ELECTROENCEPHALOGRAPHIC (EEG) COHERENCE STUDY OF WORKING MEMORY BRAIN OSCILLATIONS

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ELECTROENCEPHALOGRAPHIC (EEG) COHERENCE
STUDY OF WORKING MEMORY BRAIN OSCILLATIONS

• Dr. Simon Brežan
• Institute of Clinical Neurophysiology,
• University Medical Centre Ljubljana, Ljubljana,
  Slovenia
• coauthors Vita Štukovnik, Veronika Rutar, Jurij
  Dreo, Vito Logar, Blaž Koritnik, Grega Repovš,
  Blaž Konec, Janez Zidar
• INTERNATIONAL NEUROSCIENCE CONFERENCE, SINAPSA,
  LJUBLJANA 2005

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WORKING MEMORY (WM)

• memory processes encoding, storage, recall
• memory structure sensory memory (attention)gt
  short-term memory (rehearsal/replacement)gt
  long-term memory
• active role of short term memory working
  memory- central for intelligent goal-directed
  behaviour, coherent thoughts, language etc.
• DEFINITION complex of cognitive processes for
  time- and capacity- limited maintenance,
  manipulation and utilization of mental
  representations

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MODEL OF WORKING MEMORY (BADDELEY, 2000)

• central executive (CE) attentional control of
  subsystems, manipulation of information,
  planning, strategy selection, inhibition
• slave subsystems
• phonological loop 2 separated components
  storage and rehearsal of verbal information
• visuospatial sketchpad separated storage and
  rehearsal of visual and spatial information
• episodic buffer integration of information from
  other subsystems and episodic long-term memory

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NEUROPHYSIOLOGICAL AND NEUROANATOMICAL BASIS OF
WORKING MEMORY

• NEUROPHYSIOLOGICAL VIEW
• basic neurophysiological mechanism of WM
  repeated reverberations of electrical impulses in
  reverberational (feedback) loops (Štrucl, 1999)?
• repeated excitation of a synapse in excitational
  loopgt increase of excitatory postsynaptic
  potential (EPSP) postsynaptic facilitation.
• postsinaptic facilitation preservation/maintenan
  ce of specific information in WM?
• role of active repeating?

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• NEUROANATOMICAL VIEW
• Cell electrophysiological and functional brain
  imaging studies (Fletcher and Henson, 2001, etc.)
• various components of working memory different
  anatomically separated neuronal networks
• specific brain activity
• (pre)frontal (VLFC, DLFC, AFC) cortex
• premotor cortex
• limbic cortex and other subcortical structures
• posterior association parietal areas
• hypothetical lateralization of functions (Postle
  et al., 2000)
• verbal information (phonological loop) left
  hemisphere
• visuospatial information (visuospatial
  sketchpad) right hemisphere
• central executive heteromodal association cortex
  of (pre)frontal brain regions (Gathercole, 1999)?

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BINDING PROBLEM

• BINDING PROBLEM The mechanisms for functional
  integration (binding) of different brain areas,
  responsible for specific (WM) functions?
• Paralell and distributed processing of
  information in the brain
• Functional integration - coupling (visual
  perception, complex motor patterns, visuo-motor
  integration, cognitive functions) key for
  understanding brain functioning
• The code for functional coupling synchronised
  oscillations of neuronal networks between
  anatomically separated brain areas?
• Measure of synchronised brain oscillations EEG
  coherence

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ELECTROENCEPHALOGRAPHY (EEG), SIGNAL ANALYSIS
EEG COHERENCE AND POWER SPECTRA

• EEG repeated, periodic electrical activity of
  (pyramidal) cortical neurons
• activity of many neurons (synaptic EPSPs, IPSPs)
  ionic currentsgt field potentials, macropotential
  (EEG signal)
• intrinsic qualities of neurons, dynamic
  interactions between neuronal networks- changing
  pattern of synchronization and desynchronization
  of regional brain cells- amplitude changes of
  specific frequency bands.
• EEG - great time resolution (milisec), distinct
  patterns of activity
• brain rhythms, frequency bands for oscillations
  (delta 0,5-4 Hz, theta 4-7 Hz,
  alpha 8-13 Hz, beta 13-30 Hz, gamma 30-50 Hz)
• specific functional, behavioral, spatial
  correlates- switching neural networks between
  different functional states- activating or
  inhibiting neural systems?

