A common reference frame for movement plans in the posterior parietal cortex

To execute a movement towards a sensory target, it is necessary to transform the representation of the target from a sensory reference frame (for example, a head-centred reference frame for the auditory system) to a motor reference frame (limb or body centred). The posterior parietal cortex (PPC) is important for reference-frame transformations, and it is proposed that many parietal neurons transform sensory signals into a common, eye-centred reference frame. Neural activity in the PPC reflects movement planning, with different areas being involved in different types of movement (the lateral intraparietal area, LIP, for saccadic eye movements; the parietal reach region, PRR, for reaching; and the anterior intraparietal area, AIP, for grasping). Neurons in area LIP code the location of visual targets in an eye-centred frame of reference. Many also code the location of auditory targets in an eye-centred reference frame, and some code target location in head-centred or intermediate reference frames. Neurons in the PRR show a similar pattern of location coding for auditory and visual targets. For many of these neurons, the magnitude of response is modulated by eye, head, body or limb position. These 'gain fields', in conjunction with eye-centred representations of target location, could provide a distributed representation of target locations in different reference frames, and a mechanism for coordinate transformations. The idea that the reference-frame transformation occurs in the PPC is supported by the fact that auditory inputs to the PPC are coded in a head-centred reference frame; in other words, they are not transformed to eye coordinates before the PPC. Further support comes from network-modelling studies, in which the neural network layers that are responsible for reference-frame transformations show intermediate frames of reference, and gain fields like those seen in areas LIP and PRR. Many issues remain to be addressed, including the mechanisms of readout of the reference frames, and the task dependence of these representations.