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On the physical end of the experiment, the researchers monitored various vital signs of the participants while the tones were played. The results were fairly dramatic. Not surprisingly, a strong psychological component was identified by the researchers. When asked to rank the sounds on a scale of unpleasantness, participants that were told the sounds were from a musical composition consistently found the sounds more bearable than when they were told the true source of the sound.

The most impressive results came in the physiological measurements taken by the researchers. They found a dramatic change in skin conductivity when unpleasant sounds were played, showing a distinct physical reaction to the sound. The researchers also pinned down the frequency range of skin-crawling sound to between 2, and 4, Hz — which is the same range as human speech.

Inge Schweiger Gallo of the Complutense University of Madrid has personal reasons for studying the phenomenon. Schweiger Gallo and her colleagues began by asking Spanish speakers what grima means to them.

Stimuli that elicited grima included squeaking noises, scratching with fingernails and scratching on surfaces.

The volunteers rated grima as being less pleasant than disgust. Next, the team turned to German and English speakers, who have no word for grima in their languages. Sounds labelled as disgusting or unpleasant showed a different pattern, falling more sharply, and then returning more steadily to normal.

The effects on skin conductance — a sign of physiological changes — were similar for grima sounds and disgusting or unpleasant sounds. The team then asked Spanish volunteers to try to suppress their responses to grima. So when someone screams, or a baby cries, the amygdala takes over, heightening the activity in the auditory complex and creating a negative reaction.

This activity is not as heightened when listening to a soothing sound. Based on acoustic analysis, the scientists determined that all sounds from to Hertz were unpleasant to the subjects of the study. They hope that a better understanding of how the brain reacts to noise will help people sensitive to loud sounds, like those with some forms of autism, and people who suffer from tinnitus and migraines.