• Use 21 electrodes, but others can be added
– increase spatial resolution
– record from specific areas
– monitor other electrical activity (e.g. ECG, eye
movements)
• Odd number electrodes over left and even number
over right hemisphere
10 – 20 electrode placement
The international 10-20 system seen from (A) left and (B) above the head.
(Fyi)
The International 10/20 System
Terminology: 10/20 System
Nasion: point between the forehead and the skull
Inion:
bump at the back of the skull
Location:
Frontal, Temporal, Parietal, Occipital, Central
z for the central line
Numbers:
Even numbers (2,4,6) right hemisphere, odd (1,3,5) left
EEG channels
Channel: Recording from a pair of electrodes (here with a common
reference: A1 – left ear)
Multichannel EEG recording: up to 40 channels recorded in parallel
Participants with Electrodes
EEG in clinical diagnostics
EEG in scientific research
ELECTRODE PLACEMENT
• It is called the 10-20 system, because the
electrodes are placed at sites that are 10% or
20% of a measured length from a known
landmark on the skull.
• Percentages are used, because different
individuals have different skull sizes.
• The 10-20 system insures that electrode sites
and EEG recordings can be compared across
laboratories worldwide.
Contd..
• The 10-20 system identifies electrode sites through
careful measurements taken using standardized
procedures.
• The labeling of electrode sites is also standardized.
• Sites located over the frontal lobes are labeled `F,'
those along the midcoronal plane are labeled `C,'
while those over the parietal, occipital, and temporal
lobes are labeled `P,' `O,' and `T' respectively.
Contd…
• Electrodes located along the mid sagittal plane
have the subscript "z" as in `Cz,.‘
• Electrodes located on the left side of the skull
receive odd numbers, for example, T5. , while
electrode sites on the right side of the head
have even-numbered subscripts (e.g., P4).
A = Ear lobe, C = central, Pg = nasopharyngeal,
P = parietal, F = frontal, Fp = frontal polar,
O = occipital
Contd..
• Step 1. Measure the
distance in centimeters
from the nasion bridge of
the nose, to the inion, base
of the skull. This is the
nasion to inion distance.
• Step 2. Record your
measurement,
cm ( nasion to inion)
Contd..
• Step 3. Calculate 10% of
the nasion-inion
measurement.
• Step 4. Beginning at the
inion, and measuring
toward the top of the skull
(the vertex), place a mark
at 10% of the nasion-inion
distance, the value
calculated in Step 3. This
mark locates Oz.
•
Step 5. Beginning at the
nasion, measure toward
the top of the skull (vertex)
to place a mark at 10% of
the nasion-inion length.
• This mark locates FP.
Contd…
• Step 6. To locate the next
site, CZ, divide the nasioninion measurement by 2 to
determine 50% of the
nasioninion measurement.
• Step 6. This locates CZ,
which is half way
between the nasion
and the inion.
• FP, CZ, and OZ are
marked
Contd..
• Step 7. measure (ear-toear) measurements. (Make
certain that the tape goes
through your CZ mark.)
• Step 8. Record your ear-toear distance_______cm.
• Step 9. Calculate 20% of
the ear-to-ear distance.
• Step 10. To locate C 3, place
a mark that is 20% of the
ear-to-ear distance from
CZ.
• From C3, continuing toward
the left ear and along the
same plane, measure the
number of centimeters
calculated in Step 9 to mark
T3
• To Place a mark at T4
(repeating same on the
right side of the skull).
Contd (fyi)
Electrodes Arrangement
• Either unipolar or bipolar arrangement.
• A unipolar arrangement is composed of
number scalp leads connected to the common
point such as ear lobe.
• A bipolar arrangement is achieved by
interconnection of scalp electrodes.
• for eg: The difference of voltage between Fp2
and Fp8 are measured.
LEADS
What is Montages
• Montages are patterns of connections
between electrodes and recording channels.
• All of these combinations have inputs to
three-lead differential amplifier and use a
third connection for the reference (two ears,
forehead or nose)
Bipolar montage
• Bipolar montage
– Each channel (i.e., waveform) represents the difference
between two adjacent electrodes. The entire montage
consists of a series of these channels.
