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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.
But it doesn't need to be hard. Although its definition is still being debated, the authors argue that hallucinating can be broadly defined as seeing something that isn't there, which could include simple shapes or colours.
Some might call that illusion, but Rogers says the label doesn't really matter as long as the induced experiences can provide insights into hallucinations, pathological or otherwise.
He and UNSW colleagues Rebecca Keogh and Joel Pearson have been experimenting with flicker-induced illusions as shown in this YouTube video (warning: may cause seizure in people with photosensitive epilepsy), causing people to see patterns, dots, lines, colours and motion.
There are lots of other methods, Rogers says, that each produce different results.
In Ganzfeld-induced hallucinations you would look at a uniformly-lit surface that fills the entire visual field. This can create an illusion of geometric patterns and colours, landscape scenes like a clearing in a forest or city, people or body parts.
The "strange-face-in-the-mirror effect" involves gazing into a mirror under low light, causing, yes, strange faces to appear which can include deformations of one's own face, other people's faces or even those of animals such as cats, pigs and lions.
The paper describes a framework for scientists with useful induction techniques along a continuous spectrum ranging from reality-based perception to hallucinations, with illusions somewhere in-between.
These are distinguished by the similarity between the physical stimulation of the senses - the light that enters the eye - and the actual conscious experience or vision.
The authors explain how scientists can evaluate lab models to work out how to apply the findings to pathological states and suggest new possible research directions with induced hallucinations.
They hope further study will provide deeper, objective understanding of the underlying cognitive and neural processes as well as how and why they occur in various diseases, thus assisting with treatments.
It could also offer broader insights into human consciousness, says Rogers. "The hallucination induction techniques we suggest might also tell us a lot about how the brain and mind process sensory information, and how conscious experience is created by the brain."
