EEG recordings in man

1
EEG recordings in man

• Examples of records and results of analysis made
  by Braintune (St. Petersbuurg) hardware/software.

2
Nyquist Theorem

• The highest frequency which can be accurately
  represented is one-half of the sampling rate.
• The sampling rate here is below the Nyquist
  frequency, so the result of sampling is nothing
  like the inputaliasing.
• For practical purposes the sampling rate should
  be 10 higher than the highest frequency in the
  signal.

3
EEG is a difference in potential between two
electrodes

• If two electrodes are active, it is called
  bipolar recording.
• If one electrode is silent, it is called
  monopolar recording. The reference sites ear
  lobe, mastoid, nose.

4
Bipolar vs. monopolar recordings

• Monopolar recording is used in research, because
  it enables the researcher to localize the event
  of interest.
• Bipolar recording is used in BF, because it
  reduces shared artifacts. Electrodes should be
  placed on the sites with the strongest gradients
  of the potentials under training.

5
EMG and eye blink artifacts in EEG

• These types of artifacts can be detected by
  visual inspection.
• Eye blinks can be excluded from data analysis.
• EMG should be taken into account during spectral
  analysis.

6
EEG recorded at Cz

• EEG is a mixture of waves at different
  frequencies and amplitudes.

7
EEG recorded at T5

• At each time interval several sine-waves at
  different frequencies may be present in the
  signal.

8
Quality control of EEG recording

• 1) EEG amplifies must be calibrated with daily
  checks
• 2) acquisition parameters must be checked daily
  and keep the same
• 3) the same procedures must be employed in all
  individuals
• 4) all artifacts must be eliminated or taken into
  account prior to spectral analysis.

9
EMG Artifact

• EMG artifact starts as low as 12 Hz and ranges to
  300 Hz. Most of the spectrum lies between 30-150
  Hz.
• Sites F3, F4, T3, T4, P3, P4 can pick up EMG the
  massester and temporalis muscles.
• Posterior electrodes can pick up EMG from
  occipitalis, trapezius and supraspinal muscles.
• To avoid this type of artifact one can relax or
  position the head properly or change slightly the
  position of electrode.
• Fz, Cz, Pz can give a relatively pure EEG signal.

10
EKG artifact

• ECG artifacts occur from the electrodes that pick
  up activity from underlying pulsating blood
  vessels in the scalp.
• EKG artifact gets more prevalent with aging.

11
Ocular, blinks and electroretinal activity

• Eye movement and blinks artifacts occur in the
  delta range 0-4 Hz and occur over the anterior
  part of the scalp.

12
The noise from the standard AC electrical line
current

• This noise can be diminished by the proper
  grounding of the equipment (both computer and
  amplifies).
• It could be also eliminated by a so called notch
  filter which selectively removes 50 (60 for the
  US) Hz activity from the signal.
• This noise could be attenuated by obtaining good
  contact of electrodes with the scalp. The
  electrode impedance less than 10 kOhms is
  desirable.

13
EEG recording in man

• Eyes opened condition.
• Examples of different waves.

14
Reviewing EEG

• EEG is characterized by
• 1) voltage
• 2) frequency (is used for BF)
• 3) spatial location (is used for BF)
• 4) inter-hemispheric symmetries
• 5) reactivity (reaction to state change)
• 6) character of waveform occurrence (random,
  serial, continuous)
• 6) morphology of transient events

15
Reviewing EEG voltage

• Amplitude is the voltage in microvolts measured
  from the peak of the wave to the trough of the
  wave. Varies from 10 mcV to 100 mcV with average
  around 20-50 mcV.

16
Reviewing EEG frequency

• Spectrums reflect the amount of energy in a
  certain frequency range of EEG.
• Term monorhythmic means that a particular portion
  of EEG shows a rhythmic component in a singular
  frequency.
• Term polyrhythmic means that several rhythmic
  frequencies are present in EEG.
• The presence of large-amplitude delta-activity
  may indicate infarct or other lesion.

