Screaming can be annoying, but from an evolutionary standpoint, it is quite useful. A study shows the brain's response to "rough" vocalization.
Humans are hard-wired to interpret screams as a sign of danger, Swiss researchers have found. Reporting in Current Biology, on July 16, 2015, Luc Arnal, PhD, and colleagues at the department of clinical neurosciences at the University Hospital and the University of Geneva in Switzerland and elsewhere looked at subjects’ brain responses to “rough” vocal sounds.
Though this vocal roughness has long been considered unpleasant and annoying, it was also long believed to be irrelevant to human communication, the researchers said.
They found that this is not the case.
MRI scans of the subjects’ brains showed that rough sounds elicited a response in the amygdala, but not in the auditory cortex. Researchers concluded, “these results demonstrate that rough sounds specifically target neural circuits involved in fear/danger processing and hence provide evidence that roughness constitutes an efficient acoustic attribute to trigger adapted reactions to danger.”
Various characteristics of sound, referred to as spectro-temporal attributes, convey certain information, and are measured along the modulation power spectrum (MPS). For example, frequency conveys the gender of the speaker, and temporal fluctuations help listeners decode and parse speech. Roughness, another spectro-temporal attribute, has long been believed to be irrelevant to human communication.
A large part of the MPS is occupied by rough sounds, and the researchers demonstrated that screams, and other alarm sounds, fall in the rough range. In order for screams to be an effective form of communication they must be sufficiently different from other sounds. Researchers compared four types of sounds in two categories, which they defined as “scream” and “sentence.”
Screams consistently fell into the rough part of the MPS, and neither speech nor singing did. The researchers also tested the roughness of sentences spoken in several different languages and found no significant differences.
Researchers also found that rougher the scream, the more fear it induced. In an additional experiment, neutral vocalizations were modulated to be rougher, and listeners again perceived more fear.
Artificial sounds, those of an alarm and of instruments, were also tested, and the theory held true. The rougher the alarm sound, the more alarming it was perceived to be; and when instrument sounds were modulated to be rougher, they were also perceived as more alarming.
Finally, the researchers tested to see if the human brain can more accurately and efficiently locate rougher sounds. Both reaction time and accuracy of pinpointing location were better when participants heard sounds in the rough region of the MPS, both screams and artificial alarms.