Newswise — The sounds we encounter in our surroundings are characterized by their frequency and amplitude on a physical level. However, to us, sounds possess a significance that surpasses these measurements. We might interpret them as agreeable or disagreeable, foreboding or comforting, captivating and full of insights, or simply as noise.
The emotional 'valence' of sounds, which refers to whether we perceive them as positive, neutral, or negative, is influenced by their source. Generally, people tend to rate approaching sounds, those that move towards them, as more unpleasant, powerful, stimulating, and intense compared to receding sounds, particularly when the sounds originate from behind rather than from the front. This bias could be attributed to a plausible evolutionary advantage: for our ancestors residing in the African savannah, a sound approaching from their vulnerable back would have likely signaled a predator stealthily stalking them.
Researchers from Switzerland have recently discovered an additional impact of direction on emotional valence: our response to positive human sounds, such as laughter or pleasant vocalizations, is amplified when they originate from the left.
According to Dr. Sandra da Costa, a research staff scientist at EPFL in Lausanne, Switzerland, their study demonstrates that human vocalizations that evoke positive emotional responses elicit robust activity in the auditory cortex of the brain when they originate from the listener's left side. This phenomenon is not observed when positive vocalizations come from the front or right side.
The researchers also found that vocalizations with neutral or negative emotional valence, such as meaningless vowels or frightened screams, as well as non-human sounds, do not exhibit the same association with the left side. In other words, the enhanced response to positive human vocalizations specifically occurs when they originate from the listener's left side, while other types of sounds do not show this pattern.
From erotic vocalizations to a ticking bomb
In their study, Dr. Sandra da Costa and her colleagues employed functional magnetic resonance imaging (fMRI) to measure the brain responses of 13 right-handed volunteers in their mid-twenties, comprising both women and men without music training. The participants were exposed to sounds originating from the left, front, or right side, and the researchers analyzed the strength of their brain responses. The study encompassed six categories of sounds: positive human vocalizations (such as erotic sounds), neutral and negative vocalizations (including meaningless vowels and a frightened scream), as well as positive, neutral, and negative non-vocalizations (such as applause, wind, and a ticking bomb). By comparing the brain's reaction to these various sound categories, the researchers aimed to determine the differential impact of direction and emotional valence on neural activity.
In their study, Da Costa and colleagues specifically investigated brain regions that play a crucial role in the initial stages of sound processing. These regions include the primary auditory areas A1 and R, as well as the surrounding early-stage auditory areas and the 'voice area' (VA). It is worth noting that each of these areas exists in both the left and right hemispheres of the brain. By examining the activity within these regions, the researchers aimed to gain insights into how the brain responds to different types of sounds and how their emotional valence and direction influence neural processing
The findings of the study indicated that both the A1 and R regions in both hemispheres of the brain exhibited maximum activation when participants listened to positive vocalizations originating from the left side. In contrast, the response was significantly reduced when positive vocalizations came from the front or right side, as well as in the case of neutral or negative vocalizations, and non-vocalizations. This suggests that the brain's auditory processing areas are particularly sensitive to positive vocalizations when they are perceived from the left side, while other types of sounds do not elicit the same level of activation in these regions.
Auditory cortex discriminates in favor of positive vocalizations from left
"According to co-author Dr. Tiffany Grisendi, the primary auditory cortex in either hemisphere exhibits robust stimulation when encountering vocalizations with favorable emotional valence originating from the left. These cortical regions are the initial recipients of auditory input in the brain. Our discoveries imply that the essence, emotional significance, and spatial source of a sound are initially recognized and handled within this domain."
Furthermore, the right hemisphere's area L3, unlike its counterpart in the left hemisphere, exhibited heightened responsiveness to positive vocalizations originating from either the left or right, as opposed to those originating from the front. Conversely, the origin of the sound had no bearing on the response to non-vocal stimuli.
The evolutionary significance of our brain’s bias in favor of positive vocalizations coming from the left is still unclear.
Prof Stephanie Clarke, the senior author from the Neuropsychology and Neurorehabilitation Clinic at the Lausanne University Hospital, stated, "The timing of the primary auditory cortex's inclination towards positive human vocalizations from the left during human development remains uncertain, as does whether this is an exclusively human trait. Once we gain a comprehensive understanding, we can speculate on potential connections to hand preference or the asymmetric organization of internal organs."
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