Abstract
If spikes are the medium, what is the message? Answering that question is driving the development of large-scale, single neuron resolution recordings from behaving animals, on the scale of thousands of neurons. But these data are inherently high-dimensional, with as many dimensions as neurons - so how do we make sense of them? For many the answer is to reduce the number of dimensions. Here I argue we can distinguish weak and strong principles of neural dimension reduction. The weak principle is that dimension reduction is a convenient tool for making sense of complex neural data. The strong principle is that dimension reduction shows us how neural circuits actually operate and compute. Elucidating these principles is crucial, for which we subscribe to provides radically different interpretations of the same neural activity data. I show how we could make either the weak or strong principles appear to be true based on innocuous looking decisions about how we use dimension reduction on our data. To counteract these confounds, I outline the experimental evidence for the strong principle that do not come from dimension reduction; but also show there are a number of neural phenomena that the strong principle fails to address. To reconcile these conflicting data, I suggest that the brain has both principles at play.
Highlights
The strong principle is that dimension reduction shows us the true latent signal(s) encoded by a population of neurons, and so moves us closer to how neural circuits operate and compute
When we find single neurons that fire just before a mouse turns left, it is not because the neuron itself is “tuned” to moving left, but because it contributes most to the trajectory that means “left”
During the Aplysia’s escape gallop, we found just 5 to 8 linear dimensions (∼ 5% the size of the recorded population) is needed to account for 80% of the variance between neurons in its motor system; and adding more dimensions did not improve the decoding of motor output [17]
Summary
Funding information The time needed to think these thoughts was made available thanks to the generous support of the Medical Research Council grants MR/J008648/1, MR/P005659/1, and MR/S025944/1. Answering that question is driving the development of large-scale, single neuron resolution recordings from behaving animals, on the scale of thousands of neurons These data are inherently high-dimensional, with as many dimensions as neurons - so how do we make sense of them? The strong principle is that dimension reduction shows us how neural circuits operate and compute. I show how we could make either the weak or strong principles appear to be true based on innocuous looking decisions about how we use dimension reduction on our data. To counteract these confounds, I outline the experimental evidence for the strong principle that do not come from dimension reduction; and show there are a number of neural phenomena that the strong principle fails to address. Mark D Humphries cile these conflicting data, I suggest that the brain has both principles at play
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