Researchers at UNIGE, UNIFR have discovered how frequency and amplitude of tactile vibrations can influence how the brain interprets them.
Touch is one of the five senses that humans have mastered the most. It is still a common sense, even though it is not as well-studied, especially with the increasing use of electronic devices emitting vibrations. Any moving object emits oscillatory signals, which propagate through solid substrates. They are detected by mechanoreceptors below the skin, which transmit the information to our brain in the same way as auditory, olfactory, and visual stimuli. Researchers from Switzerland’s universities of Fribourg (UNIFR), and Geneva (UNIGE) have discovered that humans and mice do not perceive vibrations at the same frequency. This is because the brain cannot distinguish the amplitude of the vibration. This creates an illusion that shows how our perception of the world can differ from its actual reality. These results are available in Nature Communications.
Vibrations can be described as small oscillatory movements that are emitted from an equilibrium point. They can be detected by all living organisms because they propagate as waves through solid material. Two main features of vibrations are frequency and amplitude. This is the rate at which vibrations change in Hz (i.e. The number of repetitions per seconds (or amplitude) that a vibration can achieve. Daniel Huber, a professor at the Department of Basic Neuroscience, UNIGE Faculty of Medicine who led the study, said, “When our phone vibrates, it can vary more rapidly – its frequency – and more strongly – its amplitude”. How does the brain interpret these physical characteristics?” This was the question that we set out to answer in our research.
Similar perception in mice and people
The scientists conducted the same experiment with mice and humans, where they had to distinguish multiple vibration frequencies on their hands or paws. Mario Prsa is a professor at the Department of Neuroscience at UNIFR and was the first author of this study. However, mice and humans are unable to distinguish between a lower frequency from a higher one if their amplitudes do not match. It is possible to create perceptual metamers by choosing the amplitudes of different frequencies. These are physically distinct frequencies that can be perceptually undistinguishable. It’s quite amazing!” The illusion is based on a simple principle. Frequency that are higher than or lower than the most sensitive frequency (250 Hz for humans, 1000 Hz in mice) are perceived as being more similar to this frequency when their ampltude is increased. A high frequency vibration, such as 500 Hz, is seen to appear lower than it really is. A high frequency vibration (e.g. 500 Hz) appears lower than it actually is. However, a vibration with a lower frequency than the preferred one (e.g. 150 Hz appears to be higher. Mario Prsa says, “Falling prey to this psychophysical illusion means that the brain misperceives by focusing on what it knows best.” These phenomena can also be seen in other senses such as audition. Our perception of sound is often fooled by low or high volume and it rarely represents the physical attributes of sound. Instead, it is a combination of multiple stimulus characteristics.
What is the secret to this illusion in our brains? Daniel Huber says, “This is exactly the subject of our ongoing research.” “At what point does the brain stop correctly interpreting tactile stimuli and what happens at neuronal levels? Why do different species (e.g. mice and humans) misperceive the same things?
Daniel Huber’s team digs deeper into this subject: With the help of musicians and volunteers who are deaf, they transpose music into a range of vibrotactile stimuli in order to examine how deaf people perceive music.