17
Reviewing EEG frequency

• Slow (0-4 Hz) and high (more 20 Hz) frequency
  bands of EEG may pick up artifacts, such as eye
  movements and muscle activity, and therefore
  should be evaluated with caution.
• Despite the use of artifact rejection algorithms,
  the failure to accurately distinguish true
  physiological rhythmicity from the artifacts is a
  serious shortcoming of current software systems
  and requires the expert assessment.

18
Reviewing EEG transient events

• A transient is an isolated form or feature that
  stands out from the background activity.
• It is called a spike if it has the duration less
  than 70 msec.
• It is called a sharp wave if it has the duration
  between 70 and 200 msec.
• The presence of large amplitude spikes and waves
  may indicate the presence of epilepsy.

19
Maps of EEG spectrums in standards bands

• Eyes opened condition.

20
EEG recording in man

• Eyes closed condition.
• Enhancement of alpha waves.

21
Maps of EEG spectrums

• Eyes closed condition

22
EEG spectrums

• Three conditions (EC, R, M) are compared to Eyes
  Opened condition.
• Two peaks (in theta and alpha band) with
  different scalp distribution are observed.
• Reading and math produce big (but different)
  changes in alpha band and small changes in theta
  and beta bands.
• Note that alpha activities are different for all
  four conditions both in distribution and
  frequency.

23
EEG spectrums in individual bands

• Regular theta - idling rhythm
• Irregular theta - working activity
• Reading and math produce alpha rhythms that are
  different in frequency and location.

24
EEG as a sequence of micro-states

• EEG consists of series of short-lasting
  quasi-stationary epochs corresponding to what
  Lehmann et al. (1987) have called brain
  functional micro-states.
• EEG reflects the changes in the state of neuronal
  networks rather than specific aspects of
  information processing.

25
Normal distribution

• When many independent random factors act in an
  additive manner to create variability, data will
  follow a bell-shaped distribution called the
  Gaussian distribution. This distribution is also
  called a Normal distribution.
• Although no data follows that mathematical ideal,
  many kinds of data follow a distribution that is
  approximately Gaussian

26
Dysfunction as a deviation from normal
distribution

• If we measure some parameter in the population
  with some brain dysfunction, then this parameter
  must has a different, not Gaussian distribution.
• There are statistical tests that measure this
  difference.

27
Bimodal distribution in ADHD?

• Clinicians who diagnose this disorder have been
  criticized for merely taking a percentage of the
  normal population who have the most evidence of
  inattention and continuous activity and labeling
  them as having a disease. In fact, it is unclear
  whether the signs of ADHD represent a bimodal
  distribution in the population or one end of a
  continuum of characteristics. This is not unique
  to ADHD as other medical diagnoses, such as
  essential hypertension and hyperlipidemia, are
  continuous in the general population, yet the
  utility of diagnosis and treatment have been
  proven. Nevertheless, related problems of
  diagnosis include differentiating this entity
  from other behavioral problems and determining
  the appropriate boundary between the normal
  population and those with ADHD.

28
Life span normative EEG database (LNDB)

• There are at least four eyes-closed LNDB
• 1) E. Roy John et al. (1977)
• 2) Frank Duffy et al. (1994)
• 3) Robert Thatcher et al. (1987)
• 4) Barry Sterman et al. (199?)

29
Three goals of LNDB

• 1) to assess the neurological basis for the
  patients complains (the issue of organicity)
• 2) to identify the weakness of electrophysiologica
  l organization of the brain (the issue of
  neurotherapy design)
• 3) to evaluate the efficacy of treatment ( the
  issue of treatment evaluation)
• Thatcher, 1999

30
Active and passive conditions for NDB

• 1) Eyes opened and eyes closed conditions are
  often used in NDB, because of simplicity and
  relative uniformity of recording conditions.
• 2) Active tasks depend of many uncontrolled
  factors, such as intensity of stimuli, the
  subjects involvement, the distance from stimuli,
  etc. There are no standards for active
  conditions.