34 Sensorimotor Transformations in the Posterior Parietal Cortex

The posterior parietal cortex (PPC) sits at the interface between sensory and motor areas and performs sensorimotor transformations. Current research is beginning' to unravel the details of this transformation process. The first part of this chapter focuses on planning signals found in the PPC. Experiments show that the dlOUght to reach can bc read out from the parietal reach region of monkeys and used to control the position of a computer cursor without any reach movements being made by the monkeys. The second section reviews recent studies of coordinate tral15formations, which are an important aspect of sensorimotor transformations and involve the PPC. Early studies of the posterior parietal cortex (PPC) identified movement-related and sensory-related signals (:\1ountcastle et al., 1975; Robinson, 1978; Andersen et al., 1987;. Although debate continues over whether responses in PPC during sensory-guided movements are sensory or movement related, control experiments indicate that both signals are present (Andersen et aL, 1987). Moreover, there seems to be a dynamic evolution of activity, with sensory responses and responses related to movement plans occurring early in delayed-response tasks and movement-related activity occurring later (Zhang and Barash, 2000; Cohen, Batista, and Andersen, 2002; Sabes, Bremen, and Andersen, 2002). Although visual responses in the lateral intraparietal (LIP; area have been well documented for a number of years, their significance has recently been reinterpreted by Goldberg and colleagues (Powell and Goldberg, 2000; Goldberg et al., 2002; Bisley and Goldberg, 2003). They have argued that the existence of visual responses means that the signals in LIP cannot be related to movement plans. It has been further argued that the nature of responses in LIP would make it difficult for other parts of the brain to determine whether activity is related to sensory responses or movement plans. Finally, they proposed that tl1e signals must therefore be signaling salience. IUCHARD Al'lDERSEN, DANIELLA MEEKER, BIJAN PESARA"! , BORIS BREZNEN, CHRISTOPHER BU? \'E;O, and HANS SCHERBERGER Division of Biology, California Institute of Technology, Pasadena, Calif. One problem with the interpretation of Goldberg and colleagues is that the two kinds of responses can be distinguished. In LIP, activity related to eye movements can be distinguished from activity related to arm movements, even when the visual stimuli instructing these difierent movement types are similar (Snyder et al., 1997) 2000). The same is true in the parietal reach region (PRR). In LII~ when monkeys perform object-based saccades, the movement vector can be distinguished from the activity related to the object, even when the object is flashed on just prior to the eye movement. In antisaccade trials it has been claimed that there is activity related to the visual target and activity related to saccades, and thus they cannot be separated (Gottlieb and Goldberg, 1999). However, subsequent studies showed that the visual and movement activities exist in different populations of LIP cells and can in fact be easily distinguished (Zhang and Barash, 2000). Although action-related activity is well established in PPC, a legitimate question is whether this activity is related to plans to make movements or instead reflects predicted changes in sensory input arising from the integration of efierence copy. Probably both are in operation. The persistent activity in LIP and PRR on delay tasks has been shown not to reflect the sensory memory of targets or attention (Gnadt and Andersen, 1988; ~Iazzoni et al., 1996; Snyder et aI., 1997, 1998; Batista and Andersen, 2001). Since it can precede the actual eye or arm movement by many seconds, it would appear to reflect the plan to move. However, there are additional changes in activit)~ both in the spiking and local field potentials that oecur around the time of the movement, that likely reflect, among other possibilities, a predietion of the sensory consequences of movements (Andersen et al., 1987; Barash et aL, 1991; Pesaran et aL,