Movement Planning
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Movement planning is a complex activity that requires communication between both sensory stimulus and areas that control for these movements. Numerous amount of research has been carried out to decipher what areas of the brain are responsible for motor planning and intention coding, narrowing locations to specific regions in the post parietal cortex (PPC). Both fMRI and cell recording studies in monkeys have aided researchers in understanding motor planning, however the advantages and disadvantages of these methods should not be overlooked. These methods indicate a common link between monkey parietal regions that are involved in movement, and the human brain that responds similarly, (Conolly, Andersen & Goodale, 2003) but also reveal differences. Saccades and reaches are two crucial mechanisms that enable researchers to determine what areas in the brain are active during these motor tasks. An in-depth examination of these tasks, aided by the cognitive methods they employ will provide us with concrete evidence as to what processes occur during movement planning.
The aim of movement planning is to transform environmental stimuli into precise motor execution, and to achieve this, there are a number of distinct intermediate processes that need to be carried out. These processes include computations that involve coding size, location and shape of the goal-object (Milner & Goodale, 1995). Moreover, these processes are constantly under time-pressure to perform and result in accurate motor execution. Motor planning involves matching location and required actions relative to the observer, where these processes must be coded within a common frame of reference (Milner & Goodale, 1995). Processes that are triggered in the presence of stimuli, transform sensory signals that are used to direct movement into a common frame of reference. This common frame of reference is an eye-centered reference frame that controls eye-head-limb-body signals and combines information from areas that are involved in the selection of different effectors for different tasks (Cohen, Richard & Andersen, 2002). Movement planning in the PPC involves coordinate transformation (Cohen, Richard & Andersen, 2002), with areas specialized for saccadic and reach movements. The planning involved in both these mechanisms are assembled differently (Jeannerod, 1997) but are also interlinked, because to produce or orchestrate a movement towards an object with an effector, an appropriate saccade must be made to that target location, resulting in accurate movement. For movement planning, our eye-centered coordinates are crucial for enabling us to plan with the highest spatial acuity needed for movement (Synder, Batista & Anderson, 2000). Thus, preparatory processes need sensory information to be translated into motor intention and action within a contextual framework. It is hypothesized that externally directed actions such as reaching and grasping involves the PPC (Goldberg, 1985, cited in Gazzaniga, Ivry & Mangun, 1998). However, in the following discussion, the relationship between the circuits (particularly memory related activities which are thought to be internally directed) will suggest that maybe perception and action are not mutually exclusive, especially when subjects are required to plan movements to remembered locations. Thus, movement planning enables subjects to use prior sensory information to perform motor transformations.
To understand function and what areas of the brain are involved in processes such as motor planning, activity in these structures needed to be manipulated. In single cell recording, activity is recorded through a thin electrode that is implanted close to the extracellular region of a cell (or a group if multiple-recording). Electrical changes are thus measured around the region of the neuronal membrane, since inserting electrode(s) intracellularly damages the neuron (Gazzaniga, et al., 1998). Thus, changes in the activity levels in the neurons have allowed researchers to map areas (particularly sensory areas) corresponding to their functions. The prime concern for any cell recording is to manipulate some aspect of the stimulus/experimental condition and see what changes they produce in the neuron, or if there is a correlation in the firing pattern of these neurons (Gazzaniga, et al., 1998). During this recording, the location of the stimulus plays a huge role in that some cells may only fire or spike if the stimulus falls within their specific receptive field (Gazzaniga, et al., 1998) and this tends to vary. In multiple-unit recording (many neurons) specifically, the larger the tip of the electrode, the more signals picked up from the neurons within a region, which can be measured relative to a baseline condition (Pinel, 2006). The action potentials detected by the electrode are