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EEG COHERENCE AND POWER SPECTRA

• POWER SPECTRUM - degree of representation (power)
  of specific frequency band in the signal basic
  input data for coherence calculation
• gt different levels of regional cortical activity
  or different level of regional synchronization-act
  ivation or inhibition of neural networks
• EEG COHERENCE measure of degree of similarity,
  phase-locking (synchronization) of 2 distant
  signals for specific frequency band
• gt different degrees of long-range
  synchronization of oscillations between separate
  cortical regions for specific frequency band
• measure of functional coupling binding and
  communication between separated brain centers
• 2 different operational systems of the brain

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Cxy(?) coherence value between signals x and y

• Fxy(?)- value of cross-correlation power spectrum
  of signals x, y
• Fxx(?)- value of auto-correlation power spectrum
  of signal x
• Fyy(?)- value of auto-correlation power spectrum
  of signal y

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WORKING MEMORY AND SYNCHRONIZED BRAIN
OSCILLATIONS POWER SPECTRA AND EEG COHERENCE
STUDIES

• synchronous oscillations- correlation with
  specific behavioural contexts and cognitive tasks
  numerous studies
• The neurophysiological theory of (working)
  memory
• Brain oscillations in different frequency bands
  subserve specific (memory) functions and operate
  over different spatial scales.
• Multiple superimposed synchronized (coherent)
  oscillations in different frequency bands with
  different spatial patterns and functional
  correlates govern specific mental functions.

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EEG WORKING MEMORY STUDIES

• EEG COHERENCE changes
• increases mainly in theta, alpha and gamma band
  (working memory processes) (Serrien et al., 2003
  Sauseng et al., 2004 Sarnthein et al, 1998
  Jensen et al., 2002, etc.)
• changes of power spectra and coherence with
  different memory load (Gevins et al., 1997,
  Jensen, 2000, etc.)
• POWER SPECTRA changes
• decrease in lower alpha band (non-specific effect
  of attention, mental effort) (Klimesch et al.,
  1998, etc.)
• decrease in upper alpha band (correlate of
  semantic processing) (Basar et al., 2000, etc.)
• or increase in alpha band (active inhibition of
  disturbing neural networks not needed for the
  memory task) (Klimesch et al., 1998, etc.)
• increase in theta band frontal midline theta
  rhythm (memory maintenance, attention, mental
  effort) (Klimesch et al. Gevins et al. 1997,
  etc.,)
• increase in gamma band (sensory, perceptional,
  attentional, working memory processes) (Jensen,
  2000 Babiloni et al., 2004, etc.)

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SPATIAL SCALES OF COHERENCE CHANGES IN WM TASKS

• MAINLY (PRE)FRONTO- TEMPORO- PARIETAL INCREASES
  OF THETA, ALPHA COHERENCE
• MAINLY FRONTOCENTRAL INCREASES OF THETA AND GAMMA
  OSCILLATIONS- POWER SPECTRUM INCREASES (FRONTAL
  MIDLINE THETA RHYTHM) THETA SOURCE??
• ? IN ACCORDANCE WITH BADDELEYS MODEL OF WM
  SEPARATE SYSTEMS FOR STORAGE (POSTERIOR) AND
  ACTIVE MAINTENANCE, UPDATING OF INFORMATION
  (FRONTAL BRAIN AREAS) in modal specific
  subsystems
• NEED FOR INFORMATION EXCHANGE, COOPERATION,
  FUNCTIONAL COUPLING BETWEEN ANTERIOR AND
  POSTERIOR BRAIN REGIONS
• LONG RANGE- SLOW RHYTHMS, SHORT RANGE- FAST
  RHYTHMS
• TOP-DOWN CONTROL- CENTRAL EXECUTIVE

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OUR STUDY OF WORKING MEMORY AND BRAIN OSCILLATIONS

• AIM
• To examine the neurophysiological mechanisms of
  working memory processes
• To investigate task-related coherence (and power
  spectra) changes for different EEG frequencies
  during the processes of working memory
• Search for possible differences in coherence (and
  power spectra) changes between maintenance and
  manipulation processes in working memory.
• HYPOTHESES
• Increases in fronto-posterior coherence in
  working memory tasks
• Increases in bilateral (pre)frontal coherence for
  manipulation vs. maintenance-only processes of
  working memory (prefrontal central executive?)

