Figure 1 : Experimental task, biophysical model and current distribution for medial frontal tDCS.

From: Selective alteration of human value decisions with medial frontal tDCS is predicted by changes in attractor dynamics

Figure 1

(A) Trial structure. Subjects chose between two options of differing reward size (signaled using the size of the rectangle) and probability (learnt). The inset shows one of the random walks used for the reward probabilities of the two options. At the time of choice, the chosen option was indicated (orange rectangle) and the outcome displayed (red: no reward, green: reward) (cf. 5). (B) Biophysical model. Excitatory inputs to the two pyramidal populations (p1 and p2, 800 neurons each) are scaled by the expected values of each of the two choice options. Pyramidal populations inhibit each other via a common pool of inhibitory neurons (i, 400 neurons). Background input simulates random firing from regions uninvolved in the task. The effects of tDCS were simulated by changing the membrane potentials of both inhibitory and pyramidal populations (see Methods). (C) Temporal development of firing rates of pyramidal populations p1 and p2 in the biophysical model. Large expected value differences (upper panel) cause firing rates in the pyramidal populations to differentiate the two inputs earlier, compared to small differences in expected value (lower panel). Vertical black lines: time of predicted response (upon crossing of given threshold, see text for details). Shaded area: onset and duration of choice stimuli and thus the input of expected values. (D) Estimated current distributions during medial frontal stimulation (left) and lateral frontal stimulation (right, see Supplementary Fig. 2). Electrode positions are indicated by rectangular outlines (anode over Fpz for medial and over F3 for lateral stimulation condition, cathode below inion in both conditions). Upper row shows lateral view, lower row shows medial view, black lines indicate estimated current flow.