– For example, the channel "Fp1-F3" represents the
difference in voltage between the Fp1 electrode and the
F3 electrode.
– The next channel in the montage, "F3-C3," represents the
voltage difference between F3 and C3, and so on through
the entire array of electrodes.
Referential montage
• Referential montage
– Each channel represents the difference between a
certain electrode and a designated reference
electrode.
– There is no standard position at which this
reference is always placed; it is, however, at a
different position than the "recording" electrodes
Average reference montage
• Average reference montage
– The outputs of all of the amplifiers are summed
and averaged, and this averaged signal is used as
the common reference for each channel
Bipolar , unipolar and average
EEG Diagnostic Uses
• EEG changes are also apparent in patients
with sleep disorders such as insomnia ,
narcolepsy (re occuring , uncontrollable sleep
episodes),
• chronic hypersomnia (excessive sleep or
sleepiness )
• Sleep paralysis ( inability to move during full
consciousness ) , nightmares
Contd..
• EEG Pattern changes are also present with
changes in behavior.
• Depression of EEG peaks in alcoholics
• Sporadic runs of slow waves in drug addicts
Types of electrodes
• Scalp : silver pads, discs or cups, stainless steel
rods and chlorided silver wires.
• Sphenoidal :alternating silver and bare wire
and chlorided tip inserted through muscle
tissue by a needle.
• Nasopharyngeal : silver rod with silver ball at
the tip inserted through the nostrils.
Contd… (fyi)
• Electrocorticographic: cotton wick soaked in
the saline solution that rests on the brain
surface.
• Intracerebral : sheaves of teflon-coated gold
or platinum wires cut at various distance from
the sheaf tip and used to electrically stimulate
the brain.
Contd…(fyi)
• Reusable scalp disc or cup electrodes are
placed on the head using electrolyte.
• remove oil
• Contact resistance below 10 k Ω.
Activations
• Routine
– Eye opening and
closure
– Hyperventilation
– Intermittent photic
stimulation
•
•
•
•
1, 5, 10, 15 & 20 Hz
eyes open
eyes closed
eyes closure
• Optional
– Sleep deprivation
– Sedated sleep
– Specific methods of
seizure precipitation
• video games
• visual patterns
– Anti Epileptic Drug
(AED) withdrawal
Strength and Advantages of EEG
• Is a measure of brain
function; supplement
neuroimaging studies
• Provides some spatial
or localization
information
• Provides direct rather
•
than indirect evidence
of epileptic abnormality
•
• May be the only test
that shows
•
abnormalities in
epileptic patients
•
Low cost
Low morbidity
Readily repeatable
Portable / ambulatory
Limitations and Disadvantages
Of EEG
• Detects cortical dysfunction but rarely discloses its
etiology
• Relatively low sensitivity and specificity
• Subject to both electrical and physiologic artifacts
• Influenced by state of alertness, hypoglycaemia, drugs
• Small or deep lesions might not produce an EEG
abnormality
• Limited time sampling (for routine EEG) and spatial
sampling
• May falsely localize epileptogenic zone
Uses Of EEG In The Management of
Seizure Disorders
• To support a clinical diagnosis of epilepsy
• To help to classify seizures
• To help localize epileptogenic focus, especially in
presurgical candidates
• To quantify seizures
• To aid in the decision of whether to stop AED
treatment
• Not a good guide to the effectiveness of
treatment, except in absence seizures
Analyzing EEG Activities
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•
•
•
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•
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•
Morphology
Distribution
Frequency
Voltage
Duration
State of the patient
Background from which activity is arising from
Similarity or dissimilarity to the other ongoing
background rhythms
Guidelines To EEG Interpretation
• Each EEG should be read with maximum possible
objectivity
• Ideally an EEG’er should describe the findings and
make an EEG diagnosis without knowledge of the
patient's history
• Clinical significance of the findings can then be
judged by integrating the EEG diagnosis with the
history
EEG Interpretation
• Normal
– Lack of Abnormality
• Abnormal
– Non-epileptiform Patterns
– Epileptiform Patterns
Frequency bands
Type of
wave
Alpha
Amplitude
Frequency
20-200 μV
8 to 13 Hz
Beta
14 to 30 Hz
Theta and
delta
Less than
20 μV
Less than
200 μV
Gamma
2 μV
4 to 7
hz,0.5 to
3.5 Hz
High
frequency
Different Waves
Alpha
• Frequency : 8 to 13 hz
• Amp : 20 to 200 μV
• Recorded site: Most
intensely in Occipital
region.
