Synesthesia

Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiae or synaesthesiae)—from the Ancient Greek σύν (syn), "together," and αἴσθησις (aisthēsis), "sensation"—is a neurologically-based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes.

Credit : http://en.wikipedia.org/wiki/Synesthesia

If you're fanclub of Heroes TV Series. You'll notice that Emma in Season 4 has ability to perceive sound as color. That's mean she can't hear the voice of everything but only see the color's aura from voice's provenance. But it's different from "Synesthesia" we understand.

The most happening of Synesthesia is (Colored letters and numbers or Colored hearing). You will perceive the letter or symbol as color, or perceive hearing as color. But the interesting happen of synesthesia if you perceive something such as perceive the Letter A as Blue Color this will happen for a lifelong.

By the research we found a woman name's Carol Steen. She has three kinds of synesthesia.

The first is she sees colored letters and numbers.
The second is she sees color by hearing. (Just like Emma)
The third is she sees color and shape by some sensation.

The psychologist of McMaster University commented about synesthesia.
"We all born with a connection of neuron in our brain system that the cause of synesthesia to be happened. but when we grow up this connection wiil be abated or separated from each other for the most. but for the special case the connection of neuron still connected so the happen of synthesia will be happened to them until they grow up."