I was reading a book called Brain Rules by John Medina and came across some very interesting information. (BTW, I would recommend the book). It seems that babies are born with the same number of neurons and connections as adults have. And then, by the time they are 3 years old, the connections (and therefore number of neurons) have doubled or even tripled. But that doesn't last long. The brain as he puts it 'takes thousands of tiny pruning shears and trims back a lot of this hard work.'
What is remarkable is that at puberty the whole process starts over again. So, the brain explodes with neuron growth and then with vigorously trims it back again. So there is a lot of brain growth and pruning during the terrible twos and then again the the terrible teens.
"Though that might seem like cellular soldiers obeying growth commands in lockstep formation, nothing approaching military precision is observed in the messy world of brain development. ...Even a cursory inspection of the data reveals remarkable variation in growth patterns from one person to the next."
As I was reading this, I was thinking about all the stories of DS kids that decline after puberty. What if DS never get (or get much less) of that explosive neuron growth but have close to normal pruning? This would cause the neuron count to dip below that critical number that keeps coming up.
Elizabeth Goulds work showed that below a certain neuron count, the study subjects get anxiety, and depression. And the brain goes into a 'safe mode' and doesn't initiate the normal neurogenesis that should be happening. The brain some how panics and just tries to hold onto what it has.
Numbers.....could it be a numbers game?