Synesthesia is an interesting phenomena concerning how the brain receives and processes senses. For example, some people can taste colors, or see colors when they hear music, or hear sounds when they feel things. Synesthesia is basically the notion that all our 5 senses of taste, smell, sight, sound and feel are somehow inextricably connected.

It’s thought that everyone has some degree of synesthesia. This excellent documentary by Jonathan Fowler describes a theory about how synesthesia may exist in individuals.

The relevance this has with language learning is that it is speculated that the characters of an alphabet and the words of a given language were formulated by the collective synesthesia of populations.
 
For example, when you hear the letter “S”, you think or feel the form of a slow and smooth path in the shape of an S. The letter and word “I” feels like simple straight line, just like the number “1″.  The list goes on concerning the coorelation between the sound of letters and words, their shape and their meaning.