Wow, the Galton paper is amazing, thank you. To be honest I'm like the scientists that he questioned first... until I read this today, I always thought that the whole "mental imagery" concept was mainly a figure of speech, as when people say they feel butterflies in their stomach, and things like that. I can picture my breakfast table, but it's more of a conceptual sketch than an actual image. I can only focus my thoughts on one thing at a time, and e.g. I know what the colors are but I'm not really "seeing" them in my mind - the fact that people can report a "brightness" value in relation to reality makes me think that my mind really works different from that of those people, for me the brightness question doesn't even make sense.
Now I feel handicapped, although I suppose that maybe if I could conjure up these mental images, I wouldn't be so good at symbolic processing. I guess this is why I have always preferred text to diagrams and such...
By the way, what I can do is hear music in my mind - and yes, this comes with pitch, volume, timbre, polyphony and everything. In fact if I'm at a place with a constant background noise - as in a plane, or near an air conditioning machine - I sometimes think I'm actually hearing it in reality (although I have learned that I'm not). I wonder if other people also experience this.