Sentence:  On a phonemic level, phenomena of <WSD>synesthesia</WSD> have often been described and studied. Practically all children and a good many adults—though for the most part adults will deny it—spontaneously associate sounds, whether phonemes or the timbre of musical instruments, with colors and forms. 
Meaning of the word marked with <WSD> tags:  The association of one sensory perception with, or description of it in terms of, a different perception that is not experienced at the same time.