by Jalees Rehman
The Russian physiologist Ivan Pavlov is best known for his studies on classical conditioning showing that dogs repeatedly presented with a combination of food and a sound would subsequently salivate upon hearing the sound alone, in anticipation of the meal. The combination of the two stimuli – food and sound – over time "conditioned" the dogs' brains to link these two stimuli. A variation of this experiment was performed on human subjects by Ellson and published in the Journal of Experimental Psychology in 1941. In Ellson's study, 40 subjects were "conditioned" over time by hearing a sound and seeing a light. Ellson later on exposed the subjects to only the light, yet 32 of 40 subjects claimed to have also heard the sound. Ellson concluded that such conditioning could lead to hallucinations – the hearing of sounds which, objectively speaking, are not present.
Recently, the Yale University psychiatrist Philip Corlett and his colleagues conducted a very interesting variation on this earlier study by asking whether some people are especially vulnerable to having auditory hallucinations induced by conditioning. The researchers recruited four groups of study subjects: 1) Fifteen patients with severe mental illnesses who also regularly heard voices (an auditory hallucination), 2) fifteen patients with severe mental illnesses who did not hear voices, 3) fifteen individuals without any evidence of mental illnesses who also claimed to hear voices and 4) fourteen healthy individuals who did not hear voices. Group 3 consisted of voice-hearing psychics ("clairaudient psychics") who identified themselves as such via their own websites, at psychic meetings, or referrals from other psychics. Another important innovation in Corlett's study was the inclusion of brain imaging studies on all subjects, thus allowing the researchers to study functional brain responses when exposing them to auditory and visual stimuli. The researchers then repeatedly exposed the study subjects to a checkerboard image and 1 kHz tone while they were lying in the brain scanner. The subjects were asked to press one button to indicate that they heard the tone, and a second button if they did not. They were also instructed to press down the button longer, the more confident they were in having heard the tone.
After conditioning the subjects, the researchers then intermittently began to show them images of the checkerboard without playing the tone. As expected, many subjects indicated having heard the tone even when it had not been played. However, patients with severe mental illness and a history of hearing voices (group 1) as well as healthy psychics with a history of hearing voices (group 3) were significantly more likely to wrongly indicate that they had heard the non-existing tone. Members of these two groups were also more confident that their hallucination was actually real, since they pressed down the button for longer. Healthy subjects and patients with mental illness who did not have a history of hearing voices were comparatively more correct in identifying whether or not the tone was present. Importantly, when the researchers repeatedly showed the image without the tone, voice-hearing, mentally ill patients were unable to "update" their beliefs when compared to the other groups, whereas the psychics gradually recognized that the tone was non-existent.
