Think of the brain’s systems as resembling a major city’s subway lines. There will be stations where the lines cross, that are quite busy, and there are more provincial, less busy, stations along a particular line.
Anthony Demasio calls the places where systems cross convergence-divergence zones ( CDZ). CDZ regions have high proportions of mirror neurons which enable empathising and understanding the behaviour on others. Demasio believes the various CDZs compose our sense of Self.
Damage to a CDZ area say from a Stroke will cause more widespread brain dysfunction than say a similar sized Stroke localised to a particular system; for instance the occipital lobe (visual cortex).
There are three types of damage: (1) Loss of excitatory input to an area(ie: a station has a power failure)
(2)Deterioration of the overall function of the subsystem ( ie: subway line on strike)
(3)A different area of the brain attempts to do the job for the non-functioning part and decompensates. Alternatively the different part of the brain may do the new job quite well leading to good recovery of function. (ie: A new subway connection is established to replace the defunct one).