• Can be recorded in
frontal and parietal
region of the scalp
Alpha..
• Slower, and higher in amplitude
• Prominent with closed eyes and with relaxation
• Seen in all age groups but are most common in
adults.
• Ooccur rhythmically on both sides of the head but
are often slightly higher in amplitude on the non
dominant side, especially in right-handed individuals.
Alpha
• Reported to be derived from the white matter of the
brain.
• Common state for the brain and occurs whenever a
person is alert but not actively processing
information.
• They are strongest over the occipital (back of the
head) cortex and also over frontal cortex..
Contd…
• Alpha activity disappears normally with
attention (eg, mental arithmetic, stress,
opening eyes).
• In most instances, it is regarded as a normal
waveform.
Effect of Alpha
• Alpha in normal ranges: good moods, and a
sense of calmness.
• One can increase alpha by closing eyes or
deep breathing or decrease alpha by thinking
or calculating.
Biofeedback
• Alpha-Theta training can create an increase in
sensation, abstract thinking and self-control.
• When Alpha predominates most people feel at ease
and calm.
• Alpha appears to bridge the conscious to the
subconscious.
Beta
• Freq: Above 13 hz
• Recorded site: Parietal
and frontal region of
the scalp
• Beta 1 and Beta 2.
Beta
• Beta activity is 'fast' activity. It reflects
desynchronized active brain tissue.
• It is most evident in frontal region. It may be
absent or reduced in areas of cortical damage.
• It is generally regarded as a normal rhythm
and is the dominant rhythm in those who are
alert or anxious or who have their eyes open.
Beta
• It is the state that most of brain is in when we
have our eyes open , listening and thinking
during analytical problem solving, judgment,
decision making, processing information
about the world around us.
Contd…
• Beta 1 is twice the
alpha frequency they
are affected by the
mental activity.
• Beta 2 they appear
during intense
activation of the CNS
and during tension.
BETA
• The beta band has a relatively large range, and has
been divided into low, midrange and high.
Low Beta (12-15 Hz), formerly "SMR":
• Subjective feeling states: relaxed yet focused,
integrated
BETA
• Midrange Beta (15-18 Hz)
• Subjective feeling states: thinking, aware of
self & surroundings
Physiological correlates: alert, active, but not
agitated
Associated tasks & behaviors: mental activity
BETA
• High Beta (above 18 Hz):
• Subjective feeling states: alertness, agitation
Physiological correlates: general activation of
mind & body functions.
• Associated tasks & behaviors: mental activity,
e.g. math, planning, etc.
Gamma Waves
Gamma (above 36 Hz)
• Gamma is measured between 36 – 44 (Hz) and is the
only frequency group found in every part of the
brain.
• When the brain needs to simultaneously process
information from different areas, its hypothesized
that the 40Hz activity consolidates the required areas
for simultaneous processing.
• A good memory is associated with well-regulated
and efficient 40Hz activity, whereas a 40Hz deficiency
creates learning disabilities.
Gamma (40 Hz):
Subjective feeling states: thinking; integrated
thoughts
Associated tasks & behaviors: high-level
information processing, "binding"
Physiological correlates: associated with
information-rich task processing
Theta
• Freq : 4 to 8 hz
• Recorded site : parietal
and temporal region in
children.
• But they also occur
during emotional stress
in some adults,
particularly during the
period of
disappointment and
frustration.
Theta (4-8 Hz)
• Theta activity as "slow" activity.
