Biology 202
2001 Third Web Report
On Serendip

Hallucinations are defined as sensory perceptions in the absence of externally generated stimuli (6). They are not to be confused with illusions in which actual external objects are perceived but misinterpreted by the individual (6). Hallucinations can take many forms including visual, auditory, olfactory and tactile, but for this paper we will focus primarily on the visual type. Visual hallucinations can occur in a number of different situations, two of which we will discuss here: Charles Bonnet syndrome and schizophrenia. These two conditions are unique in the causes and effects of the hallucinations resulting from each, and in the types of people in whom they occur. A most interesting distinction is that Charles Bonnet patients are aware of their hallucinations while schizophrenics are not. In the next sections I will present a description of the hallucinations that occur in each condition, and some hypothesized causes of these. I will conclude with an attempt to discover why there exists an awareness of hallucinations in one that is absent in the other.

Charles Bonnet syndrome is the onset of hallucinations in psychologically healthy individuals who have become either visually-impaired, or completely blind. There are two main theories as to the cause of these hallucinations. The first and most popular is that they are "release hallucinations" that result from the, "removal of normal visual afferent input to association cortex" (7). This is supported by experiments involving direct stimulation of the temporal lobe, and fMRI's taken during hallucination events. These studies found that in the absence of visual input, activity was present in a particular visual area of the brain and that the resulting hallucination would be a type of image normally perceived by that area. For instance, a subject who hallucinated in color showed activity in the color center of the fusiform gyrus while a subject who hallucinated fences and brickwork showed activity in the collateral sulcus which responds to visual textures. (4) These areas normally respond to outside visual input, but in this case there was none. It is possible then that these areas are activated in the absence of inhibition caused by outside visual input. This would be something like the phenomenon of the chicken that runs around in circles once its head has been severed. The other theory is that hallucinations in visually-impaired individuals occur as part of a "filling in" process that is already in use by our brains.

Every human has a blind spot in each eye were the optic nerve head connects and no photoreceptors exist. However we do not experience the world with two black spots in it wherever we go. This is because our brains are able to make up material to fill in the holes based on the surrounding visual image. Thus we do not notice our blind spots because our brain is showing us what it guesses should be there. (10) It makes sense that in people who have more severely degraded vision, their brains would be acting to fill in the missing information and that this results in the hallucinations they experience. They would still be able to identify them as hallucinations because their overall clarity would probably be greater than what they could experience with their impaired vision.

But Charles Bonnet hallucinations are also recognizable by their strange content. Instances abound of patients reporting seeing things from the past or those that just did not belong in a particular situation; my favorite of these is a man who saw a macaque sitting on his doctor's knees (9). If the normal function of this filling in process is to make a best guess at the missing information using available visual cues, then how do you wind up with a jungle primate dozing on your doctor's lap in the middle of his office? Another question that begs asking here then, is why are these hallucinations experienced in individuals who are completely blind? If no outside visual cues are perceived, then how does the brain go about filling in the holes? Dr. Pascual-Leone and his colleagues (who have been conducting experiments on blindfolded individuals) have found that their test subjects experienced the same type of hallucinations as Charles Bonnet sufferers after being blindfolded for extended periods (8). Unless these subjects were in a lighted room during the experiments (in which case they may have been able to detect some differentiation in light intensity), they would have no visual cues to pick up on. So again - in the absence of all visual input, how can the brain "fill in" anything? Incidentally, when vision was restored in the subjects in the experiment above, their hallucinations ceased (8). This however could be a result of the brain not feeling the need to "fill in" any more as well as from the reinstatement of inhibition on the visual cortex. It could also be that these images are present in the brain at all times, and that in the absence of external visual stimuli they seem more real. However, I have been able to find no corroborating assertions in the available literature.

When a Charles Bonnet sufferer experiences a hallucination, they are aware of it. In fact they may not even report it to their doctors for fear of being thought of as crazy (7). Because of this many cases may go unnoticed, however there is still a substantial amount of documented evidence of patients reporting awareness of their hallucinations. In the case of the hallucination of the macaque mentioned above, the following was reported: "This subject said that he knew that the macaque was a hallucination because of its "hyper-realistic" appearance, with vivid and brilliant colors" (9).

