The gradient-like architecture mostly refers to the arrangement of visual, eye and hand-related signals, as revealed by quantitative analysis in SPL, dorsal premotor and motor cortex (Johnson et al., 1996; Burnod et al., 1999; Battaglia-Mayer

et al., 2001, 2003 for a discussion; Ferraina et al., 2009). In the caudal and rostral poles of the network, respectively in the parietal areas V6A (Galletti et al., 1995, 1997; Battaglia-Mayer et al., 2000, 2001), 7m (Ferraina et al., 1997a,b) and dorsorostral premotor cortex (Johnson et al., 1996; Fuji et al., 2000), visual and eye-related see more signals predominate over coexisting hand information (Johnson et al., 1996). In contrast, hand information dominates over visual and eye signals in the rostralmost part of the SPL (area PE; Johnson et al., 1996) and in the caudalmost part of the frontal cortex (PMdc/F2, MI; Johnson et al., 1996). In

intermediate parietal (areas MIP, PEc, PEa) and frontal (PMdc/F2) lobe regions, this website eye and hand signals coexist, with different relative strengths depending on the cortical zone considered (see Battaglia-Mayer et al., 2003, for a review). Similar trends of eye and hand information, as well as of preparatory and movement-related signals, exist in the frontal node of the parietofrontal network, across motor cortex, supplementary motor area (SMA) and pre-SMA (Alexander & Crutcher, 1990; Rizzolatti et al., 1990; Matsuzaka et al., 1992; Hoshi & Tanji, 2004; see Nakev et al., 6-phosphogluconolactonase 2008 for a recent review). Throughout the network all these signals are directional in nature (Georgopoulos et al., 1981; Kalaska et al., 1983;

Caminiti et al., 1991; Johnson et al., 1996; Battaglia-Mayer et al., 2005). Superimposed on this rostrocaudal dimension is a second gradient concerning the relative strength of different motor-related signals within the network. In fact a transition from preparatory (set- and memory-related) to genuine motor signals occurs moving from caudal to rostral in the SPL; the opposite holds true in the frontal cortex, as one moves from dorsorostral to dorsocaudal premotor cortex toward MI. However, although in different proportions, cells encoding eye and/or hand position information are ubiquitous at all rostrocaudal levels in the network, so as to form a matrix of position representation in which preparatory and movement-related signal are embedded and eventually selected for movement on the basis of task demands (Johnson et al., 1996; Battaglia-Mayer et al., 2001).