• It is seen in connection with creativity, intuition,
daydreaming, and fantasizing and is a repository for
memories, emotions, sensations.
• Theta waves are strong during internal focus,
meditation, prayer, and spiritual awareness.
• It reflects the state between wakefulness and sleep.
Relates to subconscious.
Delta
• Freq: below 3.5 Hz.
• Sometimes these waves
occur only once every 2
or 3 s.
• They occur in deep
sleep in infancy and in
serious organic brain
disease.
• They occur solely within
cortex.
Delta
• The lowest frequencies are delta.
• These occur in deep sleep and in some abnormal processes
also during experiences of "empathy state".
• Delta waves are involved with our ability to integrate and let
go. It reflects unconscious mind.
• It is the dominant rhythm in infants up to one year of age and
it is present in stages 3 and 4 of sleep.
• It tends to be the highest in amplitude and the slowest waves.
ADD
• Most individuals diagnosed with Attention
Deficit Disorder, naturally increase rather than
decrease Delta activity when trying to focus.
• The inappropriate Delta response often
severely restricts the ability to focus and
maintain attention. It is as if the brain is locked
into a perpetual drowsy state.
Delta
• Delta (0.1- 4 Hz)
• Distribution: generally broad or diffused may be
bilateral, widespread
Subjective feeling states: deep, dreamless sleep,
non-REM sleep, trance, unconscious
Associated tasks & behaviors: lethargic, not moving,
not attentive
Physiological correlates: not moving, low-level of
arousal
Contd..
• The normal EEG varies by age. The neonatal
EEG is quite different from the adult EEG.
• The EEG in childhood is generally comprised of
slower frequency oscillations than the adult
EEG.
EEG
Sleep pattern
Stages of sleep
Drowsy
Eyes are closed , produce a large amount of
rhymic activity in the range of 8 to 13 hz
Amp & freq of the waveform decreased
Fall asleep
Light sleep
Deeper Sleep
Large amplitude low frequency waveform emerges
Freq even low and higher amplitude waveform
Sleep Stage Patterns During One Night
EEG also varies depending on state..
• Stage I sleep (equivalent to drowsiness in some
systems) appears on the EEG as drop-out of the
posterior basic rhythm. There can be an increase in
theta frequencies.
• Stage II sleep is characterized by sleep spindles-transient runs of rhythmic activity in the 12-14 Hz
range (sometimes referred to as the "sigma" band)
that have a frontal-central maximum.
• Most of the activity in Stage II is in the 3-6 Hz range.
Sleep
• Stage III and IV sleep are defined by the presence of
delta frequences and are often referred to
collectively as "slow-wave sleep.“
• Stages I-IV are comprise non-REM (or "NREM")
sleep.
• The EEG in REM (rapid eye movement) sleep appears
somewhat similar to the awake EEG.
Segment of EEG activity during wakefulness. Alpha rhythm (a
continuous activity between 8 and 13 Hz) appears
During light sleep alpha rhythm disappears and from time to time sleep
spindles (a spindle-shaped waveform of limited duration at around 13hz
When sleep becomes deeper, slow waves
dominate the record.
EEG Stages in Wakefulness and Sleep
REM
• A period of high frequency that occur during
sleep is called Paradoxical sleep ,because the
EEG is more like that of an awake alert person
than one who is asleep.
• REM sleep is associated with high frequency
EEG is a large amount of Rapid Eye Movement
beneath the closed eyelids.
Contd…
• When people sleep, they experience periods of Rapid
Eye Movement.
• During this stage, which is associated with dreaming,
the brain becomes very active.
• REM sleep and dreaming are triggered by the pons
and neighboring structures in the brainstem.
• During REM sleep, the brain transfers short-term
memories in the motor cortex to the temporal lobe
to become long-term memories.
CONTD
• REM sleep in adult humans typically occupies
20-25% of total sleep, lasting about 90-120
minutes.
• During a normal night of sleep, humans
usually experience about 4 or 5 periods of
REM sleep; they are quite short at the
beginning of the night and longer toward the
end.
