Mean (+1 SEM) active lever responding for rats in the VS-Only, NIC-Only, NIC+VS and 2-Lever groups. For the 2-Lever group, responding on the infusion lever (■) is depicted separately from responding on the VS lever (□). Time-out responding is excluded. For the 2-Lever group, responding for the VS lever exceeded responding in the VS-Only group for FR2 sessions and did not differ from VS+NIC rats. Saline substitution abolished, and NIC replacement reinstated the difference between 2-Lever VS and VS-Only responding. See Palmatier et al, 2006 for details of procedures and statistical analyses. (From Palmatier et al. 2006. Copyright © 2005 by Springer-Verlag. Reprinted with permission)