Ralph,
I think you're original comment some messages back that "I seriously question people claiming that they don't require such reward." is probably closer to the mark than I would have admitted prior to this conversation. And after being tasked to critically evaluate it, I concur that your assessment of my mother's work is probably more correct than my own "idealized" memory. She got paid in the currency that she valued - approval of her peers.
All that aside, I still postulate that an "art doodling" compulsion exists that compels many artists to produce things without regard to reward, and I'll further postulate that exhibition of work prompts the artist to refine the piece - regardless of medium - in ways that are specifically done to receive the reward.
So, after several hours of thinking about this I'll take the position that we're really looking at a two tier rather than a monolithic phenomena. The expressive side, whether the agonizing child birth kind or the serendipitous enlightenment kind Jason speaks about, is done without regard to reward. But the refinement of the expression is done specifically as a reward generator.
For example, a la Jason's model, one of my best works is a rather simple picture of wooden stairs that are the dune crossover at a beach. When I made the exposure I didn't really think about it too much. I was looking for something to illustrate DOF receding from the lens. But in the contact prints it became clear to me that there was potential in the shot beyond what I originally recognized, and some judicious cropping made it into a picture that is on display at my brother in law's house. The original compulsion for the exposure was merely "that's kind of interesting." But the hours of work for the enlargement/matting/framing were seeking the reward of approval by others.
MB