Turning thought into action.

Patients with spinal cord injury, subcortical stroke, neuromuscular diseases (including amyotrophic lateral sclerosis), and limb amputation have at least two characteristics in common: a brain that wants to direct movement and a body that fails to respond accordingly. Despite intact cortical function , central motor commands in persons with these paralyzing disorders are " disconnected " from their targets, with the neural impulses of intended movement unacknowledged by the downstream central or peripheral nervous system. Pharmaco-logic, cellular (including stem-cell), and other therapies are designed to repair the injury on-site, but what if the lesion could simply be bypassed, thereby providing a new pathway for these signals to control either one's own limbs or assistive devices such as prosthetic limbs? A recent study by Vel-liste et al. 1 represents a step toward these goals. The primary motor cortex (M1) is vital for generating the fine finger and hand dexterity of monkeys and humans. Recordings of single M1 neu-rons in monkeys performing manual tasks such as gripping a joystick have revealed an association between changes in the action-potential firing rate of these neurons and the dynamics 2 and kine-matics of hand movement. In particular, some M1 cells demonstrate " directional tuning " : they fire at greatest frequency when an animal reaches in a particular direction (the " preferred direction " for that cell) and at minimum frequency when the animal reaches in the opposite direction (Fig. 1). 3 This can be modeled as a cosine tuning curve by plotting firing rate against direction of movement; knowing a cell's firing rate, one can then predict (with varying degrees of accuracy) the direction of limb movement. Not much can be gleaned from observation of the instantaneous firing rate of a single neu-ron, particularly if one wishes to predict the movement of a multijointed limb in three-dimensional space. With recent advances in technology and surgical techniques, however, tiny arrays of mi-croelectrodes can be inserted into the cortex, enabling the recording of activity from a hundred or more neurons in real time. A relatively small ensemble, or population, of directionally tuned M1 neurons allows the location of an animal's hand to be predicted, for example, while the animal moves a joystick to control a cursor in a video game. If the joystick controlling the cursor is then disconnected, that neural output can be decoded into a command signal to move a two-or three-dimensional cursor directly. …