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METHODS

• PARTICIPANTS
• 11 healthy right-handed volunteers (4 males, 7
  females) informed consent, aged between 20-35
• average number of set repetition per each
  task/person 38
• 10 min training of paradigm before recording
• EEG RECORDING
• Dark quiet room, projection of different tasks on
  computer screen- cca. 80 cm from the eyes
• EEG cap (E1-S Electro-Cap) - 29 electrodes,
  standard 10-20 International electrode system
  with extra electrodes Fp1, Fp2, Fz, FCz, Cz,
  CPz, Pz impedance below 5kO
• EEG aparat Medelec (Profile Multimedia EEG
  System, version 2.0, Oxford Instruments Medical
  Systems Division, Surrey, England)
• EOG measurement (6 additional eye electrodes,
  Croft 2000)- 20 min calibration task
• Synchroniztation signal between 2 computers
• Presentation software for paradigm programming

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• COGNITIVE WM PARADIGM
• Modified Sternberg paradigm of working memory
• CONTROL 2 EXPERIMENTAL CONDITIONS
• alternating the type of task coincidentaly
  during recording sessions!
• WAIT control task with no memory demands
  (ignore the set, fixate the cross, relax)
• TASK matching the serial position of goal
  stimulus with simultaneosly presented set of
  letters
• SET (M, D, O) TASK instruction (WAIT)
  FIXATION (5500ms) GOAL (3 D)
  ANSWER (NO)
• ANALYSIS! SET (M D O)
• MEMORIZE rehearsal (retention) of information
  in WM
• TASK matching the serial position of goal
  stimulus with rehearsed originally presented set

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DATA ANALYSIS

• Special independent computer programmes were
  designed for coherence and power spectra
  analysis
• Borland Delphi 7.0 (with EOG artefact correction-
  modified Croft correction procedure) and Matlab
  software (no EOG correction)
• .edf conversion of EEG recordings
• Analysis of 5500ms retention/fixation periods in
  all 3 types of tasks, selection of artefact-free
  epochs

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Borland Delphi 7.0 data analysis

• Our analytical procedure
• Measuring EEG volatage and recording them in .EDF
  files
• Correcting EEG voltages with the RAAA EOG
  correction method
• Dividing the 5 secod retention periods for all
  sets of every task into five 1 second periods.
• Transforming the time-domain EEG signal of all
  five 1 second periods into a frequency-domain
  signal via a Fast Fourier Transform alghoritm.
• Averaging the frequency-domain EEG signals for
  all five 1 second periods for every set of every
  task to obtain the Average-frequency-domain
  signal for that set of that task for each person.
• Calculating Power-Spectra and Coherence for every
  set of every task for each person.
• Averaging of Power-Spectra and Coherence for
  every task from all the sets in that task for
  every person. Thus obtaining the Average-task PS
  and C for every person.
• Averaging of Power-Spectra and Coherence for all
  persons for every task Thus obtaining the
  Average-person PS and C.
• Averaging the Power-Spectra and Coherence in the
  desired frequency bands. Thus obtaining the
  Average-band PS and C that are displayed in our
  results.
• Optionally comparing Average-band PS and C for
  two different tasks.

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Statistical analysis

• To calculate the border coherence-difference
  values that are significant at desired p-levels
  we created from 50.000 to 100.000 (depending on
  the frequency band) randomly distributed
  simulated EEG measurements that fit our
  experimental design exactly
• a total of 11 people, 4 with 56 sets per person,
  7 with 28 sets of one task per person
• per each set an average of five 1 second
  retention periods
• a 1 Hz resolution in the Fourier transform
• From these simulations we obtained a sampling
  distribution curve that fits our experiment
  design as accurately as possible. The
  border-coherence values that are significant at
  desired p-levels were then estimated through a
  two-tailed t-test by calculating the area under
  the sampling distribution curve that fits the
  desired p-level.
• FB 1 Delta (1-4 Hz), Theta (4-7 Hz), Alpha 1
  (7-10 Hz), Alpha 2 (10-13 Hz)
• FB 2 Beta (13-30 Hz)
• FB 3 Gamma (3050 Hz)

Border coherence-difference values at desired p-levels for different frequency bands Border coherence-difference values at desired p-levels for different frequency bands Border coherence-difference values at desired p-levels for different frequency bands Border coherence-difference values at desired p-levels for different frequency bands
p-level Coh. diff. FB 1 Coh. diff. FB 2 Coh. diff. FB 3
lt 0.5 gt 0.0186 gt 0.0091 gt 0.008
lt 0.1 gt 0.0452 gt 0.0221 gt 0.020
lt 0.05 gt 0.0542 gt 0.0263 gt 0.022
lt 0.01 gt 0.0722 gt 0.0348 gt 0.030
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RESULTS

