A coqui frog is native to Puerto Rico. Just like humans and other animals, a coqui creates sound by blowing air out of its lungs. This thrust of air moves the frog's vocal chords (two pairs of folded tissue in the throat), causing air molecules to vibrate back and forth. Result? A sound wave. The soundwave with the highest amplitude (height) makes the loudest 'ko-kee'. A coqui's vocal chords are primitive. They can't change the sound wave's frequency (number of vibrations per second) since the pitch (how high or low a note sounds) of a noise depends on the frequency of the sound wave. You can listen to a coqui here :http://www.bogartsdaddy.com/coqui.htm
The meaning of each croak is hidden in its rhythm (pattern of beats). The only difference between a distress call and a mating call is the rhythm pattern, or which beats are accented as they call. This tiny little frog has a basic grasp of rhythm which it is using to communicate to the outside world. Understanding how rhythm is percieved by the brain is anything but basic.

Daniel Levitin is the author of the book 'This Is Your Brain On Music'. Levitin explains that memory is connected to the experience of music, that one could venture to say we wouldn't have music at all if we didn't have memory. Levitin's unique contribution to research on music lies in his drawing attention to the importance of the cerebellum in music listening - including tracking the beat and distinguishing familiar from unfamiliar music. These discoveries are consistent with anatomical studies from the 1970s that found direct neural connections between the cerebellum and the hearing organ within the cochlea, which converts sound vibrations into nerve impulses. With memory, our brains formulate schemas, the patterns, organisation and rules of how music goes together. It is anticipation and expectation from schemas that is key to the emotional drive of music. Is music's true motivation centred on playing with the rules and conventions that are inherent to the brain's method of storing and organising sound?

Research performed by Petr Janata using EEG recordings shows nearly identical patterns of activity when subjects hear/percieve music and when they imagine/remember the music. These results suggest that perception entails the activation of a set of inter-connected neurons, causing them to fire in a particular pattern. Brainwaves are measured as frequencies in the unit of Hertz, the number of wave cycles per second. There are four types of brainwaves (or rhythms) that are focused on in EEGs -Alpha (8-11Hz), Beta (12hz and above), Theta (4-7Hz) and Delta (less than 3Hz), and each type can be matched with a certain state of functioning. Several studies, one of which was by Charles Gray of UCDavis and another by David McCormick of Yale U. School of Medicine have shown that the brain has its own internal rhythm. Chatter cells co-ordinate rhythmic firings of millions of cells in bursts around 30-60Hz. Some imagine that these cells link anatomically distant neural structures and are unlikely to have anything to do with musical rhythm - but wouldn't it be fascinating if they did?