for a complete beginner's intro to the fascinating world of neuroscience.
When two tones are played alternately at a fixed, slow, rate, the result is a simple melody consisting of the two alternating tones. However, if the rate of presentation is fast enough and the frequency separation between the two tones is large enough, the melody breaks down into two streams, each consisting of tones of one frequency.
The importance of onsets in auditory grouping is illustrated here with the stimuli originally introduced by Darwin and Sutherland (1984) and later studied by Roberts and Holmes (2006, 2007). The sound examples used here are courtesy of Brian Roberts.
One of the classical experiments of psychoacoustics is the measurement of the lowest sound level at which a tone is heard in the presence of a white noise masker. This sound level is called the 'masked threshold'. In the following example, the masker is played at a fixed level, and the level of the tone can be adjusted. On every repeat of the stimulus, the masker is started first, and a short time later the tone comes in.
The "Law of Continuity", one of the "Gestalt rules" thought to govern perception, stipulates that our mind will tend to interpolate or extrapolate perceptual "objects" if the edges of the objects are obscured. A visual example is shown in in the graphic here. The red line, however, is obviously broken in two as you can see the gap. However, most people would see the blue line as continuous, assuming that it continues behind the green boxes.