What does "Therein lies the cerebral beauty and the cerebral rub of plasticity" mean?
Context:
No one today fully knows what is happening in the brains of children as they learn to read while immersed in digitally dominated mediums a minimum of six to seven hours a day (Kaiser report, 2010). The present reading brain's circuitry is a masterpiece of connections linking the most basic perceptual areas to the most complex linguistic and cognitive functions, like critical analysis, inference and novel thought (ie, "deep reading processes"). But this brain is only one variation of the many that are possible. Therein lies the cerebral beauty and the cerebral rub of plasticity.

Question

Answer

"Therein lies the rub," is from Shakespeare and means to identify the cause of a problem. The paragraph is saying that a child's brain can develop in many different and complex ways, that once it is developed it will be capable of unpredictable positive things due to its complexity and adaptability.
In short, the adaptability (plasticity) of the brain is both beautiful and problematic (the rub).