Insufficient Connectivity Pruning

Synaesthesia:

Synaesthesia is a neurological condition in which stimuli of one sensory modality evoke experiences in another modality. This is thought to occur as a result of  insufficient “pruning” during development, so that most of the pathways connecting parts of the brain mediating the different senses remain in place instead of being eliminated. Consequently, there is too much cross-talk between sensory systems, such that activation of one sensory pathway leads simultaneously to activity in another.

Once believed to be extremely rare, synaesthesia is now thought to be relatively common. The cross-modal connections implicated in the condition are present in all of us, to a greater or lesser extent.