Listened to an interview at All In the Mind[link corrected] with David Eagleman, Neuroscientist, novelist.
David Eagleman: ...if I were to take a piano and I were to hit the high note and then I would hit the low note and I would ask you which one is brighter,what would you say?
Natasha Mitchell: I would say the high note.
David Eagleman: Right, and if I asked you which one is bigger what would you say?
Natasha Mitchell: The low note, weirdly enough.
David Eagleman: Yeah, right. Well it turns out everybody does that, it turns out everybody gives the same answer to that, which is really hilarious because there's no reason why auditory sound should map on to brightness or size, but it does, in all brains. And so this is the sense in which we all have this cross-talk going on in our brains that links different parts.
Clearly, synesthetic experience falls over a spectrum, from those who are intensely synesthetic to those who have a mild experience that cannot be differentiated from metaphorical mapping of experience: like mine when I listen to Bach. The music elevates me, literally. If I close my eyes, I start to fly over a landscape of colors that starts with a endless green field of waving paddy. This is repeatable. I am still not sure if I simply made up that metaphor once and stuck to it because it was enjoyable or if this is some sort of weird synesthesia. There are more than hundred kinds of synethesia documented so far, so I wouldn't be surprised.
Link doesn't work
Maybe there's no reason pitch would map to brightness, but mapping to size is a no-brainer, as big things usually make deeper sounds (consider bells, birds, boys...).
I agree with Rosie - this could be a learned or evolved response. Now why would high sounds map to brightness? Let me just brainstorm here - apes spend much of their time in the dark understory of a forest, where, if you have ever experienced this, you'll know that sounds are muffled by the plant life. Animals that want to communicate over long distances in the understory use deeper sounds. However, if we emerge into a bright clearing or a meadow, there is less muffling, and higher pitched sounds travel further, so the overall sonic palette is higher. If course that's just conjecture.
Maybe there are people for whom the higher note is not brighter?
for some people who have a music sense can differentiate the brightness diff in higher note and lower note.
The reason behind this may be, they personify or visualise music in everything.
for them a wrong note may be darkness and a nice note may be a beautiful sunrise. they figuratively describe them in terms of brightness , darkness, sunrise opr etc.,