Sounds

How does your brain distinguish between sounds that are in front of you and behind you?  Or on top, or below, for that matter.  If your head was the center of a circle on the Y-Z plane, how would your brain know where a sound is coming from on that circle?  No matter where, it would be reaching both your ears at the same intensity and time.  I think it's a purely psychological thing, where what you see and know/think influence your perception of sound as well.  I should conduct an experiment.

UPDATE:
Alright, so according to a friend of mine, the shape of your ears makes sounds in front more intense that those behind you.  So let me elaborate on the question:

What if the sound behind you was more intense that the one in front, so the shape of your ears negated the difference?