Since its introduction in the early 1980s, the concept of a "preferred direction" for neuronal discharge has proven to be a powerful means of studying motor areas of the brain. In the current paper, we introduce the concept of a "muscle-space"-preferred direction (PDM) that is analogous to the familiar hand-space-preferred direction (PDH). PDM reflects the similarity between the discharge of a given neuron and the activity of each muscle in much the way that PDH reflects the similarity of discharge with motion along each of the three Cartesian coordinate axes. We used PDM to analyze the data recorded from neurons in the primary motor cortex (M1) of three different monkeys. The monkeys performed center-out movements within two different cubical workspaces centered either to the left or right of the monkey's shoulder while we simultaneously recorded neuronal discharge, muscle activity, and limb orientation. We calculated preferred directions in both hand space and muscle space, and computed the angles between these vectors under a variety of conditions. PDs for different neurons were broadly distributed throughout both hand space and muscle space, but the muscle-space vectors appeared to form clusters of functionally similar neurons. In general, repeated estimates of PDM were more stable over time than were similar estimates of PDH. Likewise, there was less change in PDM than in PDH for data recorded from the two different workspaces. However, although a majority of neurons had this muscle-like property, a significant minority was more stable in Cartesian hand space, reflecting a heterogeneity of function within M1.
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