psychology

(Original post by nzy)
When you hear a sound with a distinct pitch, neurons in the brain fire at the same frequency as the frequency of the sound waves entering the ear.

When an instrument plays a note, the sound waves contain both the "fundamental", which is the frequency of the main pitch you hear, as well as higher "overtones" or "harmonics", which have higher frequencies equal to whole number multiples of the fundamental.
For example, if you plucked a note with a frequency of 200 Hz on a guitar, the sound would actually contain the frequencies 200 Hz (the fundamental) and 400 Hz, 600 Hz, 800 Hz and so on. This group of frequencies are known as a harmonic series.

The brain is able reproduce these harmonic series, even when they're incomplete. Say you were to hear a sound which contains the frequencies 400 Hz, 600 Hz, 800 Hz, 100 Hz, 1200 Hz etc. Neurons in the brain would fire at all of these frequencies, but the brain would recognise that they all belong to the same harmonic series that begins with 200 Hz, and neurons would also fire at 200 Hz. As a result, you would effectively hear a sound which has a pitch of 200 Hz, even though 200 Hz wasn't actually being played in the first place.

Dan Levitin quotes an experiment by the neuroscientist Petr Janata. They placed electrodes to the auditory system of a barn owl and played it a version of The Blue Danube by Strauss, but where each note of the melody had its fundamental frequency taken out and only contained the harmonics. When they played back the frequencies from the firing of the owl's neurons through a speaker, what played back was the complete melody of The Blue Danube that had been 'filled in' by the owl's brain.

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