Addendum to: Nature, published online 9 December 2009.

In this Addendum, we wish to provide a more complete and detailed description of the exclusion criteria and excluded participants to facilitate direct replication of our study procedures. Although the 0.1 conditioned response (CR) exclusion criteria outlined in the original Letter captured some excluded participants, the exclusion or inclusion of participants during the study was based on a qualitative assessment of trial-by-trial skin conductance response (SCR) as reflecting a pattern consistent with successful acquisition and extinction. In the Supplementary Information to this Addendum, we present revised Supplementary Methods, to provide a more exact characterization of the final dataset, corresponding to the qualitative evaluations made in the process of data collection and analysis.