Sampling Rate (samples per second)

The sampling rate is the number of times each second the amplifier grabs a sample of brain activity and brings it to the software.  It shouldn’t affect the frequency (Hz) or amplitude (microvolts) at all, except that perhaps in very high frequencies a very low sampling rate might show less detail or, in extreme cases, cause a problem called aliasing.

In electrical engineering there is the Nyquist Sampling Theorem, which states that a filter can only be accurate up to a frequency of less than one-half its sampling rate.  In other words, a machine that sampled at 64 Hz (there aren’t any) should theoretically be fine up to give an accurate digital picture of frequencies up to a little below 32 Hz.  A 128 sampling rate should be fine for digitizing waves of frequencies up to nearly 64 Hz.  A 256 sampling rate should allow accurate digitizing of a signal up to just below 128 Hz.

For the less technically-minded, I use an analogy to discuss sampling: Two people are placed by a window and told to count the ants crawling across it. One is allowed to see the window a certain number of times per second. The other is shown the window twice as often. The bigger and slower the ants, the more likely the two counters will come up with the same count.  When the ants are smaller and faster, however, it is likely that the person who only looks half as often will miss some.

For EEG training the difference between 256 bps and 2064 bps is meaningless.  You are training signals that complete a full cycle between 1 second (very slow delta) and 1/40th of a second.  Even at 256 samples per second, you would have more than 6 samples for each waveform at 40 Hz.  At 2048 (which might be useful for training EMG, where you might be looking at signals up to 1/500th of a second), there would be 48 readings for every waveform of 1/40th second.

All EEG amplifiers work at 256 (at least the more recent ones), even though some may oversample at 1024 or 2048.  Those then “down-sample” to 256, so it’s kind of like buying a digital camera that shoots 8 megapixel images and then putting them on the internet at a resolution of less than 1 megapixel.  You have really gained nothing by it.