Source : FISH ’S Clinical psychopathology

Functional Hallucinations

An auditory stimulus causes a hallucination but the stimulus is experienced as well as the hallucination. In other words the hallucination requires the presence of another real sensation. For example, a patient with schizophrenia first heard the voice of God as her clock ticked; later she heard voices coming from the running tap and voices coming from the chirruping of the birds.

So both the noises and the voices were audible. Patients can distinguish both features from each other and crucially, the hallucination does not occur without the stimulus. Some patients who discover that noises induce hallucinatory voices put plugs in their ears to reduce the intensity of the stimulus and hence the hallucinations. One patient recently described that she saw the mouths of her collection of dolls moving. The perception of dolls was necessary to produce the hallucination but the movement of their mouths was distinct and separate and did not represent a transformation of that perception, thus making this a functional hallucination rather than an illusion. Functional hallucinations are not uncommon in chronic schizophrenia and they may be mistaken for illusions.