Dr. Dieter Menne
07071 52176
In cooperation with Prof. Kuno Kirschfeld and Dr. Thomas Kammer at the Max-Planck Institut für Biologische Kybernetik, Tübingen
In most subjects with eyes closed, Alpha waves in the band of 8-12 Hz occur at a characteristic pattern with high amplitude. The two top charts in the screen shot show unfiltered and bandpass-filtered EEGs respectively; the subject had the eyes closed in the first part. The third chart shows the amplitude in the band, the next is the sawtooth-like representation of the signal's phase. Phase and amplitude are computed using quadrature methods; it is interesting to see how phase runs on steadily even when alpha activity is barely visible in the filtered signal. A measure of phase variance is given in the lowest chart, with a high peak indicating low stability at the transition only.
With AlphaTrigger you can design complex triggering conditions on these base signals, for example that triggers are only fired when a time-sequence of pre-conditions has been met. It uses a state machine with four states, generating a specific trigger signal at each state transition. As an example, it is possible to create the following trigger condition:
"Fire a trigger-event at an EEG-phase of 45 degrees if the amplitude was above 100µV for at least 10 seconds, and it has dropped below 50µv, but purge any pending trigger requests should phase variance go above a threshold of 1.5 units".
The system allows off-line simulation on the PC with recorded EEGs to determine a suitable set of trigger conditions. The conditions are stored in a database and can be recalled for on-line triggering, where a digital signal processor fires the transition events in real-time during EEG recording.