The experience of visual hallucinations for schizophrenics however, is quite different. Schizophrenics almost never experience visual hallucinations by themselves, rather they are virtually always in conjunction with auditory or other hallucinatory forms. In short, for schizophrenics visual hallucinations are but one part of an entire "delusional system,"(6) that results in these individuals' inability to distinguish the images they are seeing from reality.

It has been hypothesized that in schizophrenics hallucinations occur as a result of, "confusing external and internal stimulus sources" (3). In a study conducted to test this hypothesis, it was found that hallucinations might be a result of a higher level of vividness of patients' mental imagery. The findings also imply that the more impaired a patient's affinity for reality discrimination, the higher the severity of the hallucinations (3). But no one really knows (even to a point where they are comfortable making an assertive guess) what causes hallucinations in schizophrenia. Another hypothesis is that schizophrenics may have unusually high levels of serotonin in their brains. Serotonin can have the effect of blocking afferent information to the thalamus, and serotonin antagonist drugs seem to be the most effective treatment for schizophrenia (9). Pelaez postulates that schizophrenic hallucinations may be a result of a blockade of afferent information going from the prefrontal cortical areas to the thalamus, because this might trigger the kind of filling in process discussed earlier with regards to the blind spot and Charles Bonnet syndrome (9). However most of his speculations on this are in the form of a question, and the little information he does provide is largely circumstantial.

The rest of the available information is a list of characteristics in the brains of schizophrenics that are not present in the normal individuals with whom they are compared. Scans of the brains of monozygotic twins, one of whom is a schizophrenic, show a decrease in the level of activity in the frontal lobes of the effected sibling (11). A similar test shows that the ventricles in the brain of the schizophrenic twin are larger (12), and yet another test comparing 21 schizophrenics to 51 individuals of the same age found the same phenomenon but focused on the fact that they have less gray matter as a result of enlarged ventricles (2). They assert that this condition may predispose to psychiatric illness, but do not go beyond that. So there have been findings of actual structural differences in the brains of individuals suffering from schizophrenia, but nobody is really sure of exactly what these mean.

In any case, the result is a complex delusional system that schizophrenics cannot differentiate from reality (1). What they see may be as out of place as the images perceived by Charles Bonnet patients, but the way they appear is different. In Charles Bonnet sufferers, hallucinations can usually be recognized (besides by their content) by the fact that they are seen in greater clarity and more detail than what the individual is capable of given their deteriorated vision (4). A new device created by Janssen Pharmaceuticals to allow normal individuals a glimpse into the world of schizophrenics shows a more distorted picture where lines blur and bend. The images are often disturbing and may even talk to the individual, making negative comments about them or giving them commands. (5)

So why can Charles Bonnet patients perceive their hallucinations as such while schizophrenics cannot? The underlying answer may simply be that Charles Bonnet sufferers, despite their condition, are psychologically normal (7). Schizophrenics are not, and as such they are less likely to be able to confidently assert that what they are seeing isn't real. Schizophrenics also have a predisposition to paranoia which may make it easier for them to accept their hallucinations as part of a malicious plan being enacted on them by some outside force. Then too, we have the fact that in Charles Bonnet syndrome, only visual hallucinations are experienced whereas schizophrenics are bombarded with images, voices, sounds, and even smells (6). Thus, while Charles Bonnet sufferers can perceive a world outside of their hallucinations that does not correspond to those images, a schizophrenic's entire perceptive experience is effected by their condition (5). Of course I cannot claim to make a "right" answer as to why one hallucination is recognized and the other is not, but these are the factors that I believe are important to consider in the process of "getting it less wrong."

WWW Sources

2)The Schizophrenia Homepage

3)Perception, mental imagery and reality discrimination in hallucinating and non-hallucinating schizophrenic patients

4)The anatomy of conscious vision: an fMRI study of visual hallucinations

5)Channel 2000 , Device Offers Look Into Schizophrenia

6)Introduction to Clinical Psychopathology

7)Survey of Opthalmology

8)Changing Your Mind

9)Towards a neural network based therapy for hallucinatory disorders

10)Tricks of the Eye, Wisdom of the Brain

11) Schizophrenia PET Scan

12) Schizophrenia in Monozygotic Twins

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