History of the Electroencephalogram

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History of the Electroencephalogram
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• The spontaneous electrical activity of the brain
  was first observed by Caton in 1875.

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• In 1929, a German psychiatrist named Hans Berger,
  who worked in the city of Jena, published a paper
  reporting that it was possible to record the
  feeble electric currents generated on the brain,
  without opening the skull, and to depict them
  graphically onto a strip of paper. Berger named
  this new form of recording as the
  electroencephalogram (Electro for electrical
  activity, encephalo for brain and gram for
  measure-record EEG).

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EEG reflects what?

• EEG is a recording of electrical activity from
  populations of neurons.
• No consistent relationship between the EEG
  activity as recorded by macro-electrodes and the
  unit spike activity recorded by intra- or
  extra-cellular microelectrodes has been found. A
  relationship was found between the EEG and the
  EPSPs.
• The EEG seen on the scalp is an average of the
  activity of many small zones of the cortical
  surface beneath the electrode.

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How one records EEG?

• The electroencephalogram (EEG) records variable
  potential differences between two electrodes
  placed on the scalp.
• In bipolar recording and monopolar recording.
• Each pair of electrodes is connected to an
  amplifying system
• The potential difference is then displayed on an
  ink-writing oscillograph or on an electronic
  oscilloscope.

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Classification of EEG

• Oscillations in electrical potential that occur
  almost continuously between any two recording
  electrodes (i.e., EEG) differ in frequency (Hz)
  and amplitude from place to place and in
  different states of awareness.

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• Alpha waves - are those between 7.5 and 13 waves
  per second (Hz).
• Alpha is usually best seen in the posterior
  regions of the head on each side, being higher in
  amplitude on the dominant side.
• It is brought out by closing the eyes and by
  relaxation, and abolished by opening the eyes
  and/or by any alerting stimulation.

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• Beta activity is 'fast' activity. It has a
  frequency of 14 and greater Hz. It is usually
  seen on both sides in symmetrical distribution
  and is most evident frontally.

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• Theta activity has a frequency of 3.5 to 7.5 Hz
  and is classed as "slow" activity. It is abnormal
  in awake adults but is perfectly normal in
  children up to 13 years and in sleep. It can be
  seen as a focal disturbance in focal subcortical
  lesions hydrocephalus.

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• Delta activity is 3 Hz or below. It tends to be
  the highest in amplitude and the slowest waves.
  It is quite normal and is the dominant rhythm in
  infants up to one year and in stages 3 and 4 of
  sleep. It may occur focally with subcortical
  lesions hydrocephalus

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Alpha Beta Theta Delta
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EEG variations in sleep

• Non-Rapid Eye Movement Sleep -NREM sleep itself
  is conventionally subdivided into several
  different stages on the basis of EEG criteria.
• In the adult, stage 1 is observed at sleep onset
  or after momentary arousals during the night and
  is defined as a low-voltage mixed-frequency EEG
  tracing with a considerable representation of
  theta wave (four to seven hertz, or cycles per
  second) activity.
• Stage 2 is a relatively low-voltage EEG tracing
  characterized by intermittent, short sequences of
  waves of 1214 hertz (sleep spindles) and by
  formations called K-complexesbiphasic wave forms
  that can be induced by external stimulation, as
  by a sound, but that also occur spontaneously
  during sleep.
• Stages 3 and 4 consist of relatively high-voltage
  (more than 50-microvolt) EEG tracings with a
  predominance of delta wave (one to two hertz)
  activity the distinction between the two stages
  is based on an arbitrary criterion of amount of
  delta-wave activity, with greater amounts
  classified as stage 4.

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• Unlike the basic distinction between NREM and
  REM, differences among NREM sleep stages
  generally are regarded as quantitative rather
  than qualitative.
• REM sleep paradoxical high frequency beta
  waves

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Abnormal EEG

• Spike and wave - format is seen at all ages but
  most often in children. It consists of a spike,
  which is probably generated in the cortex, and a
  large amplitude slow wave (usually delta),
  thought to originate from thalamic structures,
  occurring recurrently. They may occur
  synchronously and symmetrically in the
  generalized epilepsies or focally in the partial
  ones. In the generalized types of spike and wave,
  true absence (petit mal) is characterized by 3 Hz
  spike-wave, while slow spike-wave occurs more
  usually with brain injury.

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• PLEDS - Periodic Lateralized Epileptiform
  Discharges - are a form of discharge associated
  with acute brain injury or damage. The pattern is
  also said to be most evident when acute brain
  injury is coupled with some metabolic
  derangement.

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ARTIFACTS

• Artifacts are disturbances caused by technical
  defects - usually transitory.
• Included are such things as electrode movement,
  with loss of contact, muscle activity obscuring
  the EEG, movements of the head, scratching the
  scalp, sweating etc.
• Sweat artifacts - Which represent salt solution
  between electrodes shorting them out.

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