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EEG WAVES –RYHTHMS-1

Alpha

  • 8-12 Hz
  • High voltage waxing & waning
  • Adult at rest
  • Mind wandering with eyes closed
  • Mainly in parietal-occipital

Beta

  • 18-30 Hz
  • Low voltage with harmonic alpha waves
  • Adult
  • Frontal

EEG WAVES –RYHTHMS-2

Theta

  • 4-7 Hz
  • Large amplified voltage
  • Hippocampus in experimental animals & children

Delta

  • <4 Hz
  • Large amplified voltage
  • Sleep stage 3 & 4

Gamma

  • 30-80 Hz
  • Low voltage

EEG AND SLEEP

  • Berger called the waves he claimed to have recorded  from the scalp surface i.e. skin surface of the head “EEG” using  electrodes. 
  • At first no one believed him. 
  • EEGs can also be recorded from the surface of the cortex.
  • The waves are then termed Electrocorticogram ECoG

EEG VARIATIONS

Resting rhythm & age

  • Infants- beta like
  • -Occipital slow waves 0.5-2 Hz (0-2 years)
  • Childhood-faster rhythm- adult pattern by 12 yrs

Alpha frequency-decreased by• Decreased blood glucose

  • Lowering of body temp.
  • Decreased adrenal secretions
  • Decrease in arterial PCO2-& vice versa
  • Anaesthesia
  • Hyperventilation causes “spike waves” and may trigger convulsions in some subjets

ALPHA BLOCK OR AROUSAL / ALERTING 

RESPONSE

  • No alpha-low voltage, high frequency,
  • irregular or even gamma 30-80 Hz if attention is focused
  • Sensory stimulation
  • Mental conscious work e.g. calculations
  • Is therefore termed “Desynchronisation” of EEG waves  i.e. replacement of alpha rhythm by higher frequency waves

CLINICAL APPLICATION OF EEG

Localization of lesions

  • Fluid over cortex-decreased local EEG voltage e.g. sob-dorsalis
  • Local cortical lesions-local irregular slow waves
  • Epileptogenic foci- produce high voltage localized waves

Epilepsy

  • partial or local seizures i.e. one hemisphere only
  • general onset seizures-
  • grand mal- tonic clonic spiked waves(slow before each jerk)fast EEG.
  • Petit mal-absence of seizures doublets of spikes and rounded waves. After seizure, slow waves then normal again.

MEANING OF EEG

  • Record of neuronal discharges in a volume conductor-dipole movements. Activity of most superficial layers picked up by EEG i.e. mainly dendrites & few axons, very few cell bodies.
  • Dipole set belt- dendrites & cell bodies.
  • NB. Dendrites-subject to increase & decrease polarization and electronic spread
  • Not always – action potential-all or none
  • EEG waves also seen in cerebellar cortex & hippocampal cortex thus dendritic & cell arrangement. 

MEANING OF EEG -cont

Arousal EEG

  • stimulation of specific afferent to mid brain level
  • stimulation specific thalamic relay nuclei • stimulation of cortex primary receiving areas
  • BUT high frequency stimulation of :
  • reticular formation-midbrain tegmentum base
  • non-specific thalamic nuclei i.e. RAS is responsible hence decrease midbrain tegmentum-coma-decrease of RAS

RAS

  • gets inputs that converge from:
  • ascending sensory spinal fibers
  • special senses
  • these are non-specific stimulation of RAS by any sensory input cf. modality specific sensory pathways
    • part of RAS bypasses thalamus and diffuse  projection to cortex
    • part-specific thalamic nuclei-specific areas of cortex
  • -intra laminate thalamic nuclei diffuse projection to whole neocortex.

