Do we always perceive the world the same way? The hypnosis experiment proved that we definitely don’t.
If we honestly believe our index finger is five times larger than it really is, our sense of touch improves. Researchers at the Ruhr University Bochum showed this to be the case in an experiment in which participants were placed under professional hypnosis. When the participants indicated that they understood the opposite hypnotic suggestion that their index finger was five times smaller than it was in reality, their sense of touch deteriorated accordingly. The study shows that our tactile perception is affected and can be altered by our mental processes. The scientific community has been divided on this issue. The researchers, headed by Special Lecturer Dr Hubert Densey, Professor Albert Newen and Professor Martin Tegenthoff, publish their findings in the journal Scientific Reports on April 21, 2023.
Two needles feel like one
The researchers measured the tactile perception of the 24 test participants using the two-point discrimination method. This involves resting the index finger on a device with two needles touching the finger repeatedly without pain but perceptibly. “If the needles are far enough apart, we can easily distinguish between two points of contact,” explains Hubert Dinse from the Neurological Clinic at Berufsgenossenschaftliches Universitätsklinikum Bergmannsheil. “But if the needles are very close together, we only feel touch in one place.” At a certain distance between the needles, the sensation changes from feeling two needles to feeling only one, even though two are visible. This threshold of discrimination is stable for each person given the normal daily consciousness.
If the finger is five times larger
“We wanted to find out if this sensation threshold could be changed by activating a verbally articulated idea in a person,” explains Albert Neuen from the Institute of Philosophy II at Ruhr University Bochum. The research team chose two thought cues: “imagine your index finger is five times smaller” and “imagine your index finger is five times bigger.” To specifically activate these semantic contents, the researchers used hypnotic suggestion. During a controlled state of hypnosis induced by a professional hypnotist, the participant was asked to truthfully accept the first belief for a series of tests and then the second.
Subjects participated in a total of four trials to determine the sensation threshold in each condition: under normal everyday awareness, under hypnosis without suggestion, and under two states of hypnosis with suggestions of a larger or smaller index finger.
Changes in the sense of touch
“Discrimination limits did not differ when measured during normal awareness and hypnosis without suggestion. This supports our initial assumption that hypnosis alone does not lead to changes,” says Martin Tegenthoff. “However, if the beliefs are induced as suggestions under hypnosis, we observe a systematic change in the threshold for tactile discrimination.” When the test subject imagined their index finger to be five times larger than it was in reality, their discrimination threshold improved and they were able to feel two needles, even when they were close together. When the suggestion was that the index finger was five times smaller, the discrimination threshold worsened. This means that it is beliefs that change perception. The behavioral findings were supported by parallel recordings of brain activity such as spontaneous EEGs and evoked sensory potentials.
The scientific community is divided on the question of whether or not perceptual processes can be affected by semantic content alone—experts refer to this as a matter of perceptual penetration. “Our study provides another building block in support of the idea that such downward effects of beliefs on cognition do indeed exist,” Hubert Densey asserts. “The beliefs we hold do change the way we experience the world.”