Introduction

Actions are often directed toward the location of sensory stimuli. It can be argued that a large component of our mental repertoire is dedicated toward directing motor effectors toward the location of a sensory stimulus (Snyder, 2000). The diverse computations underlying this capacity are often grouped under the rubric of the sensorimotor transformation.

In this chapter, we focus on the importance of one component of the sensorimotor transformation, reference-frame (coordinate) transformations. Reference-frame transformations are not trivial, because the location of a sensory stimulus is often coded in a reference frame that is different from the one used by the eventual motor effector. In particular, we examine the role of the posterior parietal cortex (PPC) in reference-frame transformations. We introduce evidence that an aspect of the reference-frame transformation is to represent target locations in a common reference frame in a significant population of neurons in two areas of the PPC. This common reference frame is an eye-centered representation that is “gain” modulated by such factors as eye, head, body, or initial hand position. The advantage of this representation is that it allows the PPC to maintain representations of many different reference frames that can be read out depending on the cognitive and computational needs of the ongoing task. In the next section we introduce the concept of a reference frame and reference-frame transformations, and then discuss the role of the PPC in movement planning and reference-frame transformations.