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 brain2. Ultrastructure of cerebral cortex3. Potential fields of single neurons leading to

cortical potentials4. 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 cord1. Connecting link of spinal cord and cerebral cortex2. 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 skullInion: bump at the back of the skullLocation: Frontal, Temporal, Parietal, Occipital, Central

z for the central lineNumbers: 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 nasion inion measurement by 2 to determine 50% of the nasion inion 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-to-

ear) measurements. (Make certain that the tape goes through your CZ mark.)

• Step 8. Record your ear-to-ear 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 direct rather than indirect evidence of epileptic abnormality

• May be the only test that shows abnormalities in epileptic patients

• Provides some spatial or localization information

• 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

• 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 bandsType of wave

Amplitude Frequency

Alpha 20-200 μV 8 to 13 Hz

Beta Less than 20 μV

14 to 30 Hz

Theta and delta

Less than 200 μV

4 to 7 hz,0.5 to 3.5 Hz

Gamma 2 μV 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 & surroundingsPhysiological correlates: alert, active, but not agitatedAssociated 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 thoughtsAssociated 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, widespreadSubjective feeling states: deep, dreamless sleep, non-REM sleep, trance, unconsciousAssociated tasks & behaviors: lethargic, not moving, not attentivePhysiological 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

Fall asleep

Eyes are closed , produce a large amount of rhymic activity in the range of 8 to 13 hz

Light sleep

Amp & freq of the waveform decreased

Large amplitude low frequency waveform emerges

Deeper Sleep 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 VigilanceFrequency Ranges

Beta: 14 – 30 Hz

Alpha: 8 – 13 HzTheta: 5 – 7 HzDelta: 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 berecorded 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 signal-averaging 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 has multiple peaks and valleys stretched out over a period of several hundred milliseconds after the presentation of the acoustic stimulus.

• The time period most commonly studied covers the first 10 msec after the stimulus is presented to the ear and represents the 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

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ABR

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 P1-N1-P2 sequence (50-200 ms poststimulus; originating in auditory cortex).

• N1 is the large negative wave that occurs about 80-100 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