The length of an individual trial was 60 × the length of stimulation train or 60 × 1 s, whichever was longer. For stimulation lengths of 20 pulses or greater, a new stimulation train could be earned as soon as the previous train had finished. For stimulation OSI-744 research buy lengths of ten pulses or fewer, there was a short timeout (1 s – length of stimulation train) before the next train could be earned. This was to ensure that it was physically possible for subjects to respond for all available trains regardless of the stimulation length used. At the end of each trial, the house light and the white noise were turned

off. After an inter-trial interval of approximately 1 min, the next trial began. BI 2536 mw During this 1.5 hr test session subjects were initially given 30 min of regular FR1 training. Subsequently, a within-session extinction period began. During this phase, responses at the active port were recorded but had no consequence. After 30 min of extinction had elapsed, 5 stimulation trains (each train 1 s long, intertrain interval = 1 s) were delivered to signal the renewed availability of reinforcement at the active port. For the rest of the session rats were reinforced on an FR1 basis for responses at the active port. On the day before the contingency

degradation test, subjects were given an additional 1 hr FR1 training session. The number of stimulation trains delivered during this session (dependent on subjects’ responses at the active port) was used to calculate the average rate of stimulation for the contingency degradation portion of the experiment. The next day, rats were given 30 min of regular FR1 training. Subsequently, a within-session contingency degradation period began. During this phase, stimulation trains were delivered pseudorandomly Cell press and noncontingently at a rate that was matched to each subjects’ performance during FR1 training on the previous day. After a further 30 min had elapsed, a reacquisition

phase was initiated. During this phase, stimulation was once again contingent on a response made at the active port. The total length of this test was 1.5 hr. During this experiment, whenever a stimulation train was delivered (regardless of whether it had been earned by a nosepoke response or was delivered non-contingently) the LED lights in the active port were concurrently illuminated. The behavioral data was analyzed with nonparametric tests as specified (e.g., Wilcoxon signed-rank rest for paired data, Mann Whitney test for unpaired data) coupled with Bonferroni tests to control for multiple comparisons where appropriate. This statistical approach resulted in a conservative measure of statistical significance. Given that the behavioral data was distributed in a non-Gaussian manner, and the different behavioral conditions varied greatly in their variance (e.g.