Cortical Involvment in the Recruitment of Wrist Muscles

Ashvin Shah, Andrew H. Fagg, Andrew G. Barto

{ash | fagg | barto}@cs.umass.edu
Neuroscience and Behavior Program and Computer Science Department
University of Massachusetts, Amherst

In executing a voluntary movement, we are faced with the problem of translating a specification of the movement in task space (e.g. a visual goal) into a muscle recruitment pattern. Among many brain regions, the primary motor cortex (MI) plays a prominent role in the specification of arm and hand movements. Early experiments suggested that individual MI cells contribute to movements in a visual-centered (or extrinsic) coordinate system (Georgopoulos et al., 1982). Subsequent experimental and theoretical work have argued for a muscle-centered encoding of movement by MI cells. However, in a recent two-dimensional wrist step tracking experiment, Kakei et al. (1999) observed both extrinsic-like and muscle-like task-related cells in MI. This result was interpreted as evidence for a cascade of transformations within MI from an extrinsic representation of movement to a muscle-like representation. In this poster, we present a model which examines the complexity of the transformation from the extrinsic visual space to the intrinsic muscle space that implements the wrist movements on which Kakei et al. (1999) focused. We demonstrate that, given a realistic extrinsic-like representation of movement, a simple linear network is capable of representing the transformation from the extrinsic-like cells to the necessary muscle activation pattern. This suggests that cells exhibiting extrinsic-like qualities can be involved in the direct recruitment of spinal motor neurons. These results call into question models that presume a serial cascade of transformations that terminates with MI pyramidal tract neurons (PTNs) that vary their activity exclusively with muscle activity.

Presented at 2002 Spring Meeting on the Neural Control of Movement, Conference, April 14-21, 2002, Naples, FL