Effect of B and W input correlations on the balance of D1 and D2 activity.

<p><b>(A)</b> Δ_<i>MSN</i> as a function <i>B</i> and <i>W</i> when the two MSNs population received cortical input with a firing rate of 7 Hz. Here, we considered the scenario-II and set <i>J</i>_<i>C</i>1 = 3.6 nS, <i>J</i>_<i>C</i>2 = 3.0 nS. At this input rate and with uncorrelated inputs (<i>B</i> = <i>W</i> = 0), D1 MSNS have higher firing rates. The black contour marks the region beyond which Δ_<i>MSN</i> is close to zero. All the values are concentrated below the diagonal, because of the constraint: <i>B</i> ≤ <i>W</i>. Δ_<i>MSN</i> varies non-monotonically as a function of <i>W</i> for a constant value of <i>B</i>. Dotted arc marks the <i>W</i>_<i>opt</i> for a given firing rate where Δ_<i>MSN</i> is maximal. For <i>W</i> ≥ <i>W</i>_<i>opt</i>, increasing <i>B</i> decreases Δ_<i>MSN</i>. For values <i>W</i> < <i>W</i>_<i>opt</i>, increasing <i>B</i> increases Δ_<i>MSN</i>. <b>(B)</b> same as in (A), for input rate of 23 Hz. <i>W</i>_<i>opt</i> has shifted to 0.3 as compared to 0.2 in panel <b>A</b>. <b>(C)</b> Δ_<i>MSN</i> as a function of <i>W</i> and input firing rate for <i>B</i>′ = 0.01. The space spanned by the input firing rates and <i>W</i> can be divided into three distinct regimes. Low <i>W</i> and high firing rates, Δ_<i>MSN</i> is negative(blue colours) and the output of the striatum is biased towards D2 MSNs (‘No-Go’ pathway). In the regime where <i>W</i> ≈ <i>W</i>_<i>opt</i>, D1 MSNs have higher firing rate than D2 MSNs and the striatum output is biased towards the ‘Go’ pathway. The third regime spans across very high values of <i>W</i> in which both D1 and D2 MSNs operate in a spike wasting regime and, therefore, Δ_<i>MSN</i> is very low. This regime we define as ‘Region of High-Conflict’ (RHC). Because higher firing rates for D1 and D2 MSNs are observed in non-overlapping regions in the space spanned by input correlation and rates, we argue that the striatum may act as a threshold detector and signal the state of cortical inputs by raising the relative activity of D1 or D2 MSNs over the other, respectively.