The left hemisphere of the brain processes rapidly changing sounds, while the right side processes slower sounds. When you focus on using your right hand (controlled by the left hemisphere of your brain) and listen to sounds, you are more tuned into rapidly changing sounds and vice versa.
Neuroscientists discovered this by asking subjects to pick out sounds among background noise, and indicate when they hear them, first with their left hand and then with their right hand. They found that people heard things differently depending on which hand they were using.
Neuroscientists hope that learning more about auditory organisation in the brain will to lead to new treatments for language disorders.