How does CFAM mode differ from CFM mode?
The amplitude trace on the CFAM is fully logarithmic and plots from about 0.5uv to 400uV peak to peak. The signal is clipped at about 1.2 to 1.4millivolts peak to peak unlike the CFM5330 where it is only 155uV peak to peak. Consequently the CFAM responds properly to high amplitude EEG waveforms. It also indicates the frequency content of the signal. The fully logarithmic scale has two main benefits:
1) For most of the time it converts the EEG amplitude distribution to a normal distribution.
2) If two signals are identical except that one is attenuated, the distributions of both signals about their means are identical. With the CFM system, where the distribution becomes linear below 10uV , identical morphology signals do not have the same apearance if one of them is substantially below 10uv.
The three continuous lines in the CFAM amplitude trace are, from left to right, the 90th centile the mean and the 10th centile of the signal amplitude distribution. These are averaged over time to smooth out amplitude fluctuations. The lines to left and right of the centiles are those amplitude variations that, in any 2 second epoch, exceed the 90th centile or fall below the 10th centile.
M is the scalp muscle amplitude at the recording electrodes. It plots up to 50uV peak to peak from left to right up to the next graticule.
The percentage activity per Hz in each EEG frequency band is plotted up to 100% and from right to left. B=Beta band, A=Alpha, T=Theta and D=Delta. V is sub-Delta. The small numbers between between the band letters indicate the frequencies at which the bands are divided.
S is the percent time that the signal falls below 1uV peak to peak and again plots to 100%
The electrode impedance is plotted up to 30 kohms from right to left.
L is the level of mains interference and plots up to 50uV .
There are a few other considerations :
Firstly when EEG is filtered as by the CFAM filter it is possible for amplitude and frequency distributions to vary independently, for example a change of amplitude distribution without change of frequency distribution.
Secondly it is possible in some circumstances for seizure activity to occur without there being a noticeable change in the amplitude distribution, however there may be a change of frequency distribution.
A further difference between the CFAM and the CFM is that the time constant for signal step amplitude increases and decreases are the same in the CFAM, whereas in the CFM the fall time is substantially longer than the rise time. This makes the CFAM better at detecting short EEG suppressions.
