The three types of investigations discussed above have clearly documented the remarkable plasticity of the developing brain. In the hamster retinocolli-cular system, removal of more than half of the central target and/or blockade of NMDAR-based coincidence detection have no effect on visual stimulus coding. This conservation of function occurs through compensatory compression of the retinotopic map and rearrangement of inhibitory connections. Loss of sensory input, through neonatal cochlear ablation in ferrets, results in compensatory sprouting of visual thalamic axons into the silenced auditory cortex. Such sensory substitution may underlie the expanded visual capacity of humans with early hearing loss. Cross-modal rewiring of the ferret auditory cortex results in a compensatory alteration of the structure and function of auditory cortex in a way that provides it with visual processing capacity. Taken together, these studies provide insight into how developmental mechanisms such as compensatory innervation could provide an important substrate for evolutionary changes involving alterations in sensory structures or their targets. Our work and that of many others has shown that developmental studies provide a window into the past, providing insights into how the increasing complexity of mammalian sensory systems may have evolved.
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