Direct visuomotor transformations for reaching

The posterior parietal cortex (PPC) is thought to have a function in the sensorimotor transformations that underlie visually guided reaching, as damage to the PPC can result in difficulty reaching to visual targets in the absence of specific visual or motor deficits(1). This function is supported by findings that PPC neurons in monkeys are modulated by the direction of hand movement, as well as by visual, eye position and limb position signals(2-9). The PPC could transform visual target locations from retinal coordinates to hand-centred coordinates by combining sensory signals in a serial manner to yield a body-centred representation of target location(10-12), and then subtracting the body-centred location of the hand. We report here that in dorsal area 5 of the PPC, remembered target locations are coded with respect to both the eye and hand. This suggests that the PPC transforms target locations directly between these two reference frames. Data obtained in the adjacent parietal reach region (PRR) indicate that this transformation may be achieved by vectorially subtracting hand location from target location, with both locations represented in eye-centred coordinates.