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1 Physiology, Hebrew University-Hadassah Medical School, Jerusalem, Israel; Interdisciplinary Center for Neural Computation, Hebrew University, Jerusalem, Israel
* To whom correspondence should be addressed. E-mail: ybss{at}md2.huji.ac.il.
Natural actions can be described as chains of simple elements whereas individual motion elements are readily concatenated to generate countless movement sequences. Sequence specific neurons have been described extensively, suggesting that the motor system may implement temporally-complex motions by employing such neurons to recruit lower-level movement neurons modularly. Here, we set-out to investigate whether activity of movement related neurons is independent of the sequential context of the motion. Two monkeys were trained to perform linear arm movements either individually or as components of double-segment motions. However, comparison of neuronal activity between these conditions is delicate since subtle kinematical variations generally occur within different contexts. We therefore employed extensive procedures to identify the contribution of variations in motor execution to differences in neuronal activity. Yet, even after application of these procedures we find that neuronal activity in the motor cortex (PMd and M1) associated with a given motion segment differs between the two contexts. These differences appear during preparation, and become even more prominent during motion execution. Interestingly, despite context related differences on the single neuron level, the population as a whole still allows a reliable readout of movement direction regardless of the sequential context. Thus, the direction of a movement and the sequential context in which it is embedded may be simultaneously and reliably encoded by neurons in the motor cortex.
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