Brain's primary auditory cortex is more responsive to human vocalizations

Published in the journal Frontiers in Neuroscience, the study states that the frequency and loudness of sounds we hear around us literally describe those sounds. But for humans, sounds carry significance that goes beyond those limitations. We may find sounds to be comforting or unsettling, threatening or reassuring, engaging and full of information, or merely noise.

From an evolutionary perspective, a bias can be seen as a potential adaptive advantage. In ancestral environments, where humans faced potential threats and predators, a sound approaching from behind, towards the vulnerable back, could signify potential danger. The heightened emotional response to such sounds may have helped our ancestors detect and respond to potential threats more effectively. This bias is often referred to as the "looming bias" or the "looming effect." It suggests that our perception of sounds approaching us from behind activates a more intense emotional response compared to sounds that are receding or coming from other directions.

However, it is important to note that the interpretation of sounds in specific contexts can also influence our emotional response. Factors such as familiarity of the sound, the overall environmental context, and individual differences can all play a role in shaping our emotional reactions to auditory stimuli.

The perception of looming sounds as more negative and intense may be a result of our evolutionary history, but it is not the sole determinant of our emotional responses to sounds. Multiple factors interact to create our complex emotional experiences in response to the auditory world around us.

Now, neuroscientists from Switzerland have shown another effect of direction on emotional valence: we respond more strongly to positive human sounds, like laughter or pleasant vocalizations, when these come from the left.

"Here we show that human vocalizations that elicit positive emotional experiences, yield strong activity in the brain's auditory cortex when they come from the listener's left side. This does not occur when positive vocalizations come from the front or right," said first author Dr Sandra da Costa, a research staff scientist at the EPFL in Lausanne, Switzerland.

"We also show that vocalizations with neutral or negative emotional valence, for example meaningles vowels or frightened screams, and sounds other than human vocalizations do not have this association with the left side."

Da Costa and colleagues used functional magnetic resonance imaging (fMRI) to compare how strongly the brain of 13 volunteers responded to sounds coming from the left, front, or right. These were women and men in their mid-twenties, all right-handed, and none were trained in music. The researchers compared the brain's response between six categories of sounds: besides positive human vocalizations like erotic sounds, they played back neutral and negative vocalizations, like meaningless vowels and a frightened scream; and positive, neutral, and negative non-vocalizations, like applause, wind, and a ticking bomb.

Da Costa et al. focused on brain regions known to be important for the early stages of sound processing, the primary auditory areas A1 and R, the surrounding other early-stage auditory areas, and the 'voice area' (VA). Each of these areas occurs in the left and right hemispheres of the brain.

The results showed that A1 and R in both hemispheres became maximally active when listening to positive vocalizations coming from the left, and much less when listening to positive vocalizations coming from the front or right, to neutral or negative vocalizations, or to non-vocalizations.

The auditory cortex discriminates in favour of positive vocalizations from left

"The strong activation by vocalizations with positive emotional valence coming from the left takes place in the primary auditory cortex of either hemisphere: the first areas in the brain cortex to receive auditory information. Our findings suggest that the nature of a sound, its emotional valence, and its spatial origin are first identified and processed there," said co-author Dr Tiffany Grisendi.

In addition, area L3 in the right hemisphere, but not its twin in the left hemisphere, also responded more strongly to positive vocalizations coming from the left or right compared to those coming from the front. In contrast, the spatial origin of the sound didn't impact the response to non-vocalizations.

The evolutionary significance of our brain's bias in favour of positive vocalizations coming from the left is still unclear.

Senior author Prof Stephanie Clarke, at the Neuropsychology and Neurorehabilitation Clinic at the Lausanne University Hospital, said: "It is currently unknown when the preference of the primary auditory cortex for positive human vocalizations from the left appears during human development, and whether this is a uniquely human characteristic. Once we understand this, we may speculate whether it is linked to hand preference or the asymmetric arrangements of the internal organs." (ANI)