• Increases and decreases of coherence for
  different frequency bands with differences
  between 3 tasks will be showed only for
  statistical significance p lt 0.1 FOR ALL IMAGES
• New schematic model of the head with coherence
  value TASK DIFFERENCES presented
• Colour scale
• warm colours- coherence increases between
  electrodes
• cold colours- coherence decreases between
  electrodes

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COHERENCE CHANGESMEMORIZE VS. WAIT (CONTROL)
TASK (P lt 0.1)

• ALPHA 1
• EXPLAINATION OF SCHEME!
• (Pre)fronto-central
• fronto-parietal
• fronto-occipital increases
• Interhemispheric bitemporal increases
• Temporo- parietal increases

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MEMORIZE VS. WAIT

• ALPHA 2
• Fronto-central increases
• interhemispheric frontotemporal increases
• fronto-parieto-occipital increases
• Temporo-parietal increases

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MEMORIZE VS. WAIT

• GAMMA
• Less intensive increases, dominant
• fronto-parietal
• fronto-temporal
• fronto-central increases

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MEMORIZE VS. WAIT

• THETA
• Fronto- central increases
• Fronto- occipital increases
• Interhemispheric bitemporal increases
• Frontotemporo-parietooccipital increases

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REORDER VS. WAIT

• ALPHA 1
• (pre)fronto-centro-parietal increases
• temporo- central
• interhemisheric bitemporal
• parieto-occipital increases

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REORDER VS. WAIT

• ALPHA 2
• Prefronto-central increases
• Fronto-centro-parietal
• Fronto-temporal
• Centro-temporal
• Interhemispheric bitemporal
• Temporo-occipital

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REORDER VS. WAIT

• GAMMA
• Less intensive but
• simmilar pattern of coherence increases

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REORDER VS. WAIT

• THETA
• Fronto-central
• Fronto-parietal
• Frontotemporo-occipital increases

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REORDER VS. MEMORIZE

• ALPHA 1
• Centro-temporal increase
• Occipito-temporal increase
• Decreases of coherence

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REORDER VS. MEMORIZE

• ALPHA 2
• Fronto-central
• Fronto-temporo-parietal
• Parieto-occipital increases
• Interhemispheric bitemporal increases
• Decreases of coherence

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REORDER VS. MEMORIZE

• GAMMA
• Less intensive increases
• Prefronto-centro-parieto-occipital axis increases

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REORDER VS. MEMORIZE

• THETA
• Mainly
• (Pre)fronto-parietooccipital axis increases

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ADDITIONAL DATA ANAYLSIS AND DIFFERENT WAY OF
DATA PRESENTATION

• //MATLAB SOFTWARE (no EOG correction)
• The influence of EOG correction procedures on EEG
  coherence?
• gtgtSIMMILAR TRENDS IN COHERENCE VALUES

Memorise vs. wait task THETA coherence changes
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• Reorder vs. wait task
• THETA coherence changes

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Reorder vs. memorize task THETA coherence changes
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SUMMARY OF RESULTS

• WM TASKS COHERENCE INCREASES MAINLY IN ALPHA 2,
  ALPHA 1, THETA (AND ALSO GAMMA) FREQUENCY BANDS
• FOR MEMORIZE (WM MAINTENANCE) VS. WAIT (CONTROL)
  TASK
• FOR REORDER (WM MANIPULATION) VS. WAIT (CONTROL)
• SPATIAL SCALES OF INCREASES MAINLY
  FRONTO-POSTERIOR, FRONTOTEMPORAL, BITEMPORAL
  LONG- RANGE CONNECTIONS IN THE BRAIN
• REORDER (WM MANIPULATION) VS. MEMORIZE
  (MAINTENANCE-ONLY) COHERENCE INCREASES MAINLY IN
  ALPHA2 AND THETA BAND
• SPATIAL SCALESALSO ANTERIO-POSTERIOR BRAIN AXIS,
  BITEMPORAL INTERHEMISPHERICALLY, BUT
  NO (PRE)FRONTAL INTERHEMISHERIC INCREASES
• DECREASES OF COHERENCE OTHER AREAS

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INTERPRETATION OF RESULTS

• Greatest coherence (synchronization) increases
  -retention WM periods in ALPHA AND THETA (and
  gamma) frequency bands
• widespread fronto-parietal association brain
  areas involved- in accordance with other studies
  and Baddeleys model of WM!
• temporal interhemispheric connections?
• Different EEG frequencies appear to have
  different functional correlates?
• LIJ (Lisman, Idiart, Jensen, 1998) WM MODEL!!!
• The increased theta coherence -working memory
  processes (storage, rehearsal and scanning)
• Alpha band directly involved in memory processes
  or reflects increased mental effort, attention?
  Role not known yet, results contradictive in
  different studies
• Gamma band is believed to be correlated with
  sensory processing and the very content of
  information processing, but could also reflect
  increased attentiveness.

