NeuroSPECT technique: Experts show that each one of us "hears differently"

These researchers have spent eight years submitting groups of volunteers to functional magnetic resonance imaging, aimed at understanding which areas of the human brain are activated and inhibited when given certain auditory stimuli.

Central processing

Most of the research is focussed on the peripheral organs of the system which allows us to hear: in the inner and middle ear. These Chilean doctors have opted to place emphasis on the central organ. The important aspect for them is how what is heard is processed at cerebral level; the knowledge extends to establishing that, in response to the same form of stimulation, every human being "hears differently", depending on our personal experience and history.

"Sound tells you things, and you interpret things through this sound," comments Dr. Marcos Goycoolea (above picture). "If we delve into one of our best known studies, we find examples such as this: "If you are made to listen to a piece of Bach, how do you know it is Bach's music? How do you recognise it? It is not because the ear processes the sound; you know it is Bach's music because there is a complete processing of sound stimulation going on, one in which, among other phenomena, auditory memory plays a part. So, what we have been doing is to submit volunteers to different auditory stiumuli while we measure their brain function activity."

Not only is the auditory area stimulated, but other zones too, such as those the brain uses for associating ideas and concepts, visual and frontal areas, which are related to emotions. "There is a lot of very interesting mental processing happening in interrelated areas. With sounds, there are certain areas of the brain which become excited, and others which are inhibited. Subjects who received stimulation with the same tones received stimulation in the same areas of their brains. Not all the ear captures; it is a kind of jumble of electric cables that reach up in a mechanised way. What you process in your brain is not just an electrical stimulation going up; it goes through a series of stages, and what gets up there is the final product of a lot of central processing. We drew a map of the different areas of the human brain and the results are very interesting," says Goycoolea.

Joint stimulation

Two of the most internationally famous articles are "Musical brains: a study of spontaneous and evoked musical sensations without external auditory stimuli*" and "Spontaneous musical auditory perceptions in patients who develop abrupt bilateral sensorineural hearing loss"**. Both have produced interesting and interrelated results.

"Many people listen to music, but musicians do not only listen; they also mentally modify what they hear," points out Doctor Goycoolea in relation to the aforementioned article. "Let me explain: conservatory composers and musicians relax and hear a song when there is nothing sounding; it is as if instead of switching on the radio they switch on their brains and listen by memory. In this process, they are able to modify the structures of that music within their heads. They mentally add instrument voices (violins, for example, or choirs), and they compose in this way. Schumann, Bach, Beethoven, and Mozart did this. We have now charted a cerebral map of this memory process in conservatory musicians and, among other conclusions, we realised that the same areas of the brain "light up" when these people are really hearing music as when they are composing music."