EEG: evoked potentials, clinical importance & physiological basis of consciousness, EEG changes in epilepsy

RK Goit, Lecturer

Department of Physiology

Electroencephalogram (EEG)

• is the record of electrical activity of the brain

• is recorded through the scalp

•EEG records may be bipolar or unipolar

• if the electrodes are placed on the surface of the cerebral cortex, the record is called Electrocorticogram (ECoG)

Uses of EEG

• it is the best diagnostic tool available for assessing the abnormalities of electrical activities of the brain

• it is a very helpful in diagnosing epilepsies & for studying sleep & sleep disorders

•Physiological basis of EEG• an EEG samples the summated activity of a very large

number of cortical neurons

• the record depends on the summation of excitatory & inhibitory postsynaptic potentials

Features of EEG RhythmEEG

RhythmFrequency

(Hz)Amplitude

(µV)Associated features

Alpha (α)

8-13 50-100 • found in normal adults when they are awake & in a quiet, resting state

• occur most intensely in the occipital region but can also be recorded from the parietal & frontal regions of the scalp

Beta (β)

13-30 5-10 • are recorded mainly from the parietal & frontal regions during specific activation

Theta (θ)

4-7 10 • occur normally in the parietal & temporal regions in children, but they also occur during emotional stress in some adults, particularly during disappointment & frustration

• also occur in many brain disorders (degenerative brain states)

Delta (δ)

.5-4 20-200 • occur in very deep sleep, in infancy, & in serious organic brain disease

Analysis of EEG

•The computer performs a Fast Fourier transform analysis on 4s segments of the EEG

•The smooth graphs form 4s segments are packed sequentially one down upon another

•As a result of these operations, the predominant frequency in the EEG appears as a series of tall peaks

• evoked potentials (or "evoked response") • are the electrical responses to sensory stimulation

• clinically used EPs are• brainstem auditory (BAEP)• visual (VEP)• somatosensory (SEP)

• are used mainly for• assessment for clinically silent lesions in possible multiple

sclerosis• visual loss that might be due to optic neuritis• assessment of brainstem function in neonates

•Brainstem auditory evoked potential• potentials recorded from the ear & the scalp in response to a

brief auditory stimulation

• assess conduction of the impulse through the auditory pathway up to the midbrain

•Uses of BAEPs• to assess hearing in uncooperative patients, & very young

children

• to detect degree of hearing loss in infants

• to assess the functions of the mid-part of brainstem

•Visual evoked potential• potential recorded from the occipital region in response to

visual stimuli

• a normal cortical response is recorded when the entire visual pathway is normal

• VEP can only detect the abnormalities, but cannot exactly localize the site of the lesion in the visual pathway

• Somatosensory evoked potential• potentials generated by large diameter fibers (sensory fibers)

in response to a sensory stimulus

• assess the intactness of the pathway for proprioception

• A seizure is a paroxysmal event due to abnormal excessive or synchronous neuronal activity in the brain.

• Epilepsy can be defined as a condition in which seizures are recurrent.

• Seizures may develop as a result of the spreading of EPSPs

• Seizure disorders may be idiopathic, but they also may occur as a result of brain tumors & vascular malformations, infections, drugs & metabolic problems

• can be classified into two major types:1. Focal 2. Generalized

Focal seizures

• abnormality occurs in only one part of the brain & the patients does not lose consciousness

• focal epilepsy results from some localized organic lesion or functional abnormality, such as• scar tissue in the brain that pulls on

the adjacent neuronal tissue• a tumor that compresses an area of

the brain• a destroyed area of brain tissue• congenitally deranged local circuitry

•EEG in patients with focal seizures show sharp wave

Generalized seizures

• abnormality occurs in large areas of the brain bilaterally, & patients loses consciousness

• generalized seizures may result from cellular, biochemical, or structural abnormalities that have a more widespread distribution

Petit Mal Epilepsy

• is characterized by 3 to 30 seconds of unconsciousness during which time the person has twitch-like contractions of muscles usually in head region

• this is followed by return of consciousness & resumption of previous activities

• the spike & rounded wave can be recorded over most or all of the cerebral cortex

Grand Mal Epilepsy

• last from a few seconds to 3 to 4 minutes

• are characterized by loss of consciousness, tonic phase, clonic phase

• the person remains in stupor for 1 to many minutes after the seizure attack is over, & then often remains severely fatigued & asleep for hours thereafter

• high-voltage, high-frequency discharges occur over the entire cortex

• Sleep is defined behaviorally by four criteria: • reduced motor activity• decreased response to stimulation• stereotypic postures• relatively easy reversibility

• a person goes through stages of 2 types of sleep• slow-wave (non-REM) sleep • rapid eye movement (REM) sleep

• In a normal night of sleep, bouts of REM sleep lasting 5 to 30 min usually appear on the average every 90 min

Slow-Wave Sleep

• is exceedingly restful & is associated with decreases in both peripheral vascular tone & many other vegetative functions

• the dreams of slow-wave sleep are usually not remembered because consolidation of the dreams in memory does not occur

•Non-REM sleep is divided into four characteristic stages

REM Sleep (Paradoxical Sleep, Desynchronized Sleep) • is an active form of sleep usually associated with dreaming & active

bodily muscle movements

• person is even more difficult to arouse by sensory stimuli than during deep slow-wave sleep

• muscle tone throughout the body is exceedingly depressed

• HR & RR usually become irregular

• irregular muscle movements do occur

• brain is highly active in REM sleep

• EEG shows a pattern of brain waves similar to those that occur during wakefulness-paradoxical sleep

Basic Theories of Sleep

1. area for causing almost natural sleep is the raphe nuclei in the lower half of the pons & in the medulla

• nerve fibers spread in the brain stem reticular formation & into the thalamus, hypothalamus, limbic system, & the neocortex of the cerebrum

• fibers extend downward into the spinal cord

2. stimulation of some areas in the nucleus of the tractus solitarius can also cause sleep

3. stimulation of several regions in the diencephalon (rostral part of the hypothalamus, the diffuse nuclei of the thalamus) can also promote sleep

The Functions of Sleep

• Conservation of Metabolic Energy• the metabolic rate during sleep is 15% less than during quiet

wakefulness

• Cognition• humans show little or no physiological impairment after several

days of sleep deprivation but do show impaired intellectual performance

•Thermoregulation• body & brain temperatures are usually reduced during sleep

•Neural Maturation & Mental Health• some reports indicate that REM sleep facilitates learning or

memory

Thank You