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• The neuronal synchronization (increased
  coherence) functional coupling
• role in interaction of posterior association
  cortex (where sensory information is stored), and
  (pre)frontal cortex, where relevant current
  information is held, rehearsed and updated
  (Baddeleys model, phonological loop)
• Decreases of coherence functional decoupling of
  disturbing processes, selective attention?

• LATERALIZATION?
• Verbal memory tasks seem to activate primarly
  left brain hemisphere, but visuospatial memory
  tasks activate predominantly right brain
  hemisphere- we didnt demonstrate significant
  lateralization patterns!
• possible reasons?volume conduction, low spatial
  resolution, visuospatial strategies

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Manipulation- CE function

• We found increases of coherence in fronto-
  parietal loops also compared to memorize only
• Central executive also demands funtional
  integration of anterio-posterior neural circuits-
  brain regions and not primarily prefrontal
  interhemispheric communication?
• Role of interhemispheric connections in temporal
  brain regions (alpha 2- semantic memory?)

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LIJ NEUROPHYSIOLOGICAL MODEL OF WORKING MEMORY
(Lisman, Idiart and Jensen, 1998)

• Figure --. Concept of LIJ working memory model.
• Three memory items (A, B, C) are loaded in
  memory buffer, the theta period increases by one
  gamma period with each item added. In retention
  interval (delay period), items are maintained by
  activity-dependent intrinsic properties of the
  neurons coding these items. After probe
  presentation the items can be scanned compared
  with the probe as they are activated. After
  scanning, motor response and answer can be
  initiated. RT reaction time

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• Figure --. LIJ model as a multi-item short-term
  memory buffer.
• Theta and gamma oscillations play an important
  role in the concept. An afterdepolarization (ADP)
  is triggered after a cell fires (sensory input)
  and it causes depolarizing ramp that serves to
  trigger the same cell to fire again after delay.
  These ramps are temporarily offset for different
  memories, an offset that causes different
  memories to fire in different gamma cycles. The
  key function of this buffer is to perpetuate the
  firing of cells in a way that retains serial
  order. The repeat time is determined by theta
  oscillations due to external input. Gamma
  oscillations arise from alternating global
  feedback inhibition and excitation (the cell with
  most depolarized ramp will fire again) because of
  separate firing of different memory codes.

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DISCUSSION CRITICAL APPROACH

• BETTER STATISTICAL SIGNIFICANCE?- PROBLEM OF
  CONTROL- WAIT TASK (absolute coherence values!)
  spontaneous non-intentional memory set
  repetition?- encoding before instruction
  inhibitory instruction context (WM?), working
  space preparation resting state correlated
  networks?
• PROBLEM OF VOLUME CONDUCTION- electrical charge
  flowgt voltage/ signal masking effect
• INFLUENCE OF EOG CORRECTION PROCEDURE?
• LOW SPATIAL RESOLUTION IN EEG, occipital-parietal
  transfer of signal, interhemispherically?
  NO LATERALIZATION IN VERBAL
  TASK?
• ELECTROMAGNETIC INFLUENCES, OTHER ARTEFACTS-
  SIGNAL TO NOISE RATIO

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FUTURE PERSPECTIVES

• LAPLACE CORRECTION (Nunez) FOR VOLUME
  CONDUCTION
• ADDITION OF NEW PURELY SENSORY-PERCEPTIVE
  CONTROL TASK
• ELECTRODE POSITIONING DETERMINATION
• HIGHER SAMPLING RATE
• choosing appropriate cognitive paradigms and
  neuropsychological tests- possible to study
  physiological and patophysiological aspects of
  cognitive, motor and sensory brain function!!!
• new future perspectives for possible search of
  patophysiological mechanisms and etiological
  factors contributing to many different
  neurological diseases!

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MAIN REFERENCES

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ABSOLUTE VALUES OF THETA COHERENCE-WAIT VS.
MEMORIZE TASK
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ABSOLUTE VALUES OF THETA COHERENCE- WAIT VS.
REORDER TASK