Full Idea
Connections in a baby's brain probably give the infant the experience of 'seeing' sounds and 'hearing' colours - which occasionally continues into adulthood, where it is known as 'synaesthesia'.
Clarification
'Synaethesia' is a synthesis of sense experiences
Gist of Idea
Normal babies seem to have overlapping sense experiences
Source
Rita Carter (Mapping the Mind [1998], p. 19)
Book Reference
Carter,Rita: 'Mapping the Mind' [Phoenix 2000], p.19
A Reaction
A fact to remember when discussing secondary qualities, and the relativism involved in the way we perceive the world. If you have done your philosophy right, you shouldn't be surprised by this discovery.