We applied the test to activity from either the CS or trace interval, depending on which had the largest contribution of value by ANOVA. Change point analysis was performed on scored behavior: trials in which licking (or blinking) occurred in the last 500 ms of the trace interval were scored as 1; other trials were scored as 0. We identified change points

using a threshold of p < 0.05, correcting for multiple comparisons. If multiple change points were identified, we used the change point closest to reversal. In addition, we calculated the normalized activity (Z-scored with reference to baseline firing rate) for each value-coding cell before and after the identified change point (using 12 trials of each type before http://www.selleckchem.com/GSK-3.html and after the change point) and averaged it together with all cells encoding the same valence in the same brain area (Figures 4I and 4J). We computed a “difference index” comparing each neuron’s response on each trial to the two images that reverse reinforcement contingencies (Figures 5A and 5B). We examined firing rates in the 90–590 ms after CS onset, normalizing the firing rates by subtracting the baseline firing rate and dividing by its standard deviation.

For each value-coding cell, starting ten trials of each type before reversal, we calculated the difference in the average normalized response to the two CSs in CHIR-99021 solubility dmso windows of six trials, stepped by one trial. For positive value-coding cells, we subtracted the response to Image 1 (which changes from positive to negative) from the response to Image 2 (which changes from negative to positive); for negative value-coding cells, we subtracted the response to Image 2 from the response to Image 1, so that all difference indices change in the same direction across reversal. We then averaged the difference indices for each trial across all cells in each group, and fit the average

difference indices with a Weibull function (Equation 1). Finally, for display, we normalized the fit functions and the data points by subtracting the lower asymptote of the Weibull function and dividing by the upper asymptote. Results were significant and went in however the same direction for both monkeys, so the data were combined. To quantify the time course of neural changes after reversal, we applied a sliding two-way ANOVA with main factors of image value and image identity on spike counts from a time window 90–590 ms after CS onset (Figures 5C and 5D). This window exhibited the strongest divergence of activity among neuronal subgroups, but other time windows, including the entire CS and trace interval, produced similar results. For each value-coding cell, we performed the sliding ANOVA using data from the last six trials of each type before reversal, and a group of six trials of each type from after reversal, “slid” in 1-trial steps. For example, the first ANOVA would be computed using trials 1–6 of each type after reversal; the next, using trials 2–7 of each type, etc.