When someone claims to see, or hear, or even smell, something that no one else can see, hear or smell, we define these phantom perceptions as hallucinations. But how can we determine whether a person is actually having a hallucination or a real visual or auditory experience?

Some years ago Dr Schatzman, a London Psychiatrist was faced with a female patient, Ruth, who sought treatment for hallucinations of her father. She knew she was hallucinating for her father was alive, and living overseas. Yet he kept appearing to her. Her hallucinations were so detailed that she could see the grey hairs in his bushy eyebrows, and the check pattern on his shirt. When he spoke she clearly saw his teeth.

She heard his footsteps and the rustling of his clothing as he walked. She could even smell him when he appeared. The figure obstructed her view of objects and other people "behind" the apparition. His voice was so loud that it was hard for her to hear other people talking.

She had no previous psychiatric history, and had no disorders of the body or sensory organs. Brain functions showed normal results.

Not surprisingly, when these hallucinations first occurred, they caused Ruth much fear and depression; problems which only decrease once it was shown that she was not insane, and she gained a degree of control over the appearances and behaviour of the hallucinatory figure. In time she was able to produce hallucinations of other people, including her husband, children and even her Psychiatrist.

During later treatment it was decided to try and obtain subjective evidence of her hallucinations. With the assistance of a neurophysiologist a series of tests were devised. It is possible to measure the brain waves of a subject on an EEG, while they stare at a strong strobe light, or a screen showing a changing checkerboard pattern, since the occipital cortex, (the visual area of the cortex), displays a wave consistent with this evoked stimuli. These wave changes can be used to determine genuine organic blindness from hysterical blindness.

An organically blind person will not show a visually evoked response to changing patterns. Someone who is hysterically blind may deny seeing a change in the stimuli, yet while they may deny seeing a change their EEG wave will change in response to the changing visual stimuli.

When Ruth looked at the changing check pattern her visual evoked response was normal. When told to place a hallucinatory figure in front of the pattern her visual evoked response disappeared as if something had come between her eyes and the screen and actually blocked out the stimuli. The experiment was repeated several times. She was instructed to have the apparition block part of the screen, or all of the screen. The degree to which the visual evoked response was inhibited corresponded with her estimate of how much of the screen was blocked by the apparition.

Eye movement was checked, to ensure that the reduction in the visual evoked response was not due to movement of the eyes away from the screen. It was found that she kept her eyes on the screen at all times.

Other tests involved her placing an apparition in front of the screen, then her attention was distracted. When her attention was distracted, it was found that her visual evoked response diminished. Yet it did not totally disappear as it had when the apparition blocked the screen.

In an attempt to determine where in the occipital cortex her perceptions of the stimuli was being obstructed other tests were devised. A thin beam of light was shone into her eye, and the electrical response of her retina was measured. She was then instructed to hallucinate an apparition blocking the beam of light. There was no changes in the retina at these times, indicating changes in her vision evoked response were occurring beyond the retina.

In other tests her pupils did not constrict in response to a hallucinatory light, or dilate when she obscured the light beam with a hallucinatory figure, further evidence that the "blocking" of the visual evoked response was occurring somewhere beyond the eyes.

Ruth was also tested for auditory responses. Auditory evoked responses were fed to her through headphones. She was told to hallucinate her daughter turning down the volume control of the machine delivering the auditory signals, while in reality the sound continued at a preset level. As the hallucinatory figure turned down the volume control Ruth claimed she could no longer hear them, and the EEG confirmed these responses to the auditory signals, as the EEG signals disappeared.

Obviously the hallucinations existed only in her mind, yet they were "real" enough to affect her perceptions and her brainwaves. She knew the checkerboards, the lights and the sounds were really there, yet she was able ti suppress them to the point where, to her at least they were not perceptible.

It was determined that Ruth had an unusual capacity to focus her attention, to the degree that she could exclude other real physical stimuli. This is an ability familiar to many people who are able to concentrate their attention to a high level of intensity.

Whether or not other hallucinators can inhibit their evoked responses would depend upon their ability to control their hallucinations to the same degree as Ruth.

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