Introduction

I will begin with a pair of quotes taken from the book Fundamentals of hearing by William A. Yost:

"In the most general sense when we say we hear, we usually mean that we are sensitive to the sounds in our environment... Any vibrating object with the properties of inertia and elasticity may be set into vibration and then produce a sound. ... In practice, almost any object has inertia and elasticity, and so almost any object can be set into vibration."

"Sounds constantly surround us and informs us about many objects in our world. Determining the sources of the sounds is one of our most important biological traits. Any animal's ability to locate food, avoid predators, find a mate, and communicating depends on being able to determine the sources of sounds. In order to determine sound sources we need to know that an object exists, what it is, where it is, if it is moving, and so on."

The present document is an introduction about how to hear the sounds of the brain. I am a Clinical Neurophysiologist and a substantial part of my work is obtaining EMG recordings. EMG signals are in the audible range. From the origin of the technique long time ago, a loudspeaker is connected to the output of the amplifier and in this way the patient can hear his or her own "muscle noise" in real time. The patient can hear the variation with the increase of effort or the rhythmical appearance of the discharge of Motor Unit Potentials (the basic functional units of the neuromuscular system). This activity is highly informative for the physician carrying the test. The patient sometimes asks: Are all these noises inside of me? Usually we answer that they are electrical activity transformed into noise by the loudspeaker but in certain sense what the patient is hearing is his/her own muscle noise.

Physiological signals are often outside the audible range and several approaches can be followed to hear them. These approaches are globally termed sonification. The simplest one is called audification. When we say audification we mean hearing the signal by only changing its frequency (we accelerate or reduce its frequency to accommodate to our hearing range).

The audification of a physiological signal is a process that can be carried out nowadays in a much easier way than before because of the convergence of several developments

• Physiological signals are digitized and stored as computer files using well-known formats. One of these formats (EDF: European Data Format) is widely used. So it is probable that if you have a digital polygraph you can directly export your data to EDF format. EDF files are also available on the Internet
• Computers became multimedia in the last decade. So you have tools to hear sounds. There are a myriad of applications able to handle, edit and modify sounds. Many of them allow performing spectral analysis and creating a interface between the signal and its sound
• Massive storing allows handling very long signals as well as sounds in your computer and hardware development allows cheap external storing of sounds (e.g., on CD)
• Widely available pieces of software allow a lot of procedures without programming (e.g., you can create mp3 or ogg files easily). One application developed by Bob Kemp and Marco Roessen (EDF/EDF+ to WAV) deserves further comment. It is very friendly and allows the creation of wav files from signals contained in EDF recordings.

This document is not only directed to clinical neurophysiologists but also aimed to be read by people interested in the sounds of the brain (hobbyists etc.) so I will try to introduce a little bit some neurophysiological concepts underlying the displayed signals and pay some attention to very simple details.

Most ideas contained in these notes emerged in the discussion of the audification of sleep signals with Bob Kemp and Marco Roessen. The whole procedure is described and discussed in the article Easy listening to sleep recordings: tools and examples. Sleep Medicine 2004, 5(6): 601-603