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Deconstructing action planning and action observation in parietal circuits in rats

Final Report Summary - RAT MIRROR CELL (Deconstructing action planning and action observation in parietal circuits in rats)

More than a century of work has shown that purposeful movements of the body—such as tying your shoe or getting out of bed—result from neural activity spanning several regions of the brain, including the posterior parietal cortex (PPC) and frontal motor cortex. These areas play well-studied roles in coordinating individual parts of the body, such as the hands or eyes, but the field still lacks an understanding how they control movement and posture of the body in freely behaving individuals. To address this, we recorded neural ensemble activity in freely moving rodents while tracking their head and back in 3D using a high-speed, multi-camera tracking system.
We found that PPC and frontal motor areas predominantly encode 3D posture of the body, as opposed to movement. Moreover, we found that postural tuning of the head and back is organized topographically over the cortical surface, and the signals were sufficiently robust to reconstruct an animal’s ongoing behavior. The tuning scheme also appeared optimized for metabolic efficiency—where the most commonly assumed postures were coded by fewer cells, and infrequently visited postures were represented by more of the network. Though we established that this form of tuning was independent of vision, we found that postural tuning in subsets of cells did depend on what type of behavior the animal was engaged—suggesting that the code is not absolute, but adaptable to meet behavioral demands. We also found that temporarily inactivating the frontal motor areas had no detectable effect on postural tuning in PPC, suggesting either that PPC works “upstream” of motor cortex, or that many regions of cortex encode posture. If the latter case is true, inactivating one area would likely have little effect on the others. We have also open-sourced the software for the 3D tracking platform generated during this project, as well as datasets from rats and mice.

Additional experiments sought to determine if rodents possessed so-called “mirror” neurons, discovered originally in monkeys, which fire when an animal performs an action, or merely observes a cohort perform the same action. These types of neurons are thought underlie several aspects of social behavior, including observational learning, and identifying them in rodents would allow a new era of studies to uncover the cellular mechanisms that make “mirror” neurons possible. Mice were chosen specifically because they are genetically tractable. The experiments in this project, however, found that only performed actions were represented in the brains of the animals, not observed behaviors. Nevertheless, rodents are indeed social animals, capable of observational learning, and we are continuing to research the neural mechanisms underlying this fascinating form of cognition.

Through studying how the brains of these animals represent natural patterns of behavior, including in 3D, we hope to uncover general neural coding principles of goal-directed motor behavior which could be used, for example, to improve the efficiency of brain-controlled prosthetic devices in patients suffering from paralysis. The data obtained from this work could also be applied in the optimization of biologically-inspired robotics platforms-- a field which is intent on producing machines to assist humans in hazardous environments and rescue situations.