Contd…
• A newborn baby spends more than 80% of
total sleep time in REM.
• During REM, the summed activity of the
brain's neurons is quite similar to that during
waking hours; for this reason, the
phenomenon is often called paradoxical sleep.
Different Stages of sleep
Contd
Picture of K Complex
• K complex
• K complex waves are
large-amplitude delta
frequency waves,
sometimes with a sharp
apex.
• Sometimes Associated
with Sharp Components
and followed by 14 hz.
• Amplitude is 200 μv.
Contd…
• They can occur throughout the brain and usually are
higher in amplitude.
• They occur each time the patient is aroused partially
from sleep.
• Semi arousal often follows brief noises; with longer
sounds, repeated K complexes can occur.
• K complexes sometimes are followed by runs of
generalized rhythmic theta waves; the whole
complex is termed an arousal burst.
Example of either lambda or positive occipital
sharp transients of sleep
• Lambda
• These are Monophasic,
positive sharp waves
that occur in the
Occipital location .
• Amplitude : less than
50μV
• They are related to eye
movement.
POSTS
• POSTS are triangular waves that occur in the bilateral
occipital regions as positive (upgoing) waves.
• They can be multiple and usually are symmetric.
• POSTS occur in sleeping patients and are said to be
most evident in stage 2 of sleep, although they are
not uncommon in stage 1.
• POSTS are similar or identical to lambda waves both
morphologically and in the occipital distribution.
Example of mu waveforms.
• Mu waves are runs of
rhythmic activity that have
a specific shape.
• They are rounded in one
direction with a sharp side
in the other direction
• Freq: 7-11hz with arcade or
comb shape in the central
location.
• Amp: less than 50μV
Mu waves…
• They are blocked or attenuated by contralateral movement,
thought of movement, readiness to move, or tactile
stimulation.
• Unlike alpha activity, they are not blocked by eye opening.
• They often are asymmetric.
• Mu waves are seen best when the cortex is exposed or if bone
defects (eg, post surgical) are present in the skull.
• They tend to be more evident over the motor cortex.
Example of small sharp spikes, also known as benign
epileptiform transients of sleep (BETS)
• Bets
• These are recognized by
their height, their sharp
top, and their narrow
base.
• Spikes and sharp waves
usually are abnormal.
Contd….
• They can be normal in the following settings:
Small, sharp spikes of sleep or benign epileptiform transients of sleep
(BETS) are nonpathologic.
– They occur in the temporal regions. They do not have slow-following
waves as do most of the pathologic spikes of epilepsy.
– Numerous artifacts resemble spikes, but they are distinguished by
other waves that may be present, by observation of the patient while
they are occurring, and by experience.
Benign epileptic transients of sleep
• These sharp, usually small waves occur on one or
both sides (usually asynchronously), especially in the
temporal and frontal regions.
• BETS are rare in children but are more frequent in
adults and elderly persons.
• Although they can occur in epileptic patients, BETS
often are seen in individuals without epilepsy and
can be regarded as a probable normal variant
V- Waves
• V waves are sharp waves that occur during
sleep.
• V waves tend to occur especially during stage
2 sleep and may be multiple.
• Often, they occur after sleep disturbances (eg,
brief sounds) and, like K complexes, may occur
during brief semi arousals.
• V waves are easy to recognize.
EEG in the States of Vigilance
Frequency Ranges
Beta:
Alpha:
Theta:
Delta:
14 – 30 Hz
8 – 13 Hz
5 – 7 Hz
1 – 4 Hz
EEG Amplitude
• Ranges from 1 to 100 μV peak to peak at low
frequencies(0.5 to 100 hz) at cranial surface.
• At the surface of the cerebrum signals may be
10 times stronger
• Brain stem signals measured are 0.25 μV peak
to peak(100 to 1000 hz).
EEG Waveform
EEG Waveform
ABNORMAL EEG
Epilepsy
• Brain disorder in which a person has repeated
seizures (convulsions) over time.
• Epileptic seizures result from abnormal,
excessive or hypersynchronous neuronal
activity in the brain.
