The sounds of science: what we hate to hear

Neuroscience – the study of the nervous system – can provide us with insights into how the nervous system functions, normally and abnormally, to advance our understanding of human thought, emotion, and behaviour. The Society for Neuroscience explains that through the study of the brain, spinal cord, and networks of sensory nerve cells (or neurons), as they communicate with each other through electrical signals and by releasing chemicals (called neurotransmitters) which leap across synapses (small gaps between neurons), we gain a fuller understanding of humans themselves (http://www.sfn.org).

Humans have about 100 billion neurons within our nervous system. The nervous system contains two main parts: (1) the brain and spinal cord; and (2) the peripheral nervous system containing the nerves in the neck, arms, body, legs, skeletal muscles, and internal organs. The nervous system controls functions such as vision, hearing, learning, and breathing – and ultimately, all of human behaviour (by integrating and unifying biology, chemistry, physics, physiology, and psychology.

Clinical neuroscientists use basic research findings to find out what is happening in the human brain to help understand and treat medical conditions and illnesses such as stroke, schizophrenia, autism, and Alzheimer’s disease.

In the Journal of Neuroscience, as study reports the findings of the world’s harshest sounds – that is, sounds that annoy the human brain (Features versus Feelings: Dissociable Representations of the Acoustic Features and Valence of Aversive Sounds, 10 October 2012: www.jneurosci.org).

Not surprisingly, the screech of a knife against a glass bottle is the most difficult sound to bear. The sound of a fork against glass, chalk on a board, ruler on a bottle, and nails on a chalkboard, were all especially “repugnant” to the ear and the brain. Sounds such as flowing water were much more agreeable said the study’s co-author, Tim Griffiths of Britain’s University of Newcastle.

The study measured brain activity in 16 volunteers who listened to 74 recorded sounds under an MRI (magnetic resonance imaging) machine. Each volunteer ranked each sound. The MRI scans revealed that unpleasant noises increased blood flow in the brain’s auditory (sound) cortex, and triggered the amygdala - the part of the brain that processes our emotional responses. The most disagreeable sounds had frequencies between 2,000-5,000 Hz (the part of the spectrum at which our ears are most sensitive).

The study aimed to answer three questions: (1) what does activity in the amygdala and auditory cortex represent in response to unpleasant sounds; (2) does the amygdala receive direct subcortical auditory inputs or are they relayed through the auditory cortex; and (3) how do acoustic features and valences modulate the coupling between the amygdala and the auditory cortex. The answers were: (1) the amygdala encodes both the acoustic features and valence of aversive sounds; (2) information is relayed to the amygdala via the auditory cortex; and (3) while the acoustic features modulate the forward coupling from auditory cortex to the amygdala, valence modulates backward connectivity from the amygdala to the auditory cortex.

The 16 healthy volunteers between the ages 22-35 years (7 of them female) participated in the study. They knew that they would be listening to unpleasant sounds, but did not know the type of sounds or the overall aim of the study. They listened to a set of 74 sounds for about two seconds at a time. The sound included scraping on a chalkboard to animal cries to bubbling water. Each sound was repeated four times during the experiment, and the volunteers rated them on a scale from 1 (least unpleasant) to 5 (highly unpleasant) via the press of a button. While the volunteers were listening to the sounds, the MRI machine scanned their brains to determine what was happening so that the researchers could compare the ratings with the brain’s response. The MRI scan produced images which could be saved and examined.

While researchers knew from previous studies, and intuitively, which sounds would register as the most unpleasant, it was unknown what was occurring in the brain at the same time, and what the connection was between the ear and the brain – i.e. the relationship between the activity and the emotion.

The University of Newcastle study revealed that the aversive stimuli (sound) are first processed and decoded in the auditory cortex before an emotional response is elaborated. For example, the study found that an animal cry, signalling the presence of a dangerous animal, may have different time and frequency structures relating to the size of the animal. To decode the size of the animal from only hearing the sound, the sound needs to be processed to a high level in the auditory cortex before it is evaluated in the amygdala (the emotional response area of the brain). In this study, the volunteers were not shown pictures or images accompanying the sound – only the sound.

Other studies have shown stronger and faster responses to negative sounds than to positive sounds. Brain imaging studies show different networks of brain regions may be involved in processing positive and negative stimuli. This could explain why people become vocally responsive to unpleasant sounds – and often show a wider range of facial expressions! And many people wince at the mere mention of fingernails scratching a chalkboard – sending tingles right up the spine!!

http://news.ninemsn.com.au/technology/8546798/science-reveals-worlds-harshest-sounds