Is a condition in which exposure to one sensation evokes an additional one.

The given blank can be filled with synesthesia.  

A perceptual occurrence in which activation of one cognitive or sensory pathway resulting in involuntary or automatic experiences in a second cognitive or sensory pathway is known as synesthesia.  

The one usual form of synesthesia called grapheme-color synesthesia or color-graphemic synesthesia, in which numbers or letters are observed as intrinsically colored.  

you can have just about every answer you can think of