AROUSAL AND EEG

Arousal
  • EEG arousal is not same as behavioral arousal e.g. REM sleep.paradoxical sleep with fast waves and slow voltage
    • Behavioral arousal depends on posterior hypothalamus
  • EEG arousal depends on midbrain tegmentum- RAS
    • painful stimulation in sleep-
  • behavioral arousal with sleep EEG- if midbrain tegmentum is cut
  • EEG arousal with behavioral arousal- if posterior hypothalamus is inhibited
AROUSAL EEG

CAUSED BY

  • stimulation of specific afferent to mid brain level
  • stimulation specific thalamic relay nuclei • stimulation of cortex primary receiving areas BUT high frequency stimulation of :
  • reticular formation-midbrain tegmentum base
  • non-specific thalamic nuclei i.e. RAS is responsible hence decrease midbrain tegmentum-coma-decrease of RAS

RAS

  • gets inputs that converge from: – ascending sensory spinal fibers
    • special senses
  • these are non-specific stimulation of RAS by any sensory input cf. modality specific sensory pathways
    • part of RAS bypasses thalamus and diffuse  projection to cortex
    • part-specific thalamic nuclei-specific areas of cortex
  • -intra laminate thalamic nuclei diffuse projection to whole neocortex.
  • Arousal
  • EEG arousal is not same as behavioral arousal e.g. REM sleep.-paradoxical sleep with fast waves and slow voltage
    • Behavioral arousal depends on posterior hypothalamus
  • EEG arousal depends on midbrain tegmentum- RAS
    • painful stimulation in sleep-
  • behavioral arousal with sleep EEG- if midbrain tegmentum is cut
  • EEG arousal with behavioral arousal- if posterior hypothalamus is inhibited

RAS

  • stimulation of specific afferent to mid brain level
  • stimulation specific thalamic relay nuclei • stimulation of cortex primary receiving areas
  • BUT high frequency stimulation of :
  • reticular formation-midbrain tegmentum base
  • non-specific thalamic nuclei i.e. RAS is responsible hence decrease midbrain tegmentum-coma-decrease of RAS
    • gets inputs that converge from:
  • ascending sensory spinal fibers
  • special senses
  • these are non-specific stimulation of RAS by any sensory input cf. modality specific sensory pathways
    • part of RAS bypasses thalamus and diffuse  projection to cortex
    • part-specific thalamic nuclei-specific areas of cortex
  • -intra laminate thalamic nuclei diffuse projection to whole neocortex.

RAS AND CORTICAL FUNCTION

  • Arousal:
  • Frequency and amplitude shift and 35HZ frequency from slow waves
  • Thalamo-cortical feed-back
  • Increased sensitivity of cortical neurons
  • High frequency small waves enable all cortex to receive more simultaneous sensory information from thalamus and more coordinated fire from cortical neurons.
  • When all parts of the cortex are firing in this co-coordinated manner, one experiences consciousness and conscious perception.
  • But meditation- transidential consciousness with parietal lobe.

AROUSAL AND CORTEX STIMULATION

  • Orbital surface of frontal lobe- monkeys
  • Superior temporal gyrus
  • Electrical stimulation increases RAS and EEG arousal leading to sleeping- waking up with motor activity but if awake stimulation has no effect thus neocortical events can cause arousal.
  • This type of cortical activity is emotional and psychic and not generated by external stimulation.  Cortex can sometimes wake up “alone” – no ability to move
  • NB: circadian rhythm of supra-chiasmatic nucleus has intrinsic 25-30hrs cycle modified by light from retinohypothalamic pathway.

EEG SYNCHRONIZATION( WHY WAVE 

FORMS OCCUR)

  • Waveforms come from a multitude of neurons and dendrites firing in unison. Caused by :
  • Rhythmic thalamic fire e.g. reticular nuclei
  • Influence of active cortical cells on their surrounding via collaterals reccurent inhibition followed by rebound hyper excitation.
  • To a small extent, electro-tonic spread of depolarization to parallel axons and dendrites in a volume card – dipoles. Physiology of  slow waves sleep and EEG associated with it.

