Who Said That? - Specialized neurons sort out overlapping 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.

Source of Information : Scientific American Mind September-October 2009