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Cognitive neuroscientists say they can get around this by inducing hallucinations on demand in people from the general population.
Hallucinations "can be induced in almost anyone at any time", they write in an opinion piece published in the journal Philosophical Transactions B.
Because hallucinations are a private experience that can't be independently verified, researchers usually rely on asking patients to introspect and subjectively describe their experience.
This can be biased and problematic, explains Sebastian Rogers from Australia's University of NSW: someone with dementia, for instance, may have trouble accurately reporting the episode.
They also tend to be complex and unpredictable. Visual hallucinations, for example, can include a range of different elements such as humans, faces, animals, landscapes, shapes, colours and movement.
And it can be hard to tell when someone will start or stop hallucinating, making it very difficult to study in the lab.
Comment: The stark admonitions and guidance that Solzhenitsyn gave decades ago were no less relevant then than they are right now, in the West. We'd do well to heed them to the best of our ability.