A thought I’m playing with is that there might be a relationship between synesthesia and metaphor. I had written a post some time ago about conceptual metaphor theory (http://nouspique.com/2010/02/conceptual-metaphor-theory-cmt/). Metaphor seems to engage us in a kind of leap that looks a lot like the seemingly irrational jumbling of senses.

I think of phrases like loud shirt and colourful language (I’m sure there are lots of others too) where metaphor intersects with synesthesia. Metaphor produces an irrational connect between something abstract (the esthetic quality of a shirt) and something tangible to explain that abstraction (a person shouting). Metaphor creates an equivalency between the two. I think of your synesthetic rats and wonder if they’re experiencing something similar – an equivalency between sight and sound that functions almost metaphorically.

Haven’t thought this through fully, but am wondering aloud.

I laughed at ‘when I’m being honest, I confess that “nimble” is a euphemism for “easily distracted’. I’m going to go around telling people how nimble my mind is.

These 2 things came to mind after reading your post. (They may be relevant – but I’m not sure).

1. There have been experiments where a rat’s visual nerves that normally send visual information from the eyes back to the visual cortex of the brain, are instead routed to the auditory cortex of the brain (normally the auditory cortex would receive auditory information carried by the auditory nerves). The rat could still learn to discriminate visual stimuli like different colored lights, or flashing vs constant illumination. But the info could not have been processed by the brain areas that normally produce visual sensation. I think it’s likely that these visual events gave rise to auditory experiences.

2. At the time of birth, we have a lot more neuronal connections than we do years later. Over the course of time, these connections are ‘pruned’ (like pruning branches of a hedge – since the neurons are branch-like). This fits with your idea that we might have early memories of rich synesthetic experience. I don’t know for sure whether early on, the auditory cortex of the brain actually receives input from both the visual nerves, auditory nerves, tactile nerves, etc.. but it could be.

I’ve never experienced sounds from pictures or visa versa, but I imagine the experience would be very special under the right circumstances. I do have a friend who says he sees colors when he plays the piano. He’s both a genius and very good at playing the piano.