'In the last decade and a half, animal models have found that the neurotransmitter dopamine, long known to be involved in reward processing, is involved in craving or motivation, but not necessarily enjoyment,' Treadway said.

'To date, research into reward processing in individuals with anhedonia has focused on enjoyment of rewards, rather than assessing the drive to work for them. We think this task is one of the first to do that.'

Researchers found that subjects who reported symptoms consistent with anhedonia were less willing to make choices requiring greater effort in exchange for greater reward, particularly when the rewards were uncertain.

These results were published in the Wednesday online edition of PLoS One.