• Onset of new cases occurs most frequently in
infants and the elderly.
• As a consequence of brain surgery, epileptic
seizures may occur in recovering patients.
Contd.,
• Epilepsy is usually controlled, but not cured,
with medication.
• Over 30% of people with epilepsy do not have
seizure control even with the best available
medications.
• Surgery may be considered in difficult cases.
Basic Types
• Generalized epilepsy
– Involves the entire brain at once
– Grand mal and petit mal
• Partial Epilepsy
– Involves a portion of the brain
– Some times only a minute focal spot and at other
times a fair amount of brain
Grand mal epilepsy
• Characterized by extreme discharges
originating from brainstem portion of the RAS
• Discharges spread  deeper portion of brain
or even to spinal cord  tonic convulsions of
entire body  followed near the end of attack
by clonic convulsions
Contd.,
• Lasts from few seconds to 3-4 minutes
• Characterized by post-seizure depression of
entire nervous system
• Subject may be in stupor for 1 min to a day or
more after the attack is over
Grand mal attack
• Can be recorded at any regions of cortex
• High amplitude, synchronous, periodicity
same as alpha
• Same discharge both sides of brain same time
 indicating abnormality at lower part of
brain (RAS)
Petit mal epilepsy
• Two froms:
– Myoclonic
– Absense
Myoclonic
• A burst of neuronal discharges, lasting a
fraction of a second, occurs throughout the
nervous system
• Discharges similar to those that occur at the
beginning of the grand mal attack
• Person exhibits single violent muscular jerk
involving arms or head
• Attack stops immediately before subject loses
consiousness
• Similar to grand mal except that some form of
inhibitory influence promptly stops it
• Attack progresses  more severe  grand
mal
Absence seizures
• Brief (usually less than 20 seconds) seizures
• Generalized epileptic seizures of sudden onset
and termination.
• Two essential components:
– clinically, the impairment of consciousness
(absence)
– Electroencephalography (EEG) shows generalized
spike-and-slow wave discharges.
Symptoms
• Abrupt and sudden onset impairment of
consciousness
• interruption of ongoing activities, a blank stare,
possibly a brief upward rotation of the eyes.
• If speaking: speech is slowed or interrupted
• if walking: he or she stands transfixed;
• if eating: the food will stop on his way to the
mouth.
• Unresponsive when addressed.
• In some cases, attacks are aborted when the
patient is called.
Absence type
•
•
•
•
Lasts for 5-20 secs
Spike and dome pattern
Can be recorder over entire cortex
Indicating the origin of attack  RAS
Partial Epilepsy
• Involve almost any part of brain
• Either localized regions of cortex, deeper
structures of cerebrum or brain stem
• Results from lesions of the brain
– Scar that pulls on neuronal tissue
– Tumor that compresses brain tissue
– Destroyed regions of brain tissue
• Lesions  rapid firing (1000/sec) of neurons
• Firing  localized reverberating neuronal
circuits  spread to adjacent at reduced rates
• Jacksonian march
Psychomotor epilepsy
• Low frequency rectangular waves
• 2-4 Hz superimposed on 14 Hz
• Short amnesia, abnormal rage, sudden anxiety
or fear, momentary incoherent speech, motor
act of rubbing face with hand
Multi channel EEG recording systems
• Typically 8 , 16 or 32 channels.
• Gain control or sensitivity pot (overall gain)
• High pass filter switch – selects low frequency
cutoff 0.16,0.53,1 and 5.3 hz
• Low pass filter switch – selects high frequency
cutoff usually 15,35,50,70,100 hz.
• Notch filter
External controls
•
•
•
•
•
Calibration push button 5 to 1000 μv.
Baseline pot
Individual electrode selection switch
Event marker push button
Chart speed 10,15,30 and 60 mm/s
EEG Multi channel
EEG
EEG
contd
• It is these extra cellular currents which are
responsible for the generation of EEG
voltages.
• While it is post-synaptic potentials which
generate the EEG signal, it is not possible to
determine the activity within a single dendrite
or neuron from the scalp EEG.
