[認知] How the Brain Localizes Sounds
We live in a world full of echoes. Sounds reverberate, bouncing off walls,
buildings, rocks and any other nearby surface. These sound waves pile on one
another and hurtle down your ear canals from different angles, the echoes
from one noise jumbling together with new sounds and their echoes. In spite
of that barrage, the neurons in the auditory midbrain, an area that responds
before the auditory cortex does, are able to sort out which were the original
sounds and where they came from. How they do so has long puzzled scientists,
but new research suggests the trick is simpler than expected.
In an April study, neuroscientists led by Sasha Devore at the Massachusetts
Institute of Technology tested the widely held hypothesis that specialized
cells in the brain actively suppress neuronal response to echoes. Using
electrodes in a cat’s midbrain, researchers measured cells’ responses to a
sound and its reverberations. They found that the cells that sense a sound’s
direction of origin responded more strongly to the first 50 milliseconds of
sound waves than they did to the later waves—their activity simply tapered
off after the onset of the sound. The tapering response, a much simpler
mechanism than the earlier theory of suppression, allows the brain to easily
tune in to original sounds and pinpoint who or what is making noise.
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