Continuous Representations of Speed by Striatal Medium Spiny Neurons

Area of Science:

• Neuroscience
• Motor Control
• Computational Neuroscience

Background:

• The striatum is crucial for motor output, but the precise mechanisms of movement encoding by its neurons are debated.
• Two main theories propose discrete (bursting) or continuous (graded firing rate) encoding of motor actions.

Purpose of the Study:

• To evaluate and compare discrete versus continuous encoding models of movement in the striatum.
• To determine how striatal output neurons encode and facilitate volitional movements.

Main Methods:

• Recorded single-unit and multiunit activity in the dorsomedial striatum of mice during spontaneous exploration.
• Analyzed neuronal firing patterns in relation to movement speed, distinguishing between bursting and continuous encoding.
• Utilized bulk calcium imaging in identified direct and indirect pathway neurons.

Main Results:

• Both continuous and discrete (bursting) encoding were observed, with continuous encoding being more prevalent.
• Most neurons showed nonlinear tuning to speed, peaking at moderate locomotor speeds, rather than linear responses.
• Direct and indirect pathway neurons exhibited similar speed tuning, suggesting heterogeneity is not solely due to pathway identity.

Conclusions:

• Continuous encoding of speed is a fundamental aspect of movement representation in the striatum.
• Findings support the continuous encoding model over the discrete bursting model for striatal motor control.
• This has implications for understanding movement disorders and developing therapeutic strategies.