Sounds may be physical phenomena, but to humans they also carry meaning and certain qualities. For instance, some sounds are more pleasing than others, while some are more ominous or unsettling, relaxing and soothing, or just plain noise. Now a group of Swiss researchers have discovered that our brains show a preference towards pleasing sounds made by other humans that we hear from our left-hand side.
Previous research has shown that one factor that affects the emotional “valence” of a sound – whether we see it as positive, negative or neutral – is where it comes from. Looming sounds, things that are moving towards us, are seen as more unpleasant, intense, and arousing than sounds that are receding away from us. This is especially true if the looming sound is coming from behind us, which could be explained by evolution – our ancestors may have perceived sounds approaching from behind as more of a threat.
In addition, past work has shown that our ability to identify emotional tones in human speech is impacted by the direction that we hear it. In 1985, a study demonstrated that our left ear is particularly sensitive to the emotional valence. But a recent study has added additional dimensions to this work by showing that the left-hand side is also more sensitive towards positive sounds, e.g. things that make us feel happier.
According to a team of neuroscientists at the Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne University Hospital and the University of Lausanne, positive sounds made by humans – what is referred to as “human vocalization”- causes stronger neural activity in the brain’s auditory areas when heard from the left-hand side.
“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,” Dr Sandra da Costa, a research staff scientist at the EPFL, said in a statement.
“We also show that vocalizations with neutral or negative emotional valence, for example meaningless vowels or frightened screams, and sounds other than human vocalizations do not have this association with the left side.”
In their experiment, Da Costa and colleagues examined 13 participants who listened to different sounds that came to them from the left, front, or right. They were a mixed group of men and women in their mid-twenties, and none of them had any training in music.
The researchers used functional magnetic resonance imaging (fMRI) to compare how strongly the participant’s brains responded to six categories of sounds. These included positive human vocalizations like erotic sounds, but also neutral and negative vocalizations, such as meaningless vowels and even a frightened scream. The participants also heard positive, neutral and negative non-vocalizations, such as applause, the wind, and a ticking bomb.
The results revealed that there was much more activity in regions of the brain that are associated with early stages of sound processing – the primary auditory areas A1 and R, which are present in both hemispheres of the brain – when listening to positive vocalization coming from the left. Conversely, there was significantly less activity generated by positive vocalizations coming from the front or right, as well as neutral or negative vocalizations and non-vocalizations.
“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,” added co-author Dr Tiffany Grisendi.
The team also discovered that the area L3 present in the right hemisphere, but not the corresponding region in the left hemisphere, also responded better to positive vocalizations coming from the left or right when compared to those coming from the front. When it came to non-vocalizations, however, it did not seem to matter where they came from.
Just why there is this bias towards positive human vocalization is not yet known, but the team admit that this phenomenon has not been explored in non-human species, such as primates. Further research is also needed to find out whether this preference for the left-hand side also translates into someone’s perception of the sound they are hearing.
“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,” explained Professor Stephanie Clarke, of the Neuropsychology and Neurorehabilitation Clinic at the Lausanne University Hospital. “Once we understand this, we may speculate whether it is linked to hand preference or the asymmetric arrangements of the internal organs.”
The study is published in Frontiers in Neuroscience.