Figure 9.
Activities in GPi and cortex as a function of the DA level. A, Population activities in GPi (left) and in cortex (right). The activities are averaged over 20 neurons chosen at random in the GPi or cortex in the two circuits and over 10 trials. The activity was averaged over a 200 ms window starting 200 ms after the input onset (see Materials and Methods). Solid lines, Rest state. Dashed lines, Activities in response to the input plotted in Figure 8A. The average striatal threshold and strength of corticostriatal synapses are given by Equations 10 and 11 for the solid and dashed lines. Dashed dotted line, Rest state for constant average striatal threshold. The strength of corticostriatal synapses is still given by Equation 11, but the average threshold of striatal neurons is TStr = −0.02 and no longer depends on the level of DA. Note that the GPi average rest activity is increasing with DA depletion in this case. B, Population activities (20 neurons; 10 trials) in response to the input of Figure 8A. Left, GPi; right, cortex. Black, Circuit 1; gray, circuit 2. In the normal condition (D = 100%), symmetry breaking occurs and the activities in the two circuits are different. As DA is depleted, this difference diminishes and symmetry is restored for D ≈ 70%.