Learning that certain actions lead to risky rewards is critical for biological, 
social, and economic survival, but the precise neural mechanisms of such 
reward-guided learning remain unclear. Here, we show that the human nucleus 
accumbens plays a key role in learning about risks by representing reward value. 
We recorded electrophysiological activity directly from the nucleus accumbens of 
five patients undergoing deep brain stimulation for treatment of refractory 
major depression. Patients engaged in a simple reward-learning task in which 
they first learned stimulus-outcome associations (learning task), and then were 
able to choose from among the learned stimuli (choosing task). During the 
learning task, nucleus accumbens activity reflected potential and received 
reward values both during the cue stimulus and during the feedback. During the 
choosing task, there was no nucleus accumbens activity during the cue stimulus, 
but feedback-related activity was pronounced and similar to that during the 
learning task. This pattern of results is inconsistent with a prediction error 
response. Finally, analyses of cross-correlations between the accumbens and 
simultaneous recordings of medial frontal cortex suggest a dynamic interaction 
between these structures. The high spatial and temporal resolution of these 
recordings provides novel insights into the timing of activity in the human 
nucleus accumbens, its functions during reward-guided learning and 
decision-making, and its interactions with medial frontal cortex.