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Reaching in the New Millenium: Defining Coordinate Systems

Visually guided reaching to and grasping of objects in extrapersonal space is a basic motor task that is essential for human existence and normal functioning. Defining the cognitive processes associated with programming such movements and developing an understanding of the developmental and learning sequences for such movements is a major goal for scientists studying motor control. The processes associated with performing such a task can be described as involving a series of transformations of information between sensory and motor coordinate systems. How does the brain derive visual and kinesthetic sensory coordinate systems for specifying orientation/location of external objects and upper limb segments (especially the hand and fingers) from the relevant sensory signals available? The sensory coordinate systems can be defined by their axes, which appear to be an Earth-fixed vertical and a trunk-fixed anterior–posterior axis at the perceptual level for static vision and kinesthesia (Darling, Butler, & Williams, 1996; Darling & Hondzinski, 1997; Darling & Hondzinski, 1999; Darling & Williams, 1997). However, it is not clear whether these perceptual level coordinate systems are used for the transformations associated with programming a specific movement or, indeed, whether such coordinate systems are related to how movements are carried out at the kinematic level. The observation that populations of brain neurons specify direction of the handpath of upper limb movements suggests such a relationship. Perceptuallevel sensory coordinate systems are probably used when learning movements because of the considerable cognitive effort expended to deal with sensory inputs and make adjustments in motor output during learning. A major related issue that has received little experimental study concerns coordinate systems for perceptions of direction of motion of external objects and of the hand. Humans appear to represent motion of the entire upper limb