Neural correlates of internal models.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Review of Ghasia et al. Internal models are a key concept in motor control, helping explain a myriad of adaptive behaviors, including reaching, walking, and saccadic eye movements. They come in two flavors: inverse and forward models. Inverse models (also known as controllers) take as input a desired tra-jectory and output the corresponding motor commands. Forward models take an efferent copy of the motor commands and use it to predict the future state of the body (e.g., position, velocity). Although these concepts have been extensively used in explaining behavior in psychophysical studies, their neural correlates have remained difficult to identify. To investigate the neural correlates of internal models, Ghasia et al. (2008) exploited one of their recent findings: ex-traocular motoneurons do not seem to encode for the torsion component of eye movements during horizontal and vertical smooth pursuits (Ghasia and An-gelaki, 2005). Torsion corresponds to the rotation of the eye about a head-fixed front– back axis. It represents one of the three degrees of freedom along with horizontal and vertical rotations [for review of three-dimensional (3D) kinematics of the eye, see Wong (2004)]. If the center of the fovea is fixating a point on a vertical 2D plane, according to Donder's law the position of the eye corresponds to a unique vertical, horizontal, and torsion angle. As a result, when eye movements are made from one eccentric position to another, they usually contain a torsion component, even when the fovea trajectory is strictly horizontal or vertical. Listing's law, which dictates the rotation vector, predicts that the torsional component of these trajecto-ries is influenced by the eccentricity from which the eye movement is initiated. In a nutshell, the larger the eccentricity, the larger the torsional component, with a sign change occurring at zero eccentricity. In a previous study, Ghasia and An-gelaki (2005) found that the motoneurons responsible for purely torsional movements , such as the ones induced by the roll vestibulo-ocular reflex, did not modulate their activity to account for the variation in torsion component accompanying purely horizontal or vertical smooth pursuit eye movements. That is, the motor output did not …