Screams can produce real feelings of fear in people who hear the often blood-curdling sounds. Now, researchers believe they have uncovered the underlying reason the fearsome calls exert such a hold on the human psyche.

New York University researchers carefully examined the soundwaves of screams to determine what characteristics make these sounds so powerful. Investigators found that human beings are capable of producing sounds which are exclusively used to signal distress.

"Everybody screams and everybody has an intuition about what constitutes screams - that they are loud and high-pitched. But neither turns out to be quite correct. In fact, screams have their own acoustic niche separate from other sounds. While, like some sounds, they may be high-pitched and loud, screams are modulated in a particular way that sets them apart from the rest," said David Poeppel from the Department of Psychology and Center for Neural Science at NYU.

During normal conversation, the amplitude of speech modulates at a rate of around four or five cycles a second, designated as Hertz (Hz). However, acoustical analysis of screams showed this "roughness" occurred at a much faster rate - between 30 and 150 Hz.

Subjects in the study were exposed to inarticulate screams, as well as screamed words. These were found to have the higher roughness rate not seen in spoken sentences or meaningless vocalizations. A similar characteristic was observed in some artificial sounds, including alarms, and dissonant pairings of musical notes. The characteristic was not seen in other, most pleasant, sounds. This finding would suggest that the reason people find alarms so jarring is because the roughness of the sounds resembles that of a human scream.

Participants in the study were asked to listen to a series of sounds, and note when they found what they were hearing to be alarming. Researchers found that rougher sounds were deemed to elicit higher levels of fear in subjects.

In addition to studying graphical representations of the acoustic signatures of screams and other voice characteristics, the team also examined which parts of the brain are activated by the sound of screaming. Perhaps not surprisingly, the amygdala, the area of the brain which processes fear, was shown to the be excited by the sounds of screaming, as well as alarms.

"As a whole, our findings show that screams occupy a privileged acoustic niche that, because they are separated from other communication signals, ensures their biological and ultimately social efficiency - we use them only when we need them," Poeppel stated in a press release.

Analysis of screaming and its affect on the brain was profiled in the journal Current Biology.

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