Coherence is determined by first measuring and storing the continuous stream of inter-beat-intervals (IBI), calculated from pulse data received from the finger sensor. Next, the power spectrum of the most current 64 seconds of IBI data is calculated and re-sampled every 5 seconds. The more coherent (sine-wave like) the heart rhythm the more the rhythm frequencies concentrates into a single peak in the power spectrum. If coherence is high then most of the energy will be concentrated in or around a single peak within the coherence range of the power spectrum.
The values are IBI's, accumulated in an interpolated form every 500ms for a total of 128 in the 64 sec window by comparing the power in the coherence peak to the rest of the spectrum the coherence ratio is determined. For each challenge level there are 2 coherence level thresholds for clarifying the ratio level, low/medium and medium/high. Coherence is scored as low, medium or high depending on its relationship to the threshold levels.
In general (the actual details of scoring are more complicated), points are deducted for low coherence and awarded for medium and high coherence. These points are continuously updated throughout the entire session. The current level of coherence and accumulated coherence points are reflected in the software displays.