The auditory cortex further groups the sounds coming into the ear. The main principles of grouping include:
Sounds are grouped together by proximity of frequency. Tones in the same range tend to be grouped together as originating at the same source.
Sounds are grouped together by similarity of features such as timbre.
Sounds are grouped along lines of smooth and progressive flow, particularly of frequency.
Individual frequencies are grouped together when there is a simple harmonic relationship between them. This grouping is a key process of hearing and will be examined further.
If sounds emerge, change and/or disappear at the same time, they are assumed to come from the same source.
The brain fills in sections of muffled or masked sound with plausible solutions based on acquired knowledge about the world. This process is not merely a grouping process but also a reconstructive process, which generates the impression of coherent sound in direct contradiction to the immediate sensory data from the ear.
Through the application of these processes, the nerve stimulus arriving from the cochlea is transformed into the impression of a set of separate and coherent sound sources.
(For a particularly engaging presentation of these grouping processes, with excellent examples, see: What in the world do we hear? a lecture by Professor Brian Roberts of Aston University.)