Evoked potential
• Evoked potentials are the potentials
developed in the brain as the responses to
external stimuli like sound , light etc.
• The external stimuli are detected by the sense
organs which cause changes in the electrical
activity of the brain.
• This is also called as Event – Related Potential.
EP
• Evoked Potential (EP) tests are used to check
the condition of the nerve pathways.
• They measure the brain's electrical response
to the signals sent by the nerves.
• EP tests help diagnose nervous system
abnormalities, hearing loss, and assess
neurological functions.
Major Types of Evoked Potentials
• Brainstem Auditory Evoked Potential - Checks
the pathway from the ear to the brain. The
BAEP test may help uncover the cause of
hearing and balance problems, and other
symptoms.
• Visual Evoked Potential - Checks the pathway
from the eyes to the brain. May help find the
cause of certain vision problems and other
conditions.
• Somatosensory Evoked Potential - Checks the
pathway from the nerves in the limbs to the
brain. It is a way to study the function of the
nerves, the spinal cord and brain
EP
• If light is flashed in the eye or a small electrical pulse given to
the skin over a nerve in an arm or leg, a characteristic
response - the evoked potential - can be
recorded from the brain using electrodes placed on the scalp.
• There will be a very short delay - measured in fractions of a
second - between the delivery of the stimulus and the
appearance of the electrical response in the brain.
CONTD..
• This delay corresponds to the time that it
takes for the signal to pass from the eye or
skin to the brain, along the nerve pathways.
• If there is a delay in the appearance of the
evoked potential in the brain, this may mean
that something is wrong somewhere in the
nerve pathways.
CONTD..
• For example, if there is a delay in the appearance of
the response over the scalp after a light is flashed in
one eye, this may be due to disease affecting the
optic nerve - the large nerve connecting the retina at
the back of the eye with the brain.
• Similarly, if there is a delay in the appearance of the
response in the brain after a small electrical pulse is
applied over a nerve in a leg, there may be problem a
with the spinal cord.
CONTD..
• Delays of this kind may be produced by a wide
variety of different problems - disease within the
optic nerve or spinal cord itself, or tumours pressing
on these structures from outside them and so on.
• In the past, evoked potentials were most commonly
used in the diagnosis of multiple sclerosis.
• This is a disease of the central nervous system in
which there is loss of the fatty insulation (“myelin
sheath”) around the nerves, causing them to
malfunction.
CONTD
• Loss of this fatty insulating sheath (demyelination)
causes a delay in the conduction of signals along the
nerve pathway, and this will be seen as a delay in the
appearance of the evoked potentials at the scalp if
the affected nerve pathway is stimulated.
• Evoked potential testing will also reveal whether the
optic nerve, the brainstem and the spinal cord have
been affected by the disease.
EVENT RELATED STUDIES
• Initial recording at rest. eyes open and
closed)
• Hyperventilation
• Photic Stimulation
• Auditory stimulation
• Different stages of sleep
Event related potential (ERP)
• Auditory evoked potentials (AEPs) are a
subclass of ERPs. For AEPs, the "event" is a
sound.
• AEPs (and ERPs) are very small electrical
voltage potentials originating from the brain
recorded from the scalp in response to an
auditory stimulus (such as different tones,
speech sounds, etc.).
Evoked potentials- Auditory Brainstem
Response
• Auditory brainstem
response (ABR) testing
is used to measure the
function of the central
auditory pathways.
• Recording electrodes
taped to the skull
record the electrical
activity of the brain
(EEG).
• When a brief acoustic
stimulus (e.g., a click or
short tone burst) is
presented to the ear
there is a synchronized
burst of action
potentials generated in
the auditory nerve
which spreads up the
central auditory
pathway
Contd…
• Because of its very low
amplitude (in the
microvolt range) this
wave of activity is
generally buried in the
EEG and can only be
recovered using
computerized signalaveraging techniques.
• When such methods
are employed the
complex waveform
recorded is called the
auditory evoked
potential and it includes
contributions from
many sites that are
activated sequentially in
time along the auditory
pathway.
