The problem is the theory implies that people with brain damage should experience psychological effects similar to those induced by psychedelic drugs, and we don't see anything like that.
“Brain damage” usually means gross structural damage, while psychedelics seem to operate at the neurotransmitter level.
Yes we do. For quite some effects seen in patients in psychiatry it's not too hard to come up with a drug which induces similar experiences. Take paranoia for instance. And hallucinations of course. Or, to mention an extreme and very rare example: there are people lacking a certain connection between the low-level visual processing and the parts translating that to known objects (sorry, don't remember exact terminology). It's extremely hard to imagine, but such people have normal vision yet they do not recognize based on what they see. E.g. they see a chair, but don't know it is a 'chair'. However when they touch the chair they know 'aha, chair'. Someone I know described that exactly once when on psychdelics; psilocybin IIRC.
I don't rule anything out, but the filter hypothesis seems a lot less likely to me given the evidence, than the simpler explanation: That psychedelics just mess up the normal functioning of the brain to create a different experience. This requires less assumptions than the filter hypothesis, which is akin to suggesting that the brain is actually drunk all the time, but alcohol merely removes the filter to allow us to experience the drunk phenomenology.
However, fever lack of sleep and other non localized effects can produce a wide range of Hallucinations.
If the hypothesis was true, we would expect the effects of brain damage to reliably create a richer and more psychedelic experience for the sufferer.
Why 'reliably'? Not all brain damage is the same. Maybe I'm misunderstanding the theory but I don't think it argues that, nor is it what happens in reality.
Also, why at all? Why would certain brain damage not be able to create a less rich experience? The theory doesn't say that is impossible I think?
Which brings us to why I tried to make the point about brain damage. Since we are working with the idea that psychedelic experience is the default but that the brain is filtering out most of it, it seems to follow that strategically damaging the filter should allow psychedelic experiences to flow through. You're right, not all brain damage is the same, but enough people suffer similar brain damage that I believe we ought to have seen by now some subset of patients with certain types of trauma reliably report psychedelic experiences as part of their symptoms. If such a subset exists, then I stand corrected.
Problem is that is quite hard to assess if it is sufficiently similar. That alone warrants an entire range of experiments. Also I don't know the literature enough to figure out if certain types of trauma reliably lead to similar experiences but I do think that is the case. For example electrical stimulation (not 'damage' per se but definitely altering the normal brain operation in subjects) has been repeatedly used to treat a myriad of problems like tinitus, tremors, ... And I know for a fact that during surgery, when attempting to find the right position for the electorde, it is not uncommon for patients to experience hallucinations/strong feelings of disgust/... because stimulating in the 'wrong' area instead of the proper target area. Also look at lobotomy for instance: quite a lot of similar symptoms in the unlucky subjects. Again: doesn't really prove filtration, but does indicate what you think doesn't happen (ie.strategical damage causing similar experiences) does in fact happen so it's imo not a good measure to reject the hypothesis.