Fig. 18.

Healthy behaviour: adequate response to external stimulus with a low dopamine level (${\varphi }_{\mathit{tonic}}=0.5,{\varphi }_{\mathit{phasic}}=0$) to make the BG indirect pathway dominant. Raster plots of the spiking activity of the relevant simulated populations of neurons when the action selection (direct) pathway is disabled by a low phasic dopamine level (${\varphi }_{\mathit{phasic}}=0$) while an external stimulus is applied into the cortex. The associated dopamine level, the external stimulus and the NMDA-cortical neurons’ response applied to the $D_1$-MSNs, the $D_2$-MSNs, the STN and the thalamus are given in Fig. 17. Each black dot corresponds to a spike of the corresponding neuron at a given time. For each population, the green functions on the raster plots show the average firing rate for the 20 simulations produced. The raster plots are for one of these simulations. The sparks indicate excitatory connections, whereas the circles represent inhibitory connections. The over-activity of the striatal $D_2$-MSNs suppresses the activity of the neurons of the GPe, which in turn are incapable of reducing the activity of the STN. The STN neurons start to send more excitatory inputs to the GPi to prevent the sensory input from reaching the thalamus