Development and Adaptation of Auditory Spatial Processing

The auditory system constantly monitors the environment to protect us from harmful events such as collisions with approaching objects. Auditory looming bias is an astoundingly fast perceptual bias favoring approaching compared to receding auditory motion and was demonstrated behaviorally even in infants of four months in age. The role of learning in developing this perceptual bias and its underlying mechanisms are yet to be investigated. Supervised learning and statistical learning are the two distinct mechanisms enabling neural plasticity. In the auditory system, statistical learning refers to the implicit ability to extract and represent regularities, such as frequently occurring sound patterns or frequent acoustic transitions, with or without attention while supervised learning refers to the ability to attentively encode auditory events based on explicit feedback. It is currently unclear how these two mechanisms are involved in learning auditory spatial cues at different stages of life. While newborns already possess basic skills of spatial hearing, adults are still able to adapt to changing circumstances such as modifications of spectral-shape cues. Spectral- shape cues are naturally induced when the complex geometry especially of the human pinna shapes the spectrum of an incoming sound depending on its source location. Auditory stimuli lacking familiarized spectral-shape cues are often perceived to originate from inside the head instead of perceiving them as naturally external sound sources. Changes in the salience or familiarity of spectral-shape cues can thus be used to elicit auditory looming bias. The importance of spectral-shape cues for both auditory looming bias and auditory plasticity makes it ideal for studying them together. Born2Hear will combine auditory psychophysics and neurophysiological measures in order to 1) identify auditory cognitive subsystems underlying auditory looming bias, 2) investigate principle cortical mechanisms for statistical and supervised learning of auditory spatial cues, and 3) reveal cognitive and neural mechanisms of auditory plasticity across the human lifespan. These general research questions will be addressed within three studies. Study 1 will investigate the differences in the bottom-up processing of different spatial cues and the top-down attention effects on auditory looming bias by analyzing functional interactions between brain regions in young adults and then test in newborns whether these functional interactions are innate. Study 2 will investigate the cognitive and neural mechanisms of supervised learning of spectral-shape cues in young and older adults based on an individualized perceptual training on sound source localization. Study 3 will focus on the cognitive and neural mechanisms of statistical learning of spectral-shape cues in infants as well as young and older adults.

Imagine walking down a street: even without looking, you can often tell when something is moving toward you-a cyclist, a car, or a person approaching from behind. This instinctive sensitivity to approaching sounds is known as the auditory looming bias. It allows us to detect potential danger early, long before we consciously identify what the sound is. Our Born2Hear project set out to understand how this ability develops, how the brain processes it, and whether it can be shaped by experience. We began by examining how early in life this bias appears. To do this, we compared newborns-up to four days old-to adults. Both groups listened to sounds that either moved closer or farther away. The newborns already showed stronger brain responses to approaching sounds, but only when the sound simply grew louder. Adults, in contrast, also reacted to more complex changes in sound quality that depend on the shape of the outer ear. This suggests that part of the looming bias is inborn, while another part develops gradually over time as we learn to use the full range of spatial hearing cues around us. Next, we explored why the brain treats approaching sounds so differently. Our brain recordings revealed that regions often linked to evaluating threats and preparing quick reactions became more active for approaching sounds. This happened even when listeners were busy with a different task and indicates that the looming bias likely evolved as an automatic warning system, helping us respond to potentially dangerous situations without needing conscious effort. We then asked whether this bias depends on our ability to precisely locate sounds in space. To test this, we altered the way participants heard sound in a virtual environment by giving them "virtual" ears that disrupted normal spatial cues. With these artificial cues, people could still detect changes in sound, but could no longer reliably tell where the sound was coming from. Under these conditions, the looming bias weakened significantly. This tells us that the brain's ability to detect approaching sounds seems closely linked to accurate spatial hearing. Finally, we investigated whether training people to better localize sounds could enhance or modify the looming bias. Participants did improve their ability to locate sounds after training. However, we did not find evidence for the automatic brain responses linked to detecting approaching sounds to change accordingly. This may suggest that the looming bias operates as a deeply rooted, early-warning mechanism that does not easily adapt through conscious learning. Overall, our research shows that the auditory looming bias is partly innate, shaped by experience, closely tied to spatial hearing, and deeply connected to the brain's threat detection systems.