Figure 1: Decision threshold and its dependence on the strength of the indirect pathway with CD activity satur
"Decision threshold and its dependence on the strength of the indirect pathway with CD activity saturation. (A) Illustration of activation level saturation of an AMPAergic synapse with STD. Top, Linearly ramping presynaptic input rates. Three colored lines correspond to three ramping slopes. Middle, Depression factor D and AMPAergic synaptic gating variable s as a function of time. Bottom, The activation level of an AMPAergic synapse with depression, Ds, shows a saturation depending on the slope of the ramping input. (B) Cx and CD activities with the inclusion of STD at corticostriatal synapses in the circuit model. Top, Ramping Cx activity. Middle, Averaged individual AMPAergic synaptic activation level shows a saturation. Bottom, CD firing rate saturates when the Cx ramps up. (C) CD activity for six coherence levels each averaged over 800 trials shows saturation when the activity levels for different trials are aligned with stimulus onset (top) and saccade onset (bottom). (D) The range of threshold values shows a similar dependence on g(CD-GPe) as that for original model, and saturates when g(CD-GPe) exceeds 8 nS, the value at which the indirect pathway balances the direct pathway in this case. Error bars indicate SD. Note that the synaptic efficacies along the direct and indirect pathways are doubled to achieve a disinhibition at the SNr before saccade onset despite CD saturation."
One way to introduce saturation for CD activity is through the inclusion of STD at cortico-striatal synapses. The input strength to the SNr is weakened due to the CD saturation, and SNr activity remains very high, which would lead to a failure of disinhibition before saccade onset and prolong the decision time. Simply doubling the synaptic efficacies along both the direct and indirect pathways, however, restores the action selection mechanism and the dependence of threshold on g(CD-GPe). Taking into account CD saturation does not impair the mechanism for effective threshold mechanism.