It is complex, but the brain is stimulated to grow far more by the cacophony of the natural world than it is by the bare, low bitrate information we get from the world most inside people live through.
The modern world caters to consciousness, which can process about 120 bits of information a second. The natural world caters to the body's sensory complex, which takes in a billion bits a second, and when you count all the information created by first-order processing, it's closer to 11 billion bits a second.
A nervous system grows as a direct reaction to stimulus. More stimulus means more growth.