Scraping a

chalkboard A blackboard or a chalkboard is a reusable writing surface on which text or drawings are made with sticks of calcium sulphate or calcium carbonate, better known as chalk. Blackboards were originally made of smooth, thin sheets of black or da ...

(also known as a blackboard) with one's

fingernails A nail is a protective plate characteristically found at the tip of the digits (fingers and toes) of all primates, corresponding to the claws in other tetrapod animals. Fingernails and toenails are made of a tough rigid protein called alpha-ke ...

produces a

sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

and feeling which most people find extremely irritating. The basis of the innate reaction to the sound has been studied in the field of

psychoacoustics Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound including noise, speech, ...

(the branch of psychology concerned with the perception of sound and its physiological effects).

Physiological response

Brain stem reflex

In response to audio stimuli, the mind's way of interpreting sound can be translated through a regulatory process called the reticular activating system. Located in the

brain stem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is co ...

, the reticular activating system continually listens, even throughout delta-wave sleep, to determine the importance of sounds in relation to waking the cortex or the rest of the body from sleep. Chalkboard scraping, or noises that elicit an emotional response have been known to trigger tendencies from the fight or flight response which acts as the body's primary self-defense mechanism.

Emotional response

Distinct emotion

A study published in 2017, in ''

Frontiers in Psychology ''Frontiers in Psychology'' is a peer-reviewed open-access academic journal covering all aspects of psychology. It was established in 2010 and is published by Frontiers Media, a controversial company that is included in Jeffrey Beall's list of " ...

'', found that the emotion elicited by this sound and similar sounds such as a knife scraping a plate, was similar to but distinct from disgust. The physiological response was a very slight initial fall in heart rate followed by a sharp rise, returning to normal after about 6 seconds. This pattern differed from that elicited by disgust and was shared by speakers of Spanish (which has a word for the emotion: ''grima'') and speakers of German and English (which have no single word for the emotion). Spanish volunteers who were asked to suppress their response to ''grima''-inducing sounds were able to do so to some extent but there was no change in response to sounds that induced disgust. Some volunteers said that ''grima'' could be induced not only by sounds but also by the feel of some objects, such as sponge rubber or cork.

Primate heritage hypothesis

One explanation for the adverse reaction is that the sound is similar to the warning call of a

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

, as would have been commonly heard among human ancestors in prehistoric times. However, a study using

Cotton-top tamarin The cotton-top tamarin (''Saguinus oedipus'') is a small New World monkey weighing less than . This New World monkey can live up to 24 years, but most of them die by 13 years. One of the smallest primates, the cotton-top tamarin is easily reco ...

s, a species of

New World monkey New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboi ...

, found that they reacted similarly to both high-pitched sounds similar to fingernails on chalkboard, and to amplitude-matched

white noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used with this or similar meanings in many scientific and technical disciplines, i ...

. In contrast, humans are less averse to the white noise than to scraping. A 1986 study used a tape-recording of a three-pronged garden tool similar to a

fork In cutlery or kitchenware, a fork (from 'pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tines with which one can spear foods either to h ...

being " grided" across a chalkboard, which roughly reproduces the sound of fingernails on chalkboard. The recording was then manipulated, removing pitches at the extremities and the median. The results were then played back. It was determined that the median pitches are in fact the primary cause of the adverse reaction, not the highest pitches as previously thought. The authors hypothesized that it was due to predation early in human evolution; the sound bore some resemblances to the alarm call of macaque monkeys, or it may have been similar to the call of some predator. This research won one of the authors, Randolph Blake, an

Ig Nobel Prize The Ig Nobel Prize () is a satirical prize awarded annually since 1991 to promote public engagement with scientific research. Its aim is to "honor achievements that first make people laugh, and then make them think." The name of the award is a ...

in 2006. More recent research contradicts this hypothesis.

Physical hypothesis

In a 2011 study, musicologists Michael Oehler and Christoph Reuter hypothesize that the unpleasantness of the sound is caused by

acoustic resonance Acoustic resonance is a phenomenon in which an acoustics, acoustic system amplifies sound waves whose frequency matches one of its own natural frequencies of vibration (its ''resonance frequencies''). The term "acoustic resonance" is sometimes u ...

due to the shape of the human

ear canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the auricle to the eardrum and is about in length and in diameter. S ...

which amplifies certain frequencies, especially those in the range of 2000 to 4000 Hz (the median pitches mentioned above); at such a level that the sound would trigger pain in human ears.

References

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