Chance, music, and the ear of the beholderBy studying the way that the brain's auditory cortex responds to certain types of changes in sound, researchers have found evidence for how the brain is attuned to be especially sensitive to degrees of change in sound that the brain is most likely to encounter in the natural world.
The work is reported by a team of scientists led by Dr. Jan Schnupp at the Oxford University Auditory Neuroscience group.
The world we inhabit is constantly subjected to countless random influences that may occasionally conspire to bring about large and abrupt changes in our environment, but that more often will cancel each other out, and thus provoke only little or no change. Imagine a boy playing a game where, on every round, he tosses twenty coins, and for every head he will take a step to the left, while for every tail he will take a step to the right. In most rounds, the number of heads will be very similar to that of the tails, and the boy's net position will change only a little, but occasionally it might be almost all heads or all tails and the boy will make a sudden large leap to the left or right. Many things in nature are pushed around, much like the boy in this game, by a combination of random influences, from dust grains in a drop of water pushed about by the collision with water molecules to flocks of animals roaming across a pasture. Scientists would say that their movement is "approximately 1/f-distributed," which means simply that there is an inverse relationship between the size of a change and the likelihood of a change. Small changes occur often (at a large frequency), while large, abrupt changes occur with a proportionately smaller frequency.
In the 1970s, the American scientists Voss and Clarke noticed that even music, speech, and natural soundscapes appear to behave according to the 1/f rule. Large, abrupt changes in loudness or in musical pitch are proportionally less frequent than small, gradual ones, and when human volunteers are asked to rate computer-generated random melodies, they instinctively prefer those melodies that obey the 1/f rule to those that do not. Melodies that change more abruptly than the 1/f rule would sound unpleasantly erratic. In contrast, melodies that change more slowly sound boring.
Now, in the new study, the researchers have discovered that this preference for 1/f-distributed soundscapes appears to be hard-wired into the mammalian brain. Individual neurons in the auditory cortex of ferrets were found to respond more strongly and more reproducibly to stimuli with random pitch and loudness fluctuations that conformed to the 1/f rule than to other stimuli that did not. This selective sensitivity results in a more accurate representation of the 1/f-distributed sounds within the brain. 1/f-distributed behavior has a strong random element, but is nevertheless not entirely unpredictable, and it appears that our sensory systems have evolved to expect this partial predictability as "natural," and to exploit it to become particularly efficient at processing sensory inputs that exhibit "just the right amount of randomness."
The researchers include J.A. Garcia-Lazaro, Bashir Ahmed, and J.W.H. Schnupp of the University Laboratory of Physiology in Oxford, United Kingdom. This work was supported by Biotechnology and Biological Sciences Research Council (UK) grant 43/S1959 to J.W.H.S. and studentships from Consejo Nacional de Ciencia y Tecnologia (National Council for Science and Technology, Mexico) and Deafness Research UK to J.A.G.-L. Michigan probes were kindly provided by the Center for Neural Communication Technology, Michigan, with funding from the National Institutes of Health/National Center for Research Resources (grant code P41 RR09754).
Garcia-Lazaro et al.: "Tuning to Natural Stimulus Dynamics in Primary Auditory Cortex." Current Biology 16, 264–271, February 7, 2006. DOI 10.1016/j.cub.2005.12.013 www.current-biology.com
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