An important idea in learning is the notion that dopamine signalling from the midbrain represents a prediction error that can guide associative learning. This idea has been around for a while, but evidence so far has been correlational rather than causal. Moreover, some data has suggested that dopamine represents reward and/or reward saliency rather than prediction error. A new report from Steinberg et al provides some causal evidence supporting the idea that dopamine release from the midbrain really does signal prediction error [cite source=’pubmed’]23708143[/cite]. Specifically, the lab used optocogenetics to enable light-activated midbrain dopamine release. They then used activation to overcome the classic effect of blocking. The primary paper is above as well as a good summary [cite source=’pubmed’]23799468[/cite].