Researchers usually assume alpha brain waves to behave relatively similarly over time. In a new study, led by Chris Benwell and just accepted for publication in NeuroImage, we find that this is not necessarily true for the 1-2 hrs that a typical EEG experiment lasts.
In a re-analysis of data from two previous EEG experiments we saw that alpha underlies two major trends – alpha power increases (an effect previously described) and alpha frequency decreases consistently over time. Both effects maybe related to growing fatigue, the depletion of cognitive resources or a transition from a volitional to a more automatic task performance.
We also found that different sources of alpha rhythms in the brain can show different trends. While some showed both, power increases and frequency decreases, others showed only one if the two trends. Further analysis revealed that both trends do not necessarily depend on each other.
Figure We usually assume that alpha varies “spontaneously” around a mean and higher alpha leads us to observe a different behaviour. This relationship however could simply be a consequence of both, alpha and behaviour changing “deterministically”, i.e. trending over time. In experiments, we likely observe a “mix” of both sources of variance (also see Benwell et al. 2017 or here).
These findings need to be taken into account when testing for links between brain rhythms (brain state) and behaviour – a link might be reported accidentally just because both, brain state and behaviour change over time – and when attempting to manipulate alpha through stimulation – stimulation frequencies may need to be adapted on the fly, stimulation will need to target a specific alpha generator while leaving others unbothered.