The next time you whisper sweet-nothings into someone's ear, you might want to target their left side.
Neuroscientists at the Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne University Hospital and the University of Lausanne in Switzerland have discovered a strange bias in our perception of pleasing voices.
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According to the brain scans of 13 adults, positive human sounds, like laughter, trigger stronger neural activity in the brain's auditory system when they are heard from the left-hand side, suggesting the human auditory cortex is specially tuned to the direction of sounds that make us happy.
Just why there's a preference at all isn't clear – the experiments focused only on changes in activity in the auditory cortex. How such a change translates into someone's perception of those sounds is unknown and would need to be tested in future research.
That said, previous studies have shown the left ear can more easily identify the emotional tone in someone's voice, hinting at some underlying specialization.
Because the left ear feeds information to the right hemisphere of the auditory cortex first, it was assumed that the right side of the brain must be better at processing emotion than the left side.
But these recent results suggest that may not be the right answer.
When the study's participants listened to happy human vocalizations from three different directions – either the left, center, or right – both sides of their auditory cortex activated.
Recordings heard on the left side alone, however, elicited a much stronger neurological response.
"This does not occur when positive vocalizations come from the front or right," says neuroscientist Sandra da Costa from EPFL.
"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."
The direction of a noise can obviously impact the quality of that sound – think of an ambulance siren as it travels toward you and then away. And it can also impact our perception.
Previous studies have shown that looming sounds are often perceived as more ominous and arousing than receding sounds. And evidence suggests a person is more easily aroused when a sound comes from behind.
Heightened sensitivity to certain noises coming from certain directions makes broad evolutionary sense. A human's survival in millennia past would no doubt have depended on being extra suspicious of sounds sneaking up from behind.
But a left-handed bias to the emotion in human voices is not so easily explained.
Some brain functions are known to reside more on the left side of the brain than on the right, and vice versa, but in this particular case, that doesn't seem to explain the results.
While the right hemisphere of the auditory cortex showed a stronger response to happy human voices in one region called L3, both sides of the brain were activated by the sounds in experiments.
"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," says neuroscientist Stephanie Clarke.
"Once we understand this, we may speculate whether it is linked to hand preference or the asymmetric arrangements of the internal organs."
The study was published in Frontiers in Neuroscience.
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