This is a preliminary release, still experimental and liable to change.
New facilities in this version.
'Field' displays the electric potential field over the head at a given instant. Place the cursor over a feature of interest and select 'Field' from the cursor choice control.
'Spectrum' allows you to obtain an estimated display of the frequency spectrum of a segment of any channel of the displayed page. Point to the start of the desired segment and drag to the right (minimum 4 seconds). A box will appear and on releasing the mouse button the estimated spectrum will appear.
"GFS" - for details see Appendix at the bottom of this page.
Unless you are familiar with the system please click here - How to use the viewer - for detailed instructions.
Note: The 'How to use the viewer' system opens in a separate window.
Otherwise click on
I hope that these records will constitute a useful resource
particularly for students of EEG. This is why I have tried to make
the experience as close to that of reporting a routine EEG as
possible.
Limitations of Web browsers mean that the resolution (particularly
the sampling rate) and the speed of the display, cannot approach
that of commercial EEG apparatus.
Nevertheless I think it is valuable to be able to scan the whole
record (not merely the 'interesting bits') and to be able to
examine pages in different montages, to alter the timescale (paper
speed) and gain (amplification) as well as applying high and low
frequency filters.
When you have selected a record for the first time there is a pause of up to 30 - 60 seconds while the display system loads, longer if you are using a dialup connection.
You can then :-
and so on . . .
Horace Townsend
A very interesting technique is termed Global field synchronization (GFS). This is an estimate of the amount of phase-locked activity in all the EEG channels at a given frequency. GFS has been shown, by Dr T Koenig, to decrease in correlation with increasing cognitive impairment in patients with Alzheimer's disease (and surprisingly the same is said to be true of Schizophrenia). GFS is based on empirical observations - surprisingly large areas of the cortex show phase-locked activity for quite long periods of time and this synchronisation breaks down progressively as cognitive performance is impaired. The technique should be robust because it uses all the electrodes on the head.
Select "GFS" from the Cursor menu. Click on a point in the display. A two second epoch is selected and subjected to Fourier analysis and the results are shown in the graph. Each dot represents the amplitude and phase of the activity at a given frequency associated with each electrode. The vertical and horizontal axes represent the amplitudes of sine and cosine components respectively. The frequency is selectable from the control which replaces the "Display Gain" button - centre bottom. Default value is 10c/s).
The dots form a cloud which more or less approximates to a straight line and from this cloud the degree of synchronization is estimated by 'principal component analysis'. The angle of the line of dots depends on the relation between the epoch selected for analysis and the phase of the rhythm being analysed. Note that GFS is independent of this phase relation. As the cloud of dots becomes more and more diffuse the value of GFS decreases. The result can vary from 0 to 1 and may be interpreted as a measure of functional connectivity of brain processes during the selected epoch.
In Koenig's studies of decreased EEG synchronization in Alzheimer's disease the computed values of GFS are averaged over 20 epochs. The resulting measures are again averaged in bands of frequencies at 0.5c/s intervals (e.g. alpha 1 - 8.5c/s to 10c/s; alpha2 - 10.5c/s to 12c/s) to give an estimate of overall Global Field Synchronization.