Just went to Jeremy's defense. It was good, he used retrograde markers to label L5 spinal cord projecting pyramidal cells that projected either to C4 or C8 which control proximal and distal forearm muscles respectively. Upon training of a distal muscle task he recorded and tested to see if C4 populations and C8 population were more connected to each other after learning the motor skill.
Since it was a primarily distal forelimb behavior only C8 projecting neurons showed differences in their structure and connectivity after training. They formed more connections (from about 2% connectivity to 6%), but they were weaker on average (but that may because there are just so many more new connections). No changes C4<->4 or C4<->8.
Its particularly interesting because motor cortex has basically no layer 4. In fact he said that layer 5 in M1 is 5a and 5b, but no layer 4. Layer 5 seems to be a major "output" layer of cortex, and perhaps it is the layer that can set up these arbitrary patterns (like the heteroclinic channel layer, or the CPG layer). It seems that layer 5 projects mainly up the hierarchy. L5 from V1 goes to higher-order thalamus, L6 goes feedback to LGN.
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