Contd…
• An averaged waveform
• The time period most
has multiple peaks and
commonly studied
valleys stretched out
covers the first 10 msec
over a period of several
after the stimulus is
hundred milliseconds
presented to the ear
after the presentation
and represents the
of the acoustic stimulus.
electrical activity
evoked in neurons in
the auditory nerve and
brain stem
Contd
• This technique is very
useful in studying
hearing loss of central
auditory origin, as may
be caused by a lesion
affecting the brainstem
(e.g., acoustic neuroma
or multiple sclerosis).
• It is also helpful in
documenting the
hearing loss in infants
who cannot cooperate
with a behavioral-based
audiometric exam.
ABR
ABR
174
AEP
• The AEPs that are recorded from the top of the head
originate from structures within the brain (e.g., the
auditory cortex, the auditory brainstem structures,
the auditory VIIIth cranial nerve).
• They are very low in voltage: from 2-10 microvolts
for cortical AEPs to much less than 1 microvolt from
the deeper brainstem structures.
• Their low voltage combined with relatively high
background electrical noise requires the use of highly
sensitive amplifiers and computer averaging
equipment
AEP
Contd…
• The Auditory Brainstem
Response ("ABR"; 1.5-15 ms post
stimulus), which originates in the
VIIIth cranial nerve (waves I and
II) and brainstem auditory
structures The Middle Latency
Response ("MLR", 25-50 ms
poststimulus), includes waves Na
(negative wave following ABR
wave V, originates in upper
brainstem and/or auditory cortex)
and Pa (positive wave at about 30
ms, originates in the auditory
cortex bilaterally).
Contd..
• The "Slow" cortical auditory
ERPs, which include the P1N1-P2 sequence (50-200 ms
poststimulus; originating in
auditory cortex).
• N1 is the large negative
wave that occurs about 80100 ms after the stimulus. It
originates primarily in the
auditory cortex bilaterally.
Visual Evoked Potentials (VEP)
• Visual evoked potential (VEP) tests evaluate
how the visual system responds to light. VEP
tests are used to evaluate optic neuritis, optic
tumors, retinal disorders, and demyelenating
diseases such as multiple sclerosis .
• The patient is then asked to stare at a strobe
light or checkerboard pattern on a television
screen.
VEP
• For visual evoked potential (VEP), you are placed in front of a computer
screen, which shows a pattern of white and black squares like a
chessboard, and a red dot in the middle that you are supposed to focus
your eyes on with minimal movement.
• The procedure is done one eye at a time, with the eye that is not being
tested blocked off with an eye patch. During the actual procedure, these
squares alternate (white ones become black, black ones become white) at
a rate of several times a second, which produces responses in the visual
cortex, which is picked up by your skull electrodes.
• Since the computer controls the exact timing of the changes of the square
colors, and receives the exact timing of the electric response in the
corresponding electrodes, it is able to determine precisely the amount of
time it takes for the visual stimulus to reach the visual cortex.
Somato sensory evoked potentials (SEP)
• For the upper SEP (arms), two
stimulus electrodes are attached
on the inside wrist, closer to the
thumb. These electrodes will
receive timed electric pulses that
will produce an involuntary twitch
of the thumb.
• An additional sensor electrode is
applied on the back of your
shoulder, close to the attachment
point of the clavicle.
• Similar to the VEP, the computer
times the electric pulses (which
come at a rate of several times a
second) and gets the responses
from the appropriate skull
electrode, thus determining the
exact time it takes for the
stimulus to reach the
intermediate point on your
shoulder, and then the brain.
• The same is repeated for the
other arm.
Lower (SEP)
• For the lower SEP (legs),
two stimulus electrodes are
attached to the inside of
your ankle, in such a way as
to produce an involuntary
twitch of the big toe.
• Additional sensor
electrodes are placed at the
back of the knee (closer to
the outside), on the spine of
the lower back, and on the
spine of the upper back.
• Electric pulses are then sent
at a rate of several times a
second, and the responses
are recorded in the same
manner as above.
Evoked potential
Response