Neural correlates of learning and trajectory planning in the posterior parietal cortex

Elizabeth B Torres,Christopher A Buneo,Rodrigo Quian Quiroga,He Cui

doi:10.3389/fnint.2013.00039

Abstract

The posterior parietal cortex (PPC) is thought to play an important role in the planning of visually-guided reaching movements. However, the relative roles of the various subdivisions of the PPC in this function are still poorly understood. For example, studies of dorsal area 5 point to a representation of reaches in both extrinsic (endpoint) and intrinsic (joint or muscle) coordinates, as evidenced by partial changes in preferred directions and positional discharge with changes in arm posture. In contrast, recent findings suggest that the adjacent medial intraparietal area (MIP) is involved in more abstract representations, e.g., encoding reach target in visual coordinates. Such a representation is suitable for planning reach trajectories involving shortest distance paths to targets straight ahead. However, it is currently unclear how MIP contributes to the planning of other types of trajectories, including those with various degrees of curvature. Such curved trajectories recruit different joint excursions and might help us address whether their representation in the PPC is purely in extrinsic coordinates or in intrinsic ones as well. Here we investigated the role of the PPC in these processes during an obstacle avoidance task for which the animals had not been explicitly trained. We found that PPC planning activity was predictive of both the spatial and temporal aspects of upcoming trajectories. The same PPC neurons predicted the upcoming trajectory in both endpoint and joint coordinates. The predictive power of these neurons remained stable and accurate despite concomitant motor learning across task conditions. These findings suggest the role of the PPC can be extended from specifying abstract movement goals to expressing these plans as corresponding trajectories in both endpoint and joint coordinates. Thus, the PPC appears to contribute to reach planning and approach-avoidance arm motions at multiple levels of representation.

Highlights

The process of planning reaching movements to visual stimuli begins with an image on the two retinas and ends with a complex spatiotemporal pattern of arm muscle activations
Many previous studies have pointed to a role for the posterior parietal cortex (PPC) in movement planning though its precise role in trajectory planning computations remains unclear
Studies of LIP and medial intraparietal area (MIP) suggest that PPC activity reflects high level motor intentions, i.e., abstract movement plans specifying the spatial goal of movement as well as the effector or effectors used to attain that goal

Summary

Introduction

The process of planning reaching movements to visual stimuli begins with an image on the two retinas and ends with a complex spatiotemporal pattern of arm muscle activations. Subsequent computations require converting this abstract task-level representation of movement into a spatially and temporally organized sequence of hand positions required to achieve the reach goal, a process termed “trajectory formation” (Hoff and Arbib, 1993; Torres and Zipser, 2002). This mapping is non-trivial as there are an infinite number of possible hand paths consistent between two points in 3D space. These latter transformations are highly non-linear and understanding them will require advances in computational methods as well as insights gained through neurophysiological investigations of the brain areas involved in visually-guided reaching

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Conclusion