Neural basis of limb selection in unimanual reaching

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Abstract The central goal of this proposal is to understand the neural systems subserving the decision to use the left or right hand1 – a simple choice that is central to our daily functions. Here we focus on parietal mechanisms of limb selection. Despite it being one of the most common decisions faced in everyday behavior, a largely neglected aspect of decision making is deciding which hand to use to achieve a goal. For instance, when deciding which hand to use to flip a light switch upon entering a room, the choice is influenced by many factors. These factors include what each hand is already doing (e.g, holding a phone), proximity of each hand to the switch, and future goals (e.g., desire to subsequently pick up an object with a particular hand). Our long- term goals are to elucidate the fundamental mechanisms of limb selection and movement coordination. Because limb selection is present in most behaviors, this work will provide insights broadly applicable to motor control. The posterior parietal cortex (PPC) is known to be involved in the planning and control of actions in an effector-specific manner. An emerging concept is that the same PPC regions that are involved in movement preparation and control also participate in action selection. We recently demonstrated that a parietal region known to contribute to the planning of arm movements and dubbed “the parietal reach region” (PRR), codes the spatial targets for reaches with the contralateral arm but receives information about the ipsilateral arm from the opposite hemisphere. PRR has also been implicated in the decision to reach versus saccade to a spatial target. In our first aim, we characterize limb selection behavior and determine the factors that drive limb selection. In our second aim we test the hypothesis that PRR is involved in limb selection during unimanual reaching. Next, we examine interhemispheric communication as a function of limb choice. Finally, we use reversible inactivation of the callosal fibers connecting PRR in each hemisphere to determine the role of interhemispheric competition in determining which limb to use. Collectively, these studies will have a broad impact on the field by illuminating the roles of PPC and callosal connections in a fundamental choice motor behavior. The results will lay the foundation for future studies that probe the larger bilateral frontoparietal network involved in movement control. The work may inform rehabilitation approaches for patients with neurological disorders including stroke and traumatic brain injuries in whom limb use is affected. 1 We will use the terms limb, arm, and hand interchangeably.