Our thoughts alter our tactile perception: Neuroscience — ScienceDaily

If we sincerely believe that our index finger is five times bigger than it really is, our sense of touch improves. Researchers from the Ruhr University Bochum demonstrated that this is the case in an experiment in which participants were subjected to professional hypnosis. When the participants signaled that they understood the opposite hypnotic suggestion that their index finger was five times smaller than it actually was, their sense of touch was impaired 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. Led by PD Dr Hubert Dinse, Professor Albert Newen and Professor Martin Tegenthoff, the researchers published 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 the index finger being relaxed on a device with two needles that repeatedly touch the finger painlessly but perceptibly. “If the needles are far enough apart, we can easily distinguish two points of contact,” explains Hubert Dinse from the Neurological Clinic of the Berufsgenossenschaftliches Universitätsklinikum Bergmannsheil. “But if the needles are close together, we only feel the touch in one place.” At some distance between the needles, the sensation changes from feeling two needles to feeling just one, even though two are present. This discrimination threshold is stable for each person given normal everyday consciousness.

If the finger was five times bigger

“We wanted to find out if it is possible to change this sensation threshold by activating a verbally articulated thought in a person,” explains Albert Newen from the Institute for Philosophy II at the Ruhr University Bochum. The research team chose two thought tracks: “Imagine your index finger is five times smaller” and “Imagine your index finger is five times larger.” 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 sincerely accept the first belief for a series of tests, and then the second.

Subjects participated in a total of four experiments to determine the sensation threshold in each case: under normal everyday consciousness, under hypnosis without suggestion, and under two hypnotic conditions with the suggestion of a larger or smaller index finger.

Changes in the sense of touch.

“Discrimination thresholds did not differ when measured during normal consciousness and hypnosis without suggestion. This supports our preliminary assumption that hypnosis alone produces no change,” says Martin Tegenthoff. “However, if the beliefs are induced as suggestions under hypnosis, we observe a systematic change in the tactile discrimination threshold.” When a test person imagined that her index finger was five times larger than it actually was, his discrimination threshold improved and he could feel two needles, even when they were closer together. When the suggestion was that her index finger was five times smaller, the discrimination threshold worsened. This means that it is the beliefs that change the perception. The behavioral results were supported by parallel recordings of brain activity, such as spontaneous EEG and sensory evoked potentials.

The scientific community is divided on the question of whether or not perceptual processes can be influenced by semantic content alone; experts refer to this as the question of the cognitive penetrability of perception. “Our study provides another building block that supports the idea that such top-down influences of beliefs on perception really do exist,” Hubert Dinse stresses. “The beliefs we hold really change the way we experience the world.”