how do intial paths to "regions" of the brain develop?

For example the eyes are "close" to the brain and there is an optical region in the brain which is established very early on. But what about taste etc. which are more remote. Other more advanced areas of the brain are never directly connected.
It appears that active paths don't consolidate, but may branch to a common nerve to create a more complex stimulus. metaphorically 2 colours joining to make a third but continue themselves as well.
Are the higher order parts of teh brian nothing more than an unused bundle in the beginning and simply through stimulation become more complex stimulus.

In these cases how do two nerves determine to combine an stimulus? At a high level sight and taste can be reasoned together for a food item. At some point they interact.