Monday, 11 May 2015

Visual hallucinations imply 'whole brain' dysfunction; auditory hallucinations not so much

It is well known that 'hearing voices' (when nobody is there) is a common sign of the 'functional psychoses' of schizophrenia, mania and psychotic depression - when patients may appear to be alert and orientated; but that true visual hallucinations are rare except when there is delirium, acute confusional state, 'clouded consciousness' - when the patient is distractible, perplexed, either hyper-excited or drowsy, and disorientated  (does not know where they are, the time or who are the people in their environment).

I suggest that the reason may be related to the necessity for 'global', multi-system brain dysfunction before visual illusions become real and convincing - and this is because humans are highly visual animals such that about half of the brain seems to be involved - in some way or another - with the visual system.

This massive input from the visual system has bee posited as a major driver in the evolution of sleep - a state when eyes are closed and the brain largely cut-off from the environment, presumably to allow internal processing - since an animal's sleep needs seem to correlate with the volume and complexity of its visual input:

This implies that for a person to become convinced that an internally-generated visual illusion is actually real, would require that a great deal of the brain be functioning abnormally, and this would imply a global brain dysfunction with multiple deficits, such as delirium or widespread dementia - which probably is itself exacerbated by delirium:

When only a part of the brain is involved in generating a visual illusion, as happens with some instances of Temporal Lobe Epilepsy, the experience is of a visual illusion which is known to be false, not really happening in the environment - i.e. a 'vision', rather than a visual hallucination which is experienced as real.

So a temporal lobe seizure may create a stereotyped 'vision' (probably an activated memory) of small human figures performing actions, rather like a video replay; but this is not experienced as really happening in the environment so it is not a true hallucination.

In other words, when the input from the various parts of the visual system is inconsistent - so that one input contradicts another - then the experience is of an illusion, not an hallucination.

By contrast, the auditory system involves, as I understand, a much smaller proportion of the brain than the visual system. We may infer that in the functional psychoses, a much more localized and partial brain abnormality, involving the totality of auditory regions, may generate auditory material such as voices that are experienced as real because the auditory information in self-consistent, un-contradicted.

The results is that people suffering only from auditory hallucination are more partially impaired, and therefore less globally impaired, than those who suffer from visual hallucinations.



Anonymous said...

I think you are onto something. I have had visual hallucinations, but only in very specific circumstances. It has happened when I have been extremely tired, "too tired to sleep."

In every instance, I have been dreaming with my eyes open.

In normal sleep, all of the senses are running on "imagination." However, when the eyes are open, the optic nerves are sending data to the brain. However, the brain is dreaming (i.e. making up events) so perhaps it fills in the blanks by imposing imagination on the actual data it is receiving.

This is just a hunch based on experience, but may be worth investigating.

Bruce Charlton said...

@rt - Thanks for your comment.

I have tended to assume that when sleeping with eyes open, the brain cuts-off the visual input; rather like lifting open the eyelids of a sleeping dreaming person - the eyes don't see, and I don't think it influences the dream.

Ben P said...

@BC "I have tended to assume...."

That may be generally true, though I do have a related anecdote.

I was dreaming once that I was in a place in which I couldn't see very well. It wasn't too dark, but rather I had the impression that I just couldn't open my eyes up enough to see very well.

After some struggle to get my eyes open, the scene changed completely to a rather abstract scene of light and dark regions.

A short time later I awoke and opened my eyes. To my surprise, the view before me matched exactly the abstract scene I had seen in my dream. The dark regions were the underside of the upper bunk and part of a wall, while the light areas were sunlight coming in through a window onto the rest of the wall.

I had, by sheer force of dream-will, opened my eyes and perceived what was seen!

Dexter said...

"This implies that for a person to become convinced that an internally-generated visual illusion is actually real, would require that a great deal of the brain be functioning abnormally, and this would imply a global brain dysfunction with multiple deficits, such as delirium or widespread dementia."

Whenever people report religious visions, the Left dismisses them as hallucinations. Yet these are usually reported as totally convincing interactive visual and auditory experiences, and furthermore, those who experience them (generally) do not display the symptoms of massive global dysfunction afterwards. They return to their normal lives, as one would not expect them to do if "a great deal of the brain was functioning abnormally".

Or is it possible to have a massive brain dysfunction once, and never again? ("I had a global brain dysfunction" - pause - "I got better".)

In short this strongly suggests that religious visions are NOT the product of mental disease.

Bruce Charlton said...

@Dexter- The main thing to whether you believe that religious visions are possible. If they are possible then some (not all, maybe not even most) religious visions could be genuine.

But whether they are genuine does not really depend on brain dysfunction/ altered states of consciousness. I mean, a vision could be genuine even when consciousness is altered - for example Joseph was told about Mary during sleep.

My opinion is that most genuine revelations and visions are probably received in a trance state or sleep - but unlike most dreams and daydreams which are forgotten or regarded as trivial, they are remembered and have a profound impact on that person.