EEG,%20SLEEP,%20EVOKED%20POTENTIALS

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EEG, SLEEP, EVOKED POTENTIALS
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EEG
Registration of electrical brain potentials It
reflects function properties of the brain
Richard Caton 1875 1. Registration of ECoG and
evoked potentials
Hans Berger 1929 human EEG, basic rhythm of
electrical activity alfa (8-13Hz) and beta (14-30)
After 1945 EEG as a clinical inspection
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EEG activity is mostly rhytmic and of sinusoidal
shape
rhythm ? 8-13 Hz
Rhythm ? 14-30 Hz
rhythm ? 4-7 Hz
rhythm ? 3 and less Hz
rhythm ?, rolandický rytmus 8-10 Hz
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Normal EEG lokalization of graphoelement types
Frontal - ? activity
Sevrení pesti
Uvolnení pesti
parietal ?, rolandic rhythmus
Temporal - ?,? activity
Otevrení ocí
Zavrení ocí
Temporo-parieto- occipital - ? activity
Podle Faber Elektroencefalografie
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Epilepsy
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Epilepsy seizure petit mal (absence)
Spike and wave activity
The seizure was clinically manifest as a staring
spell
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SLEEP
The age-old explanation until 1940s sleep is
simply a state of reduced activity
Nathaniel Kleitman in early 1950s made remarkable
discovery Sleep is not a single process, it has
two distinct phases REM sleep is characterized
by Rapid Eye Movements Non-REM sleep
Moruzzi in late 1950s studied reticular
formation rostral portion (above the pons)
contributes to wakefulness. Neurons in the
portion of RF below pons normally inhibit
activity of the rostral part
Sleep is an actively induced and highly organized
brain state with different phases
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Sleep follows a circadian rhythm about 24 hours
Circadian rhythms are endogenous persist
without enviromental cues pacemaker, internal
clock suprachiasmatic ncl. hypothalamus Under
normal circumstances are modulated by external
timing cues sunlight retinohypothalamic tract
from retina to hypothalamus (independent on
vision)
Resetting of the pacemaker Lesion or damage of
the suprachiasmatic ncl. animal sleep in both
light and dark period but the total amount of
sleep is the same suprachiasmatic ncl. regulates
the timing of sleep but it si not responsible for
sleep itself
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Average evoked potentials
Event-related potentials Routine procedure of
clinical EEG laboratories from 1980s Valuable
tool for testing afferent functions
EEG changes bind to sensory, motor or cognitive
events
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• Electrical activity electrodes placed on the
  patients scalp
• Evoked electrical activity appears against a
  background of spontaneous electrical activity.
• Evoked activity a signal
• Background activity a noise
• Signal lower amplitude than noise, it may go
  undetected (hidden or masked by the noise)
• Solution
• - by increasing amplitude of the signal
  intensity of stimulation
• by reducing the amount of the noise

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Signal averaging Mixture of electrical activity
composed of spontaneously generated voltages and
the voltage evoked by stimulation
Segments or epochs of equal duration Start
coincides with the presentation of
stimulus Duration varies from 10 to hundrets
milliseconds
Brains spontaneous electrical activity is random
with respect to the signal sum of many cycles
will tend to cancel out. (to zero) The polarity
of the EP will always be the same at any given
point in time relative to the evoking
stimulus Evoked activity will sum linearly
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• How to reduce the amount of the noise
• Superimposition

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How to reduce the amount of the noise
Simplified diagram illustrating how coherent
averaging enhances a low level signal (coherent
EP time locked to the evoking stimulus)
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Description of waveforms peaks (positive
deflection) troughs (negative deflection) Measure
s 1. Latency of peaks and troughs from the time
of stimulation 2. Time elapsing between peaks
and/or troughs 3. Amplitude of peaks and
troughs Comparison of the patients recorded
waveforms with normative data
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Visual-evoked potentials (VEP) Anatomical basis
of the VEP
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Visual-evoked potentials (VEP) Electrical
activity induced in visual cortex by light stimuli
Rods and Cones
Retina
Anatomical basis of the VEP
Bipolar neurons
Ganglion cells
Optic nerve
Anterior visual pathways
Optic chiasm
Optic tract
Lateral geniculate body
Optic radiation
Retrochiasmal pathways
Occipital lobe, visual cortex
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Visual-evoked potentials (VEP)
Stimulus checkerboard pattern on a TV
monitor The black and white squers are made to
reverse A pattern-reversal rate from 1to 10 per
second
Electrodes - 3 standard EEG electrodes placed
over the occipital area and a reference elektrode
in a midfrontal area
Analysis time (one epoch) is 250 ms Number of
trials 250 , 2 tests at least to ensure that the
waveforms are replicable
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Normal VEP
VEPs to pattern-reversal, full-field stimulation
of the right eye
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Abnormal VEPs
Absence of a VEP Prolonged P 100 latency -
demyelination of the anterior visual pathways
Amplitude attenuation - compressive lesions
Prolonged P 100 only on left or right eye
stimulation lesion of the ipsilateral optic
nerve Excessive interocular difference in P 100
latency lesion of the ipsilateral optic nerve
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VEPs as a tool in the diagnosis
of multiple sclerosis Excessive interocular
difference in P100 latency Prolonged absolute
latency Decreased amplitude
Compression of optic nerve, optic chiasm (tumor
of pituitary gland or optic nerve
glioma) Decreased amplitude Prolonged latency of
P100
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Brain-stem auditory-evoked potential BAEP
Short-latency somatosensory-evoked potential SSEP
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Short-latency somatosensory-evoked potential SSEP
Left median nerve study