depicts the cerebral connections of n=418 subjects with a frequency-parameter k: for any k=1,2,...,n one can view the graph of the edges that are present in at least k connectomes. if parameter k is decreased one-by-one from k=n through k=1 then more and more edges appear in the graph, since the inclusion condition is relaxed. the surprising observation is that the appearance of the edges is far from random: it resembles a growing, complex structure, like a tree or a shrub (visualized on the animation on the left). it is hypothesized in that the growing structure copies the axonal development of the human brain: the earliest developing connections (axonal fibers) are common at most of the subjects, and the subsequently developing connections have larger and larger variance, because their variances are accumulated in the process of axonal development. synapse elimination main article: synaptic pruning several motorneurons compete for each neuromuscular junction, but only one survives until adulthood. competition in vitro has been shown to involve a limited neurotrophic substance that is released, or that neural activity infers advantage to strong post-synaptic connections by giving resistance to a toxin also released upon nerve stimulation. in vivo, it is suggested that muscle fibres select the strongest neuron through a retrograde signal or that activity-dependent synapse elimination mechanisms determine the identity of the "winning" axon at a motor endplate. mapping brain mapping can show how an animal's brain changes throughout its lifetime. as of 2021, scientists mapped and compared the whole brains of eight c. elegans worms across their development on the neuronal level and the complete wiring of a single mammalian muscle from birth to adulthood. adult neurogenesis main article: adult neurogenesis neurogenesis also occurs in specific parts of the adult brain.