One of the hallmarks of human evolution is the extraordinary degree to which we can manipulate the physical world with our hands or with tools that extend or amplify portions of our forelimb and digits. This manual dexterity coevolved with an expansion of posterior parietal cortex (PPC), which contains areas involved in programming voluntary movements, coding reach targets in multiple reference frames, and decision making. To allow our body to interact with our physical surroundings, these fields must also construct an internal model of the physical self: our body's configuration, the boundary between our body and external physical objects, and the temporary expansion of that self as we wield a tool that extends our reach and manual capabilities. Such comprehension of where and what the self is and even the ability to manipulate objects and use them as tools did not evolve de novo in humans, but rather emerged from simple networks likely to be present in early mammals. The overarching goal of this proposal is to use a multileveled comparative approach to determine how simple networks associated with reaching and grasping were modified to produce the sophisticated abilities associated with the human condition. In four important animal models (rats, tree shrews, prosimian galagos, and macaque monkeys) we will use electrophysiological recording techniques, intracortical microstimulation (ICMS), and neuroanatomical tracing techniques to define homologous areas in PPC. We, and others, have proposed that PPC generates movements guided by the integration of multisensory inputs occurring in PPC. During movements, neurons in motor areas and movement-specific domains of PPC coordinate their activity to generate unique sequences of body, forelimb and hand postures necessary for context-dependent target acquisition and other movements. This hypothesis will be tested in two ways: (1) We will reversibly deactivate motor (M1) cortex in macaque monkeys and rats and examine the effects on movement domains elicited by ICMS in PPC, and (2) We will reversibly deactivate M1 and cortical areas in PPC in macaque monkeys during a natural, bimanual target acquisition task to reveal how these cortical areas work together to generate accurate and contextually appropriate reaching and grasping. By combining connectional, functional, and behavioral data from multiple species, these studies will provide a rich understanding of the role of these complex brain networks in the planning and execution of movement.
Strokes can cause paralysis and severe deficits in normal hand use with varying degrees of recovery over time, indicating that there is a complex network of functional connections between PPC and motor cortex that is plastic. However, we understand very little about this network, which has hampered our ability to maximize the recovery of hand use with specific remedial therapies. While many studies in monkeys have provided keen insights into the structure and function of PPC, we still do not understand how these results apply to humans. Our proposed studies will begin to establish if there are underlying PPC cortical networks that are shared by four key species of mammals (including two different primates) to better understand basic processing networks in human PPC. Our studies will illuminate the contribution of these posterior parietal fields to the initiation of reaching and grasping and other movements, as well as the consequences on these manual behaviors following stroke.
| Stewart, Courtney E; Kanicki, Ariane C; Altschuler, Richard A et al. (2018) Vestibular short-latency evoked potential abolished by low-frequency noise exposure in rats. J Neurophysiol 119:662-667 |
