Neural activity is redundant: many states in motor cortex can generate similar movements. When we record from motor cortex, we capture only a small fraction of the total neurons. Redundancy makes it possible to observe the overall state of motor cortex from limited observations, but might also impair the generalization performance of a linear decoder.
Consider two neurons, $A$ and $B$, that combine linearly to produce movement $C{=}\alpha_1 A{+} \alpha_2 B$. (Perhaps both neurons drive the same targets in spinal cord.) An animal could use any linear combination of activations of units $A$ and $B$ to perform behavior $C$, so long as the sum $\alpha_1{+}\alpha_2$ is constant. What if there is an unobserved variable $\gamma$ that sets whether neuron $A$ or $B$ is used more (Fig. 1)?
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| Figure 1: (simulated hypothetical scenario) Neural signals $A$ and $B$ combine linearly according to weight $\gamma$ to form behavioral output $C=\gamma A + (1-\gamma) B$. Parameter $\gamma$ modulates sinusoidally between $0.25$ and $0.75$. |
