autotrophic flagellates (2–20 mm), consumed by micro and mesozooplankton; picoplankton (0.2–2 mm), consumed by heterotrophic nanoflagellates; and inedible phytoplankton .20 mm. The uptake of nutrients (NO 3 , NH 4 and PO 4 ) have been decoupled from the carbon assimilation processes by including dynamic nutrient kinetics 22 , whereby nutrient uptake is dependent on both the level of intercellular storage and external nutrient concentrations. The microbial food web contains bacteria, heterotrophic flagellates and microzooplankton, each with dynamically varying C:N:P ratios and is described in ref. 23. Bacteria consume DOC, decompose detritus and can compete for inorganic nutrients with phytoplankton. Heterotrophic flagellates feed on bacteria and picoplankton and are consumed by microzooplankton and mesozooplankton. Microzooplankton feed on diatoms, autotrophic and heterotrophic flagellates and are consumed by mesozooplankton. 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Competing interests statement The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to A.H.T. (e-mail: aht@pml.ac.uk). .............................................................. Direct visuomotor transformations for reaching Christopher A. Buneo, Murray R. Jarvis, Aaron P. Batista* & Richard A. Andersen Division of Biology, California Institute of Technology, Mail Code 216-76, Pasadena, California 91125, USA ............................................................................................................................................................................. 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. The problem that we address here is shown in Fig. 1a. Although the execution of movement requires the specification of a detailed pattern of inputs to the muscles, movement planning is believed to involve the computation of higher level movement parameters, such as the direction and/or distance that the hand must move to reach the target (vector M) 10 . This is due to the fact that movement goals, as well as evidence of our success in achieving these goals, are largely expressed in high level terms, that is, as visually perceived discre- pancies between the position of the hand and target or deviations from a desired path 13 . Hereafter we use the term ‘target position in hand coordinates’ to describe vector M in Fig. 1a, although the terms ‘movement vector’ and ‘motor error’ could also be used. This information could be derived by subtracting the sensed location of the hand (vector H) from the sensed location of the target (vector T), as long as hand position and target position are coded in a common frame of reference. However, although target position appears to be coded in eye-centred (retinal) coordinates in the early stages of reach planning 14 , hand position is derived from both visual and proprioceptive signals, and can conceivably be coded in eye- centred coordinates, body centred coordinates (that is, with respect to the torso), or both. It is unclear therefore whether the operation shown in Fig. 1a is achieved by subtracting the position of the hand from the position of the target directly, using eye-centred coordi- nates (Fig. 1a, b), or by transforming target locations from eye- to head- to body-centred coordinates, and then subtracting the body- centred position of the hand 10,11 (Fig. 1c). We have approached this problem by analysing the reach-related activity of neurons in the PPC, while varying target position, hand position and gaze direction. Single cell recordings were obtained from area 5 (Fig. 2a, b), a subdivision of the PPC that projects directly to cortical and subcortical motor structures 15–17 . In an initial experiment, 89 neurons from two monkeys were studied under four experimental conditions (Fig. 2c). In two conditions, gaze was held constant at the centre position of a vertically oriented * Present address: Howard Hughes Medical Institute, and Department of Neurobiology, Stanford University School of Medicine, Fairchild Building, Room D209, Stanford, California 94305, USA letters to nature NATURE | VOL 416 | 11 APRIL 2002 | www.nature.com 632 © 2002 Macmillan Magazines Ltd