The provided code models the basal ganglia (BG) network, specifically focusing on the dynamics of dopamine-modulated selection and control pathways based on the Humphries & Gurney (2002) extended BG model. The basal ganglia are a group of subcortical nuclei critical for action selection, motor control, learning, and reward processing. Below is a concise biological explanation of the elements modeled in the code: ### Basal Ganglia Circuitry 1. **Striatum (D1 and D2 pathways):** - **D1 Pathway (Direct Pathway):** The striatum contains D1 dopaminergic receptors, which facilitate the initiation of desired actions. This path is often associated with the "Go" pathway in action selection. - **D2 Pathway (Indirect Pathway):** Contains D2 dopaminergic receptors, playing a role in inhibiting undesired actions, referred to as the "No-Go" pathway. The code modulates these pathways using dopamine levels (`DA_sel` for selection and `DA_cont` for control) to simulate their influence on action potential, adhering to the model's parameters. 2. **Subthalamic Nucleus (STN):** - The STN integrates cortical inputs and exerts excitatory projections to the globus pallidus internus (GPi) and globus pallidus externus (GPe). It is crucial for adjusting action plans based on cortical feedback. 3. **Globus Pallidus (GPe and GPi):** - **GPe (Globus Pallidus externus):** Acts as an inhibitory relay, managing the balance between excitation and inhibition in the BG circuitry by interfacing with the STN. - **GPi (Globus Pallidus internus):** Provides inhibitory outputs to the thalamus, which directly influences motor cortical activity, thereby playing a critical role in action regulation. 4. **Thalamus and Motor Cortex (MCtx):** - The thalamus relays motor and sensory signals, while the motor cortex integrates cortical and thalamic inputs to control voluntary movements. ### Dopamine and Gating - **Dopamine (DA):** Involved in modulating synaptic plasticity and neuronal excitability in the BG. In this model, it acts as a gain modulator, affecting how salience inputs are processed within the striatal pathways. - **Gating Mechanisms:** DA levels modify the sensitivity of neurons to their inputs (gain), dynamically adjusting action selection thresholds. The model considers both 'hard' and 'soft' switching, simulating how various levels of DA might influence the competition between possible actions. ### Model Parameters and Output - **Output Functions:** Include 'ramp_output' and adaptation for dopamine-influenced activity levels ('DA_ramp_output') to simulate the neural response's non-linearities in channel outputs. - **Winner Selection:** Determines which action(s) are selected based on the computed outputs from the GPi, using thresholds (`theta`) to signify action readiness. ### Significance This model is intended to reflect the biological and functional organization of the basal ganglia, emphasizing the roles of dopamine modulation and neural competition in regulating motor actions. The code is structured to represent how different nuclei within the basal ganglia interact, facilitating a range of movement-related decisions and behaviors influenced by dopamine-dependent mechanisms.