Comparing Hallucinations in Schizophrenics and Sufferers of Charles Bonnet Syndrome

A Comparative Look at Hallucinations in Schizophrenics and Sufferers of Charles Bonnet Syndrome, and Their Corresponding Reality Discrimination Abilities

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.