07 December 2016

Within striatal circuits the neuromodulator dopamine is considered by many  to act as a learning signal, controlling neural plasticity (60). One of the main  lines of evidence has come from recordings of midbrain dopamine neurons  whose axons project to wide regions of forebrain (61)—including the striatum,  which has a very high density of dopamine receptors. In macaque monkeys such  cells often show brief (~0.1 s) increases in firing rate, particularly to unexpected  rewards, or to unexpected cues that signal upcoming rewards (reviewed in  62). The ability of dopamine cells to ignore fully expected rewards and shift  firing to cues that predict rewards has a strong resemblance to “error signals”  in certain formal learning theories (63) and certain computational models of  “reinforcement learning”(64). Both classes of model refer to situations in which  the extent of associative learning is controlled by a simple signal that provides  feedback on the overall success or failure of an action or expectation. Because  the signal does not provide detailed information on the exact nature of any errors  committed, it is sometimes described as a “critic” rather than a “teacher” (65).