Top-down processing in this example might help you identify the object as a black box rather than a box-shaped hole in the white background. Once information is processed to a degree, an attention filter decides how important the signal is and which cognitive processes it should be made available to.  For example, although your brain processes every blade of grass when you look down at your shoes, a healthy attention filter prevents you from noticing them individually.  In contrast, you might pick out your name, even when spoken in a noisy room.  There are many stages of processing, and the results of processing are modulated by attention repeatedly.