Functional Brain Signal Processing: EEG & fMRI

Lesson 4

Kaushik Majumdar

Indian Statistical Institute Bangalore Center

kmajumdar@isibang.ac.in

M.Tech. (CS), Semester III, Course B50

Delta Band EEG

0 – 4 Hz. Originates in frontal cortex, hippocampus and thalamus.

Associated with slow wave sleep. Associated with declarative memory

consolidation. Implicated in attention during performance of

a complicated mental task like artihmatic calculation.

Theta Band EEG

4 – 8 Hz. Associated with Rapid Eye Movement (REM) sleep.

Associated the mnemonic processes in our brain, where step by step detail of a process up to the sequential details has to be taken care of.

Implicated in working memory. Implicated in long-term potentiation (LTP) leading

to facilitation of long-term memory and learning.

Alpha Band EEG

8 – 12 Hz. Magnitude increases when eyes closed. Diminishes when eyes are open.

Alpha power often inversely varies as theta power in healthy brain. This indicates good cognitive ability.

Alpha power is positively correlated with brain maturity.

Alpha power is positively correlated with good memory performance and speed of information processing.

Mu Band EEG

8 – 12 Hz. Originated from motor cortex and morphology is distinct from alpha.

It selectively augments and diminishes during motor related task demands.

Important for Brain Computer Interface implementation.

en.wikipedia.org

Beta Band EEG

12 – 30 Hz. Associated with waking stage in general and cognitive and emotional processes in particular.

Hyper beta activity at the sleep onset has been associated with insomnia.

Different EEG Bands

http://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2012/cwm55/cwm55_mj294/

Gamma Band EEG

30 – 80 Hz. 40 Hz is considered representative (taken as 37 – 43 Hz). Implicated in sensory information processing.

It has been implicated in formation of memory, linguistic processing, internal thoughts (apparently without any outside stimuli) and behavior, particularly motor actions and planning for the actions. It has also been implicated in attention, arousal and object recognition.

Event Related Potential (ERP)

http://en.wikipedia.org/wiki/Event-related_potential

http://cognitrn.psych.indiana.edu/busey/temp/eeglabtutorial4.301/maintut/data_averaging.html

ERP: Definition

When the scalp potential is evoked in response to an event or stimulus presentation in the environment the evoked potential is called event related potential (ERP). Of course in scalp EEG ERP may be embedded in artifacts and neuronal activities not evoked by the event of interest.

Single Trial and Trial-Averaged ERP

motorbehaviour.wordpress.com

Time-Frequency Analysis

Short-time Fourier transform.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 120

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Normalized Frequency ( rad/sample)

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Welch Power Spectral Density Estimate

Epileptic ECoG of 500 time points

0 5 10 15 20 25 30 35 40 45 500

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Absolute value of FFT on 100 time points

Event Related Spectral Perturbation (ERSP)

ERSP measures average dynamic changes in amplitude in the broad-band EEG frequency spectrum as a function of time. That is, the ERSP measure the average time course of relative changes in the spontaneous EEG amplitude spectrum induce by a set of external events.

ERSP

Plot generated by Pradeep D. Prasad with EEGLAB

ERSP Computation

Choose a sliding temporal window with suitable size and overlap for the selected EEG epoch.

Do FFT in the window and determine the power spectral density for a frequency bin size of Fn (1 Hz or slightly larger).

Smooth by a moving average window of suitable size in order to eliminate the random jitters.

ERSP Computation (cont)

All smooth window-wise spectral estimates obtained by 2 and 3 are to be normalized by dividing by the spectral estimate of the first window (EEG epoch to be selected such a way prior to 1 that this first estimate will correspond to a pre-stimulus or baseline spectrum).

Measure of ERP Amplitude

Peak amplitude measure. Mean amplitude measure.

Peak Detection

A

B

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D F

E

0)1()( msms0)()1( msms

))1(,1( msm

))(,( msm

))1(,1( msm

0))1()(()()1()('' msmsmsmsms

0))1()()(('')( msmsmsmP

0))()1()(('')( msmsmsmP

0)(''&0)(&0)( msmPmP

ERP Latency

Peak latency. Fractional area latency.

Peak Latency

Latency at which the peak is occurring. Has to be measured with following cautions:

1. Filter out the high-frequency noise in the EEG.

2. Rather than taking the maximum peak alone, take other local peaks also (possibly with some threshold), because the maximum peak may not always be due to an ERP waveform.

Peak Latency (cont.)

3. When different waveforms are compared they must have similar noise level.

Fractional Area Latency

References

Majumdar, A brief survey of quantitative EEG (under preparation), Chapters 1 and 4, 2013.

S. J. Luck, An Introduction to Event-Related Potential Technique, MIT Press, Cambridge, Massachusetts, 2005.

THANK YOU

This lecture is available at http://www.isibang.ac.in/~kaushik