Where Does the Population Vector of Motor Cortical Cells Point during Reaching Movements?

Visually-guided arm reaching movements are produced by distributed neural networks within parietal and frontal regions of the cerebral cortex. Experimental data indicate that (I) single neurons in these regions are broadly tuned to parameters of movement; (2) appropriate commands are elaborated by populations of neurons; (3) the coordinated action of neu(cid:173) rons can be visualized using a neuronal population vector (NPV). How(cid:173) ever, the NPV provides only a rough estimate of movement parameters (direction, velocity) and may even fail to reflect the parameters of move(cid:173) ment when arm posture is changed. We designed a model of the cortical motor command to investigate the relation between the desired direction of the movement, the actual direction of movement and the direction of the NPV in motor cortex. The model is a two-layer self-organizing neural network which combines broadly-tuned (muscular) proprioceptive and (cartesian) visual information to calculate (angular) motor commands for the initial part of the movement of a two-link arm.