REFER ENCE WIKIPEDIA

  • German physiologist and psychiatrist Hans Berger (1873–1941) recorded the first human EEG in 1924.[6]
  • Berger also invented the electroencephalogram (giving the device its name), an invention described “as one of the most surprising, remarkable, and momentous developments in the history of clinical neurology”.[7]
  • In 1934, Fishfirst demonstration of EEG epileptiform “spikes”. 
  • In 1935 inter ictal (ictus means epileptic seizure) “spike waves” and the 3 cycles/s pattern of clinical absence seizureswere described
  • In 1936 the interictal spike as the focal signature of epilepsy. The same year, the first EEG laboratory opened at Massachusetts General Hospital.
  • Then followed the development of the earliest EEG machines and recorders

SOURCE OF EEG ACTIVITY

  • The brain’s electrical charge is maintained by billions of neurons. Neurons are electrically charged (or “polarized“)
  • The brain behaves like an electrical volume conductor. 
  • Waves of depolarization and repolarization in the mass conductor picked by electrodes placed on the scalp, as a constantly changing voltage The Recording trace of these voltage changes are the EEG. The voltage generated by an individual neuron is far too small to be picked up by EEG electrodes 
  • The  EEG is produced by the summation of the synchronous activity of thousands or millions of neurons that have similar spatial orientation such as the cortical pyramidal cells. 

EEG CHANGES WITH CHANGES IN STATE OF BRAIN ACTIVITY

  • EEG activity shows oscillations which have a variety of frequencies which and wave forms which g=have been classified as Alpha, Beta, Delta, Gamma waves . 
  • These waves spatial distributions and are associated with different states of brain functioning (e.g., 
  • waking and the various sleep phases or stages). These synchronized oscillations represent activity over a network of neurons. 
  • The neuronal networks underlying some of these oscillations are jnown(e.g.,
    • the thalamocortical resonance underlying sleep spindles that are seen some stages of NON_REM sleep,
    • The PGO (Ponto Genicolo Occipital lobe) spikes of REM sleep
  • Others are not (e.g., 
  • The Gamma waves (in alert person) and Delta waves( in stage four of deep non-REM sleep) are associated with cerebral neuron electrical activity. 

CLINICAL USES OF EEGs

A routine clinical EEG recording involves recording from scalp electrodes. 

Electroencephalography is used in the following clinical circumstances:

  • Distinguish epileptic seizures from other similar conditions e.g.:1. psychogenic non-epileptic seizures,
    1. syncope (fainting) 
    2. sub-cortical movement disorders and
    3. migraine variants.
  • Differentiating real encephalopathy or delirium from primary psychiatric syndromes such as catatonia
  • Serving as one of the main test for diagnosing brain death
  • Predicting the likely  eventual outcome of treating some coma patients
  • Determining whether to gradually withdraw of anti-epilepsy medication
  • Continuous EEG monitoring including recording of siezure EEGs sometimes done to get s better picture of the patiens . This can be complimented with simultaneous audio-visual recordings.) c
  • This technique gives amore comprehensive information on the condition  including possible localization of the focal point of the

CLINCAL ISE OF EEG

EEG EPILEPSY MONITORING USED FOR:

  • distinguishin epilepsy seizures from other conditions e,g.
  • psychogenic non-epileptic seizures,
  • syncope (faintin) 
  • sub-cortical movemet disorders and
  • Different types of migraine
  • determining the typeof epilepsy so as to determine the right treatment
  • Determinung whether the epilepy had a distinct focus which could be removed  surgically. 

OTHER CONDITIONS AMENABLE TO EEG MONITORING

  • depth of anesthesia
  • cerebral perfusion in operation for widening artheroma-clogged carodit arterial system
  • Effect of  amobarbital injection into each carotid in turn for testing cerebral dominance (WADA TEST)
  • brain function in ICUs patients 
  • non-convulsive seizures including non-convulsive status epilepticus
  • the effect of sedative/anesthesia in patients in medically induced coma 

(for treatment of increased intracranial pressure or unmanagable seizures

CLINICAL USE OF ECoG

FOR MORE ACCURATE LOCALIZATION OF 

SIEZURE LOCUS BY NEUROSURGEONS

  • Direct recording from the surface of the brain
  • Deeper recording through implanted electrodes
  • Craniotomy or “burr holes”  used before such procedures are performed…..explain

METHOD

  • Electrodes placed on point on the scalp lubricated with a conductive gel or paste. Number of electrodes vary according to intensity of information  required
  • Each electrode is attached to an individual wire
  • Special head Caps or Nets used to hold the electrodes in orderly
  • Electrode locations and names are specified by the International 10–20 system
  • In most clinical applications, 19 recording electrodes (plus ground and system reference) are used. Fewer canbe used in babies etc.

