They provide a 2.4 Hz stimulus and then measure frequency-matched activity across brain networks. They suggest some novel methods of measuring how the signal traverses the brain, but they don't suggest why it does, which is good. They do say this is unlikely to be entrainment, I'll get into that more in a bit.
We shouldn't be surprised that auditory stimulation produces frequency-matched activity across distributed brain regions. The auditory system naturally routes information across multiple interconnected networks. The auditory system picks it up, but the auditory system is also not siloed into a single area of the brain. No brain systems are, we have replication, and this is just showing the the nervous system is passing the signal throughout the brain. In no way does it suggest that this is related to thought, consciousness, focus, or that these frequency-matched responses reflect any functional change in brain state.
When people talk about entrainment, that is a real thing. But the word itself describes when systems synchronize, not that they will.
I guess I'm cautious about papers like these because of our work in neurostimulation and sleep where we use phase-targeted auditory stimulation to enhance slow-wave activity. Basically increasing sleep's restorative function.
In our work it isn't this sort of "gentle tones to help you sleep", or "activating networks to alter brain activity", which is an area I see a lot of snake-oil and nonsense.
The way closed-loop phase-targeted slow-wave enhancement works is by "interrupting" the brain during the synchronous firing of neurons, which (it is believed) triggers a protective mechanism in the brain and as a response, the brain increases the synchronous firing of neurons. We're talking about very short (50ms) interruptions.
I get my back up a bit when I hear about this idea that reading electrical activity of the brain and making broad assumptions about what they "mean". I've been invited to speak on a panel July 2nd with Australia's Commissioner of Human Rights to discuss ethical safety around EEG data, and while I do believe we need to protect bio-data, I don't believe in the "electrical activity means we can read or alter your thoughts" camp.
If you want to know more about our work, you can check out https://affectablesleep.com, and if you're in Sydney, and want to come to the talk, I can't find a link atm, but it's at the Sydney Knowledge Hub on July 2nd., part of the Sydney Neurotech Meetup