Temporal encoding of movement in motor cortical neurons.

Editor's Note: These short reviews of a recent paper in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to mimic the journal clubs that exist in your own departments or institutions. For more information on the format and purpose of the Journal Club, please see Review of Hatsopoulos et al. Although it is broadly accepted that primary motor cortex (M1) plays a critical role in controlling volitional arm movement , considerable debate remains about the details of the implementation. Most studies assume that individual M1 neu-rons encode time-invariant movement features within a direction, velocity, or force coordinate frame (Scott, 2003). For example, a neuron that encodes hand movements away from the body does not change this relationship over the duration of a reach. A recent article in The Journal of Neu-roscience by Hatsopoulos et al. (2007) questions this notion of static encoding. The authors contend that M1 neurons encode time-dependent trajectories, termed " pathlets, " that can be temporally extensive and spatially complex. Within their model, a neuron that encodes hand movements away from the body early in a reach may encode movements toward the body at some later time. Although the concept of pathlets, or motor fragments, has a long history in motor control (Leyton and Sherrington, 1917), Hatsopoulos et al. (2007) are the first to provide a sophisticated model that calculates pathlet shape and duration. Furthermore, they show that pathlets better capture the tuning properties of M1 neurons than previous models using time-invariant movement features. Hatsopoulos et al. (2007) trained three macaque monkeys to perform various reaching tasks while neural activity was recorded from the arm area of motor cortex with a microelectrode array. In the first task, monkeys made hand movements from a central location to one of eight peripheral targets (5–7 cm distance). The results confirmed previous observations that preferred hand directions of M1 neurons change over the course of a center-out reach (Sergio and Kalaska, 1998). This confirmation is important because it provides empirical evidence that preferred directions (PDs) change over time. In fact, Hatsopoulos et al. (2007) report that many neurons exhibited dramatic rotations in PD over a 400 ms time window [Hatsopoulos et al. a result inconsistent with a static movement-encoding model. PDs were also calculated in a second reaching task called random target pursuit (RTP). In RTP, movements are made to sequentially appearing targets located randomly in the …