Workshop 2: EEG

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Workshop 2 EEG

• Keith Clements

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The Electroencephalogram

• Rhythmic voltage changes recorded from the scalp
  surface.
• Generated by synchronised activity of large
  numbers of neurons.
• Traditionally divided into different bands
• Delta 0.5-4 Hz
• Theta 4-8 Hz
• Alpha 8-13 Hz
• Beta 13-30 Hz
• Gamma 30-50 Hz

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Sources

• Not generated by action potentials? High
  frequencies are poorly conducted.
• Mainly thought to reflect synaptic potentials
• Alpha produced by thalamo-cortical links,
  generated by the thalamus.

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Technicalities

• Low voltage signal. Good electrical contact is
  essential
• Recording may be bipolar (between two active
  sites) or monopolar (using a neutral reference
  site).
• Sources of artifact
• 50Hz hum
• Muscle potentials
• Eye movements

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Alpha

• Around 70 of EEG activity
• Alpha blocking referred to as desynchronisation.
• Typically attenuated when the eyes are open

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Applications

• Sleep stages
• Distinct changes in the EEG during sleep

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Applications

• Hemispheric differences e.g. Davidson et al
  (1990) found that positive emotions were
  associated with activation of frontal areas of
  the left hemisphere, while disgust was associated
  with right hemisphere activation.
• Depression associated with reduced activity
  (increased alpha) in the left frontal lobe
  (Henriques Davidson, 1990).
• Valence or approach/avoidance?
• Trait anger associated with increased left
  frontal activity (Harmon-Jones Allen, 1998)

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The workshop

• Well be recording EEG using bipolar leads from
  the occipital lobe.
• Well record during three states relaxed eyes
  closed, relaxed eyes open and during mental
  arithmetic.
• The signal will be filtered to separate activity
  in each of four bands (delta, theta, alpha
  beta)
• As well as standard deviation, as a measure of
  amplitude, we will also measure frequency,
  calculated from the length of one cycle during
  the initial segment.

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SD x 4
SD x 4
SD x 4
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One cycle, gives frequency. Zoom in as needed for
different bands. Repeat three times to allow you
to work out an average frequency.
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References

• Davidson, RJ. et al. (1990). Approach/withdrawal
  and cerebral asymmetry Emotional expression and
  brain physiology. Journal of Personality Social
  Psychology, 58, 330-341
• Harmon-Jones, E. Allen, J.J.B. (1998). Anger
  and frontal brain activity EEG asymmetry
  consistent with approach motivation despite
  negative affective valence. Journal of
  Personality Social Psychology 74, 1310-1316.
• Henriques J.B. Davidson, , R.J. (1990).
  Regional brain electrical asymmetries
  discriminate between previously depressed and
  healthy control subjects. Journal of Abnormal
  Psychology, 99, 22-31.