A new study led by scientists in The Netherlands has revealed the mechanisms through which the brain creates “auditory continuity illusion”, where a physically interrupted sound is heard as continuing through background noise; thus when we try to listen to conversation in a noisy room, the brain fills in the gaps between interrupted sound fragments to create what we perceive as a continuous sound.

The study was the work of senior author Dr Lars Riecke from the Department of Cognitive Neuroscience at Maastricht University and colleagues and was published online in the journal Cell Press on 25 November.

It is quite common for us to “hear” sounds that aren’t really there: human hearing is a constructive process.

The brain has the ability to take auditory fragments and generate an overall “image” of the sound. It is like assembling a jigsaw puzzle with some of the pieces missing but still having a good enough impression of the overall picture.

In this new study, Riecke and colleagues have revealed some new insights into how the brain is able to do this.

Riecke told the media that:

“In our day-to-day lives, sounds we wish to pay attention to may be distorted or masked by background noise, which means that some of the information gets lost. In spite of this, our brains manage to fill in the information gaps.”

For the study, he and his colleagues investigated the timings of when the brain encodes interrupted sounds and when it produces the auditory image or illusion of the continuous sound.

They got participants to rate the continuity of a tone while they monitored the electrical activity in their brains.

The researchers discovered that when a sound was restored perceptually (ie when the participant “heard” it), slow brain waves called theta oscillations, which occur when the brain is encoding boundaries of sound, were suppressed during the interruption of that sound.

They wrote that:

“When physically interrupted tones are perceptually restored, stimulus-evoked synchronization of cortical oscillations at [approx] 4 Hz is suppressed as if physically uninterrupted sounds were encoded.”

Riecke said:

“It was as if a physically uninterrupted sound was encoded in the brain.”

This restoration-related suppression was most obvious in the right auditory cortex, said the researchers, both during the illusorily filled gaps and also just before and after the gaps.

The findings reveal a new mechanism that helps us better understand human hearing.

In conclusion, Riecke said that their results showed that:

“Spontaneous modulations in slow evoked auditory cortical oscillations may determine the perceived continuity of fragmented sounds in noise.”

One point that particularly interested them was the fact that the suppression occurred before the gap and reached a maximum just afterwards, suggesting that the mechanism works very fast, and perhaps even anticipates gaps, thus making it possible for us to hear fragmented sounds in natural environments.

The researchers proposed that these findings may also help in the design of better hearing aids.

“Hearing Illusory Sounds in Noise: The Timing of Sensory-Perceptual Transformations in Auditory Cortex.”
Lars Riecke, Fabrizio Esposito, Milene Bonte, Elia Formisano.
Neuron , 25 November 2009 (Vol. 64, Issue 4, pp. 550-561).

Source: Cell Press.

Written by: Catharine Paddock, PhD