Why is the sound of nails on a blackboard (and to some of us, even the thought of it) enough to make us wince and cover our ears? Do our ears respond differently to them?
It has been previously known (by brain imaging studies) that in addition to the auditory part of our brain (the auditory cortex), the emotional center of the brain—the amygdala—is also activated by unpleasant sounds. Now how does this dual response work? How are unpleasant sounds represented in the brain? Do specific auditory signals go to the amygdala from the auditory cortex or does it receive them directly? Researchers at Newcastle University have performed a series of experiments seeking to determine precisely this.
The brain scans of 16 participants were obtained in response to a variety of pleasant (bubbling walter, a baby laughing) and unpleasant sounds (a fork scraping glass, nails against a blackboard). Based on these scans, the research team could chalk out the series of events that lead to the disturbed response exhibited upon hearing unpleasant sounds.
The sound signals are first processed by the cortex and then transferred to the amygdala. The amygdala, upon recognizing the signal as being ‘unpleasant’, kicks in to transfer a signal back to the auditory cortex, driving it to perceive the sound as even more unpleasant. What this means is that the intense discomfort you experience when you hear an electric drill is driven by an emotional response. “It appears there is something very primitive kicking in,” says Dr Sukhbinder Kumar, the paper’s author from Newcastle University. “It’s a possible distress signal from the amygdala to the auditory cortex.”
The authors also noted that most sounds we classify as unpleasant belong to a high frequency range of the auditory spectrum, the same range in which the sound of a human screaming can be found. Perhaps our aversion to these sounds are thus evolutionary.
This study could be extended to learn more about the generality of this result. Is this specific to unpleasant sounds? What about words with negative connotations? What about words that are merely ‘negative’ but not distinctly unpleasant? The same research methodology can be extended to uncover the mechanisms by which our brain processes auditory information.
You can read about this research here.