A patient on a hospital ward says that every time the bedside fan switches on, she hears a voice arguing somewhere within the noise; when the fan is off, there is no voice. She can hear the fan itself completely normally throughout, and confirms this happens every single time. Before revealing the answer: what is this finding, and why is it not simply an illusion?
This is a functional hallucination. It is not an illusion because the fan itself is not being misperceived or distorted in any way — she hears it accurately and normally throughout. The abnormality is a separate, additional hallucinated voice that reliably accompanies the real sound, rather than a transformation of the real sound into something else.
References
- Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 3, 'Disorders of Perception'.