Left-Side Bias: Brain Responds Strongly to Positive Sounds from Left

Summary: Positive human vocalizations, like laughter, coming from the listener’s left side trigger stronger activity in the brain’s auditory cortex.

Researchers used functional magnetic resonance imaging (fMRI) to monitor the brain’s response to different types of sounds from varying directions in right-handed participants. They observed the highest activation in the primary auditory cortex when participants listened to positive vocalizations coming from the left, compared to front or right.

This finding may imply that the nature, emotional valence, and spatial origin of a sound are initially processed in the primary auditory cortex.

Key Facts:

  1. Positive human sounds, such as laughter, coming from the left side result in stronger activity in the brain’s auditory cortex.
  2. This strong activation due to sounds with positive emotional valence occurs in the primary auditory cortex of either brain hemisphere.
  3. The brain’s apparent preference for positive vocalizations from the left is as yet unexplained in terms of its evolutionary significance or appearance during human development.

Source: Frontiers

Sounds that we hear around us are defined physically by their frequency and amplitude. But for us, sounds have a meaning beyond those parameters: we may perceive them as pleasant or unpleasant, ominous or reassuring, and interesting and rich in information, or just noise.

One aspect that affects the emotional ‘valence’ of sounds – that is, whether we perceive them as positive, neutral, or negative – is where they come from.

Most people rate looming sounds, which move towards them, as more unpleasant, potent, arousing, and intense than receding sounds, and especially if they come from behind rather than from the front.

Credit: Neuroscience News

This bias might have a plausible evolutionary advantage: to our ancestors on the African savannah, a sound approaching from behind their vulnerable back might have signaled a predator stalking them.

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.”

From erotic vocalizations to a ticking bomb

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 hemisphere 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.

Auditory cortex discriminates in favor 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 favor 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.”

About this auditory neuroscience research news

Author: Mischa Dijkstra
Source: Frontiers
Contact: Mischa Dijkstra – Frontiers
Image: The image is credited to Neuroscience News

Original Research: Open access.
Emotional sounds in space: asymmetrical representation within early-stage auditory areas” by Sandra da Costa et al. Frontiers in Neuroscience


Emotional sounds in space: asymmetrical representation within early-stage auditory areas

Evidence from behavioral studies suggests that the spatial origin of sounds may influence the perception of emotional valence.

Using 7T fMRI we have investigated the impact of the categories of sound (vocalizations; non-vocalizations), emotional valence (positive, neutral, negative) and spatial origin (left, center, right) on the encoding in early-stage auditory areas and in the voice area.

The combination of these different characteristics resulted in a total of 18 conditions (2 categories x 3 valences x 3 lateralizations), which were presented in a pseudo-randomized order in blocks of 11 different sounds (of the same condition) in 12 distinct runs of 6 min. In addition, two localizers, i.e., tonotopy mapping; human vocalizations, were used to define regions of interest.

A three-way repeated measure ANOVA on the BOLD responses revealed bilateral significant effects and interactions in the primary auditory cortex, the lateral early-stage auditory areas, and the voice area.

Positive vocalizations presented on the left side yielded greater activity in the ipsilateral and contralateral primary auditory cortex than did neutral or negative vocalizations or any other stimuli at any of the three positions.

Right, but not left area L3 responded more strongly to (i) positive vocalizations presented ipsi- or contralaterally than to neutral or negative vocalizations presented at the same positions; and (ii) to neutral than positive or negative non-vocalizations presented contralateral.

Furthermore, comparison with a previous study indicates that spatial cues may render emotional valence more salient within the early-stage auditory areas.