"Growing" Brain Tissue A key insight is to take into account the fractal nature of how the brain  grows  (like any other tissue) from a single stem cell. Model structure should reflect the way cells grow and connect rather than relying on mathematical structures like matrices. Connections  look  random but have an underlying fractal structure. On the other hand, the model needs to reflect the well known principle that "cells that fire together wire together". The fractal structure "explains" this rule in a physical way (cortical columns) but the brain can wire itself up in a way that might be seen as 4 dimensional. Connections are formed due to "closeness" in space as well as some kind of time function.  The model should assume some level of continuous growth. In the brain, new neurons are being created and others are dying all the time. It's not known how these new neurons find their place in the structures of the brain. Similarly, we know