EEG and Brain Connectivity: A Tutorial - Bio-Medical Instruments, Inc.
EEG and Brain Connectivity: A Tutorial - Bio-Medical Instruments, Inc.
EEG and Brain Connectivity: A Tutorial - Bio-Medical Instruments, Inc.
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frame” of consciousness (Thatcher <strong>and</strong> John, 1977; John, 2005). At about<br />
300 – 500 msec the match miss-match resolution of expectation <strong>and</strong> received<br />
inputs is completed. Associations <strong>and</strong> connections in time occur from about<br />
200 msec to minutes of time. Thus, operant conditioning of <strong>EEG</strong><br />
biofeedback is likely to work best when the interval of time between an<br />
“<strong>EEG</strong> Event” is greater than 100 msec <strong>and</strong> around 1 – 2 seconds, with a<br />
operating curve yet to be produced. When accurate measurements are made<br />
of the optimal interval of time between a brain event <strong>and</strong> the feedback signal<br />
<strong>and</strong> not active stimulation, then one can expect that 500 msec to 1 sec would<br />
be a good interval of time for associations to occur using operant<br />
conditioning <strong>EEG</strong> biofeedback. For active stimulation <strong>EEG</strong> biofeedback<br />
then phase reset can occur <strong>and</strong> many other phenomena that can easily be<br />
measured can occur. However, modern <strong>EEG</strong> science easily h<strong>and</strong>les event<br />
related potentials (ERPs) if one knows the instant in time when the stimulus<br />
was delivered or the instant in time when the movement of the subject<br />
occurred. Spontaneous <strong>EEG</strong> <strong>and</strong> ERPs are related in that the background<br />
<strong>EEG</strong> is the “mother” of the ERP (electrical field) at a given moment of time.<br />
The powerful <strong>and</strong> rhythmic background <strong>EEG</strong> are the summation of millions<br />
of excitatory EPSPs oscillating in loops but only firing on the rising phase of<br />
the oscillation. This results in a “quantization” of neuron excitability as<br />
reflected by the rhythms of the <strong>EEG</strong>. The idea of “quantization” of neural<br />
action potentials time locked to the rising phase of the <strong>EEG</strong> is old <strong>and</strong> is well<br />
supported by recent evidence (Buszaki, 2006).<br />
15- What is Phase Difference?<br />
Coherence <strong>and</strong> phase difference (measured in angles) are linked by<br />
the fact that the average temporal consistency of the phase difference<br />
between two <strong>EEG</strong> time series (i.e., phase synchrony) is directly proportional<br />
to coherence. For example, when coherence is computed with a reasonable<br />
number of degrees of freedom (or smoothing) then the phase difference<br />
between the two time-series becomes meaningful because the confidence<br />
interval of phase difference is a function of the magnitude of the coherence<br />
<strong>and</strong> the degrees of freedom. If the phase angle is r<strong>and</strong>om between two time<br />
series then coherence = 0. Another way to view the relationship between<br />
phase consistency (phase synchrony) <strong>and</strong> coherence is to consider that if<br />
Coherence = 1, then once the phase angle relation is known the variance in<br />
one channel can be completely accounted for by the other. The phase<br />
relation is also critical in underst<strong>and</strong>ing which time-series lags or leads the<br />
other or, in other words the direction <strong>and</strong> magnitude of the difference.