A central issue in neuroscience is to understand the mechanisms underlining the stereotyped patterns of neuronal connectivity. Studies of neuronal development in both vertebrates and invertebrates have established two distinct processes involved in neuronal patterning, reviewed in (1 ). At early stages, guidance mechanisms that are independent of neuronal synaptic activity establish connections that have a rather crude topographical accuracy. Later, activity-dependent mechanisms remodel these initial connections into the refined circuits of the mature nervous system.
