Whispering neighbours in the cinema, clinking cups in the café or a multitude of simultaneous conversations at the family coffee table: even if we don't always notice it, we are constantly busy blocking out noise so that we can concentrate on the essentials.
"In order to process auditory information, we have to isolate it. If this doesn't happen, we perceive different acoustic stimuli that don't fit together. As a result, we don't understand either the one or the other piece of information correctly," explains hearing researcher Dr Sebastian Puschmann. In the Department of Biological Psychology, headed by Prof Dr Christiane Thiel, he is investigating the role the brain plays in hearing and understanding as part of the Hearing4all Cluster of Excellence. Specifically, he is researching what happens in the brain when we deliberately try to overhear something. As part of a study, a team led by Puschmann has now visualised brain activities that take place when a test subject hears two speakers at the same time, but only pays attention to one of them.
For the current study, 45 test subjects between the ages of 19 and 35 with normal hearing were placed in a magnetic resonance imaging (MRI) scanner and listened to ten-minute fairy tale stories - either without distracting background noise or overlaid with a second fairy tale story. "This is one of the most difficult situations imaginable for the brain," says Puschmann about the two-speaker scenario. At the same time, it is closer to reality than previous listening experiments, in which researchers often only played individual syllables or words to test subjects in an artificial background noise. "If we want to better understand the brain's ability to hear and understand, we need to carry out experiments in realistic scenarios," explains the hearing researcher. Only then can the brain also perform tasks that are not directly related to hearing, but do have something to do with understanding - for example, when it recognises words or parts of sentences from the context, even if part of the acoustic signal was perhaps not audible.
Using functional MRI images, the Oldenburg team has now succeeded in visualising the brain activity of the test subjects when listening and ignoring and recognising revealing patterns. What looks like a flickering grey brain scan to the uninitiated gives researchers clues as to the moments when oxygen saturation is particularly high in the various areas of the brain. "A high oxygen content indicates a high level of neuronal activity," explains Puschmann. If many test subjects show a high level of activity in a brain region at the same time as the fairy tale being played in the MRI, it can be assumed that this is a reaction to what they are hearing.
The main aim of the study was to gain a better understanding of how focussing attention on a speaker influences how auditory stimuli are transmitted in the brain. "We discovered that there is not one area of the brain or one processing stage at which unwanted information is filtered out. Rather, it is a gradual process. The signals that we try to ignore are filtered out, so to speak," explains Puschmann. Particularly at the first processing stage of auditory stimuli in the cerebral cortex, the primary auditory cortex, the researchers also found clear patterns of brain activity that could be traced back to the speaker being ignored. "In this area of the brain, the disturbing noise still arrives relatively unfiltered - no matter how hard someone concentrates on avoiding it," says Puschmann.
In addition to new insights into where the brain filters the story that is being followed attentively from the distracting story, the study results also provide the researchers with methodological advantages for the future. For example, they have selected an analytical approach that enables them to obtain usable measurement results with a remarkably short magnetic resonance imaging (MRI) examination. "This means that in future we will also be able to examine test subjects who we don't want to subject to a long measurement," explains Puschmann.
The next step is to focus on test subjects who have a problem that affects many people in old age: They may not feel they have poor hearing, but they still have problems understanding. Conversations in noisy environments are therefore particularly difficult for them. "We want to investigate which brain reactions we can measure in them when they try to block out background noise," says Puschmann. This could shed light on the extent to which hearing problems in noisy environments can be attributed to the fact that it is more difficult to filter out background noise in old age.
