Motor cortical representation of hand translation and rotation during reaching

Previous studies have shown that hand translation is well represented by neuronal activity in the proximal arm area of primary motor cortex (M1). However, little is known about cortical representation of hand rotation in M1. In this study, single-unit activity was recorded from monkeys while they performed a "center-out with rotation" task. When reaching for a target, subjects had to match four separate kinematic parameters: three-dimensional location and one-dimensional orientation of the target. Among the 512 neurons modulated by hand movement, 446 were tuned to hand translation, 326 were tuned to hand rotation, and 260 neurons were tuned to both hand translation and hand rotation. Approximately half of the neurons that encoded both translation and rotation did so in a nonlinear manner. This nonlinear interaction can be modeled as a gain-field type of encoding whereby hand rotational velocity modulated the hand translational cosine tuning curves in a multiplicative manner. Furthermore, this study demonstrated that both hand translation and hand rotation can be decoded simultaneously from a population of motor cortical neurons.