There are very experienced meditators and teachers with decades of practice like Kenneth Folk, Shinzen Young, Culadasa, Michael Taft that are not 100% convinced about the ultimate truth     I recommend watching the entire interview with him on Buddha at the Gas Pump https://deconstructingyourself.com/podcast/dy-004-feather-light-paper-thin-guest-shinzen-young  Shinzen Young says that this is not something he can say with great confidence LSD study shows decreases in alpha, beta, theta and delta brain waves but what about gamma? A neuroscientist in a video where Martin Ball smokes 5-MeO-DMT said "consistent with a lot of the other research that's been done, gamma tends to increase" so if the fastest brainwave increases, isn't that in line with what materialist paradigm would suggest?
General anesthesia
There is an argument that if you become sufficiently conscious (master Samatha meditation), you can have some consciousness even in deep dreamless sleep. This is true, however, general anesthesia seems to knock out even those people completely to non-consciousness https://youtu.be/AyFdmhwCpOM?t=23m49s
Nirodha samapatti AKA anupādisesa nibbāna. There is some controversy on whether or not the version of nibbāna where there is an experience can be called nibbāna, but what about this sort of classic version of nibbāna where there is no experience at all, no consciousness, no non-dual/being infinite? I mean exactly like a materialist would imagine a death to be like. It requires very good meditation skills, furthermore, it's traditionally said to be only possible for non-returners (anāgāmi) and Buddhas (arahants) so only highly awakened with great meditation skills, we may say very conscious people. All this training to discover a materialist paradigm fitting extinction of all mental activity. Many believe that this is precisely what Buddha meant by Nirvana.