Fig 1.
Simplified cortico-striatal circuit.
(A) The diagram of a simplified cortico-striatal circuit. Each arrow denotes the connection from one population to another, where the arrowhead is excitatory, and the circlehead is inhibitory. In the circuit, MSNs receive direct excitatory inputs from the cortex and relayed inhibitory inputs via FSIs. Ma and Mb denote two MSN groups differentiated by their cortical sources as in C Right. Background inputs maintain the spontaneous activities of the striatum. The inhibitory projections from FSIs to MSNs are much stronger than the recurrent connections between MSNs, denoted by the arrow widths. (B) The type of connectivity that may emerge is a function of the relative size of a neuron population (x-axis) and its probability to connect (y-axis) to other neurons in the network. (C) Illustration of the feedforward connections from FSIs or cortical neurons to MSNs. (C:Left) Sharing of FSI inhibition, where the colored nodes denote FSIs, and gray nodes denote MSNs. The color of FSI to MSN projections corresponds to the high sharing (red) and low sharing (purple) scenarios shown in B, assuming indegree is fixed. (C Right) Sharing of feedforward excitation. Each MSN received a fixed number of cortical inputs. The presynaptic pools of the two MSN populations (blue and green) partially overlap. Nctx denotes the size of the private pool, and Nol denotes the shared pool of presynaptic cortical neurons for the two sources.
Fig 2.
Across-trial variability of MSN activities during evoked states.
(A) Top: raster plot (2 different trials denoted by red and blue) of MSN activities with . Bottom: population rate of MSNs over time. The small panel shows spontaneous activity without any cortical input for 0.1s. (B) Same as A but for Nfsi = 25. (C) The color denotes the average population firing rates of MSN across trials given different FFE sharing (Win) and FFI sharing, i.e., the number of FSIs (Nfsi). (D) Same as C, but for the across-trial variability of population rate trajectories.
Fig 3.
Modulation of correlation by sharing of FSIs in the striatum during evoked states.
(A) Effective correlation settings for the cortical inputs. (B) Output statistics of MSNs for Nfsi = 0. Left: output correlation (averaged across 100 trials for each input setting) of MSNs’ spiking activity. The x-axis denotes mean pairwise correlation within the MSN group (Wmsn) while the y-axis denotes mean pairwise correlation between two MSN groups (Bmsn). The color corresponds to A. Right: all trials of output correlations. (C) Each subplot compares the correlation of MSN activities between the control case (no FSI) and the studied case (a certain number of FSIs or degree of FFI sharing). Each line corresponds to a particular input correlation (color code same as in panel A). The starting and ending points of the lines denote the trial-averaged output correlation for Nfsi = 0 and Nfsi > 0, respectively. From left to right, the value decreased from Nfsi = 250 to Nfsi = 25 (or the sharing of FFI increased). (D) Same as in panel C, but in this case the starting point of each line is the output correlation measured for Nfsi = 250.
Fig 4.
Downstream effect of FFI sharing on a GPe neuron.
(A) We describe the state of MSN population activity as average pairwise correlation (x-axis) and across-trial variability (y-axis). Each line starts at a point indicating average pairwise correlation and across-trial variability for and ends at a point indicating average pairwise correlation and across-trial variability for Nfsi = 250. A filled circle marks the ending of a line. Each line corresponds to a specific input correlation (see Fig 3A). The correlation level and across-trial variability of population firing increase with stronger sharing of feedforward inhibition (Nfsi changing from 250 to 25). (B) The bursting index (x-axis) and across-trial variability (y-axis) of a GPe neuron in response to MSNs’ activity during evoked states. Each line starts at a point indicating average bursting index correlation and across-trial variability for
and ends at a point (marked with a filled circle) indicating average bursting index and across-trial variability for Nfsi = 250.
Fig 5.
Possible functional consequences of FSI modulations.
(A) During the learning phase, the high-sharing of FSI induces variable population responses across trials, which facilitates exploration behavior; during the performance phase, FSIs are functionally decoupled, leading to stable responses across trials, which facilitates exploitation behavior. (B) Different groups of MSNs receive cortical inputs from distinct but overlapped sources. In contrast to MSNs, FSIs receive inputs from both sources. Whether the signals get integrated (high correlation between the activities of two MSN groups) or differentiated (low correlation) is modulated by the sharing level of FSI projections to MSNs.
Table 1.
Neuron parameters.
Table 2.
Network connectivity parameters.