Evidence that the delay-period activity of dopamine neurons corresponds to reward uncertainty rather than backpropagating TD errors

We previously demonstrated the presence of delay-period activity in midbrain dopamine neurons, and provided evidence that this activity corresponds to uncertainty about reward.An alternative interpretation of our observations was recently put forth in which it was suggested that the delay-period activity corresponds not to uncertainty but to backpropagating TD prediction errors.Here we present evidence that supports our original proposal but appears inconsistent with the alternative interpretation involving backpropagating errors.