A Dynamical Basis Set for Generating Reaches

The motor cortex was the one of the first cortical areas to be explored electrophysiologically, yet little agreement has emerged regarding its basic response properties. Often it is assumed that single-neuron responses reflect a preference for a particular movement or movement variable. It may be further assumed that movement is generated by (or at least accompanied by) a growing population-level preference for the relevant movement. This view has been attractive because it provides a canonical form for the single neuron, a link between preparatory and movement activity, a way of interpreting the population response, and a platform for designing analyses and couching hypotheses. However, this traditional view yields predictions that are at odds with basic features of the data. We discuss an alternative simplified model, in which outgoing commands are produced by dynamics that generate different output patterns as a function of the initial preparatory state. For reaching tasks, we hypothesized simple quasioscillatory dynamics because they provide a natural basis set for the empirical patterns of muscle activity. The predictions of the dynamical model match the data well at both the single-neuron and population levels, and the quasioscillatory patterns explain many of the otherwise odd features of the neural responses.