When the auditory system blurs the bodily representation and vice versa

When the auditory system blurs the bodily representation and vice versa
When the auditory system blurs the bodily representation and vice versa

Imagine this little test: you close your eyes and extend your arms forward, while a voice in a stationary speaker located two meters away pronounces sentences. You must now walk in place for 60 seconds and try to stay in the same place by orienting yourself using the voice that continues to speak to you.

Where will you be after 60 seconds, when you open your eyes?

Despite your efforts to stay put, you will have veered to the right in a curve that will have caused you to deviate by 26 degrees (on average) from the initial position of the speaker, which was 90 degrees from your right ear .

The people responsible for this deviation are your hearing function and your body representation system, which disrupt each other.

This is what Daniel Paromov observed in his doctoral work carried out under the direction of François Champoux, from the School of Speech Therapy and Audiology at the University of Montreal, the results of which were published in the journal iScience.

Disturb the senses to better understand them

Daniel Paromov

Credit: Joëlle Simard-Lapointe

The auditory system is one of the different senses that allows one to locate oneself in space, contributing to bodily representation. And although different studies have demonstrated the influence of the auditory system on body perception and movement-related activity, the influence of body representation on spatial hearing remained unexplored.

In short, we know that what we hear can affect the way we feel and move, but we do not know whether the way we feel and move can have an effect on what we hear.

This is what Daniel Paromov explored by recruiting 20 people to give them an experience during which he disrupted the processing of auditory information.

Being “tricked” by your hearing system

To begin, we placed the speaker at one of the three positions tested (i.e. at 0, 45 and 90 degrees), at a distance of two meters, as a standard measurement. At this stage, all the subjects clearly identified where the voice was coming from: they determined its position with enough precision.

Then, they were asked to walk in place with their eyes closed at a rate of two steps per second, for 60 seconds, maintaining the same angle relative to the direction of the voice. Everyone was subjected to the same experiment three times.

In all cases, when the speaker was facing them (0 degrees), the average deviation was 13 degrees. With the speaker positioned at 45 degrees, subjects moved forward and deviated 24 degrees, then 26 degrees when the sound came from a 90-degree angle to the right.

Their body rotation reached 77 degrees.

An unbalanced interaction

Without sensory input from vision, the interaction between the auditory and sensorimotor systems is unbalanced. So that the change in body orientation leads to illusory changes in sound localization.

“To know where it is in space, the brain brings together information from different senses and, through this process, creates a coherent and apparently fluid perception of the external world,” explains Daniel Paromov. Usually, when the brain combines information from our five senses, it comes from events that are related, but when that information doesn’t quite match what’s happening, it can cause misleading perceptions.”

He adds: “We must above all remember that, by creating a simple illusion in the representation of the body in space, we disrupt auditory perception. We were thus able to observe a reciprocal effect between the auditory system and bodily representation: they influence each other, which was unknown to us.

According to the director of the study, François Champoux, these data reconfigure the way we think about spatial auditory perception.

“The laws of psychoacoustics no longer hold when we put them in relation to the other senses,” underlines the professor. Our data show, among other things, that non-auditory sensory variables are necessary for auditory localization and, without these variables, auditory localization simply seems impossible. Most of what we know about the auditory system is taken in isolation, and it’s high time we start exploring how the auditory system interacts with the other senses if we really want to understand some of the issues surrounding perception.”

A strong illusion with potentially concrete consequences

According to François Champoux, the auditory illusion created by Daniel Paromov is the strongest observed to date in a localization task.

“You have to understand that we are normally very precise in locating a sound in space, so a deviation of 26 degrees is enormous! he exclaims. What is striking about this illusion is the simplicity of the task and the enormity of the result: everyone can see what is being revealed quite categorically.”

If this fundamental research project has demonstrated this reciprocity effect, different applications could result from it, according to Daniel Paromov.

“Our results allow us to see the possibility of better understanding certain effects attributable to the senses, including motion sickness, or the way in which the brain reacts in space when subjected to the illusion created by virtual reality », concludes the doctoral student.

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