Figure 5. Description and performance of drift diffusion model of delayed discounting.

A figure describing how each decision boundary is altered by changing the delay (d) and value of the reinforcer (a), red lines denote delay (lower) boundaries and blue lines denote the immediate (upper) boundaries (A). Representative random walks are shown that find the delay (red) and immediate (blue) boundaries (dashed lines) (B). The response time distribution for all trials acquired in the experimental data are shown in (C1) and the response time distribution for all walks in the model data are shown in (C2). Notice the similarity in the shape of each distribution. The value (mean±SEM) of the immediate lever at each trial in the experimental data is shown in (D1; trial*delay interaction F(115, 3450)=6.84, p<0.0001). The mean value of the immediate boundary is shown for the model data in (D2). The color of each line in both D1 and D2 corresponds to the same delay. Notice that each line eventually reaches a unique asymptote (i.e. indifference point) depending on the delay. Simulations were run using the k values from experiments in each rat population in (E). The discounting function (mean±SD) is plotted for simulated Wistars (blue), P rats (red), and the mean k value from P’s and Wistars (black). The inset in E shows the distribution of k values generated by the model for 500 bootstraps (blue bars=simulated Wistars, red bars=simulated P’s). The arrows denote the experimental k values for each rat population; notice they are much lower than simulated values.