METHODS…cont

  • Each electrode is connected to one input of a differential amplifier (one amplifier per pair of electrodes
  • a common system reference electrode (“earth terminal”) is connected to the other input of each differential amplifier)
  • In analog EEG, the signal is then filtered (next paragraph), and the EEG signal is output as the deflection of pens as paper passes underneath the EEG signal is output as the deflection of pens as paper passes underneath. 
  • For modern digital systems, the amplified signal is digitized via an analogto-digital converter
  • The digital EEG signal is stored electronically and can be filtered for display During the recording, a series of activation procedures may be used such . 
  • procedures for inducing normal or abnormal EEG activity that might not otherwise be seen. These procedures include 
    1. Hyperventilation
    2. Photic (light) stimulation at certain frequencies (using a strobe light)
    3. Eye closure and opening
    4. Mental activity like mental arithmetic 
    5. Sleep and sleep deprivation. 
    6. In epilepsy monitoring in hospitalized individuals,  seizure treatment may be deliberately withheld  to induce seizure or study any other effects

EEG ANALYSIS

  • A typical adult human EEG signal is about 10 µV to 100 µV in amplitude when measured from the scalp and is about 10–20 mV when measured from subdural electrodes (i.e. ECoG)
  • The EEG recording can be analysed using various programs; 

e.g., using free open-source software or commercial software packages

  • EEG voltage signal represents a difference between the voltages at each of the pairs of electrodes used 
  • Therefore, the display of the EEG for the reading encephalographer may be set up in one of several ways
  • The representation of the EEG channels is referred to as a montage.

COMMON MONTAGES

SQUENTIAL MONTAGE

  • Each channel (i.e., waveform) represents the difference between two adjacent electrodes. The entire montage consists of a series of these channels. 

REFERENTIAL MONTAGE

  • Each channel represents the difference between a certain electrode and a designated reference electrode. There is no standard position for this reference; it is, however, at a different position than the “recording” electrodes. Midline positions are often used because they do not amplify the signal in one hemisphere vs. the other. Another popular reference is “linked ears,” which is a physical or mathematical average of electrodes attached to both earlobes or mastoids

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.

LAPLACIAN MONTAGE

  • Each channel represents the difference between an electrode and a weighted average of the surrounding electrodes.
  • When analog (paper) EEGs are used, the technologist switches between montages during the recording in order to highlight or better characterize certain features of the EEG. 

DIGITAL EEG:

  • All signals are typically digitized and stored in a particular (usually referential) montage; since any montage can be constructed mathematically from any other, the EEG can be viewed by the electroencephalographer in any display montage that is desired.

INTERPRETATION OF RESULTS

  • The EEG is read by a clinical neurophysiologist or neurologist (depending on local custom and law 
  • Done through  visual inspection of the waveforms  (graphoelements). 
  • Computer signal processing of the EEG—so-called quantitative EEG—also used.
BANDFrequency (Herts)lLocationNormallyPathologically
DELTALess than 4Hz•Frontally in adults•Posteriorly in children •High amplitude waves•Adult slow wave sleep •In babies •During some continuousattention tasks•Sub-cortical lesions •Metabolic •Encephalopat hy•Hydrocephalu s•Deep midline lesions
THETA4-7 Hz•Found in locations not related to task being performed•Higher in young children •Drowsiness in adults •Idling•Focal subcortical lesions •Metabolic encephalopath31 y e,g. in renal 

COMPARISON TABLE OF EEG RHYTHMIC ACTIVITY FREQUENCY BANDS(Wikipedia)

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