The integration of brain functional areas is fulfilled by their mutual innervation through axon prolongation and new synapse formation, through which associative memory cells (AMC) are recruited including primary and secondary AMC. The following papers provide conceptive evidences. 

Thank you for your answer.

Good question. There are a number of non-exclusive proposals out there, but no definitive answer.

(1) convergence onto other higher level areas

(2) synchronization of activity. This can take place in different frequency ranges from low frequency (e.g. resting state fluctuations in BOLD) to high frequency (e.g. gamma oscillations)

(3) interaction in functional networks (partly overlapping with 2)

(4) no literal integration needed, as it is directly translated to action

As so often, the truth will be somewhere in between, but not in the middle. 

Best, Peter

Prof Peter,

Thank you for your answer.