Neural population dynamics during reaching

Abstract

Most theories of motor cortex have assumed that neural activity represents movement parameters. This view derives from what is known about primary visual cortex, where neural activity represents patterns of light. Yet it is unclear how well the analogy between motor and visual cortex holds. Single-neuron responses in motor cortex are complex, and there is marked disagreement regarding which movement parameters are represented. A better analogy might be with other motor systems, where a common principle is rhythmic neural activity. Here we find that motor cortex responses during reaching contain a brief but strong oscillatory component, something quite unexpected for a non-periodic behaviour. Oscillation amplitude and phase followed naturally from the preparatory state, suggesting a mechanistic role for preparatory neural activity. These results demonstrate an unexpected yet surprisingly simple structure in the population response. This underlying structure explains many of the confusing features of individual neural responses. It has long been thought that individual neurons in the motor and premotor cortex are tuned for parameters of movements such as direction. But despite decades of work, the exact nature of the represented parameters is still unclear, as are the mechanisms through which such representations could support the complex movements made in the course of every day life. Here, Churchland et al. propose an alternative theory — that population dynamics could underlie motor control. They show that reaching movements are associated with oscillatory population activity in the monkey motor cortex, despite there being no periodic component to this behaviour. The amplitude and phase of the oscillation followed naturally from the preceding state, suggesting a role for preparatory neural activity in reaching.