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Computational Models of Proprioception

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Computational Models of Proprioception Available online at www.sciencedirect.com ScienceDirect A leg to stand on: computational models of proprioception 1,4 2,4 Chris J Dallmann , Pierre Karashchuk , 3,5 1,5 Bingni W Brunton and John C Tuthill Dexterous motor control requires feedback from interacts with motor circuits to control the body remains proprioceptors, internal mechanosensory neurons that sense a fundamental problem in neuroscience. the body’s position and movement. An outstanding question in neuroscience is how diverse proprioceptive feedback signals An effective method to investigate the function of sensory contribute to flexible motor control. Genetic tools now enable circuits is to perturb neural activity and measure the targeted recording and perturbation of proprioceptive neurons effect on an animal’s behavior. For example, activating in behaving animals; however, these experiments can be or silencing neurons in the mammalian visual cortex [4] or challenging to interpret, due to the tight coupling of insect optic glomeruli [5] has identified the circuitry and proprioception and motor control. Here, we argue that patterns of activity that underlie visually guided beha- understanding the role of proprioceptive feedback in viors. However, due to the distributed nature of proprio- controlling behavior will be aided by the development of ceptive sensors and their tight coupling with motor con- multiscale models of sensorimotor loops. We review current trol circuits, perturbations to the proprioceptive system phenomenological and structural models for proprioceptor can be difficult to execute and tricky to interpret. encoding and discuss how they may be integrated with existing models of posture, movement, and body state estimation. Mechanical perturbation experiments Early efforts to understand the behavioral contributions Addresses 1 of proprioceptive feedback relied on lesions and mechan- Department of Physiology and Biophysics, University of Washington, Seattle, WA, USA ical perturbations. A classic example of a proprioceptive 2 perturbation experiment is the use of vibration to artifi- Neuroscience Graduate Program, University of Washington, Seattle, cially excite primary muscle spindle afferents. In humans, WA, USA 3 muscle vibration creates the illusory perception that a Department of Biology, University of Washington, Seattle, WA, USA muscle is being stretched [6]. If a person is walking, Corresponding author: Tuthill, John C ([email protected]) vibrating their hamstring produces an increase in forward 4 Co-first authors. 5 walking speed, while vibrating their quadriceps has little Co-senior authors. effect on walking kinematics [7] (Figure 1, top left). During backward walking, however, these effects are Current Opinion in Physiology 2021, 21:100426 reversed, suggesting that proprioceptive feedback acts This review comes from a themed issue on Proprioception in a context-dependent manner. Edited by Tim Cope and Leah Bent Longer-term mechanical perturbations have also been used to investigate the dynamics of sensorimotor adapta- tion. Ba¨ssler et al. [8] inverted the sign of feedback signals https://doi.org/10.1016/j.cophys.2021.03.001 from the femoral chordotonal organ in the stick insect leg 2468-8673/ã 2021 Elsevier Ltd. All rights reserved. by surgically crossing the receptor tendon (Figure 1, bottom left). This manipulation causes a walking stick insect to either ‘salute’ or ‘drag’ her leg, and a standing stick insect to rhythmically wave her tibia. Over the course of a month, Ba¨ssler and colleagues observed that the walking salute and dragging remained unchanged, Experimental perturbations probe the role of while the waving movements gradually decreased. This proprioceptive feedback in motor control observation suggests that the postural feedback control All animals possess specialized sensory neurons that system recovered, while the control system for leg move- monitor the mechanical consequences of their actions. ment did not. These sensors, known as proprioceptors, are essential for coordinating body movement and maintaining body pos- These examples illustrate several hazards to consider ture [1,2]. While it is possible for some isolated motor when executing and interpreting proprioceptive pertur- circuits to generate structured output in the absence of bation experiments. First, the effects of proprioceptive proprioceptive feedback, behaviors driven by purely feedback on motor output are often context-dependent, feedforward motor signals are often clumsy and ineffec- such as during forward and backward walking. Second, tual [3]. Understanding how proprioceptive feedback proprioceptive signals are typically used for controlling www.sciencedirect.com Current Opinion in Physiology 2019, 1:100426 2 Proprioception Figure 1 Mechanical Neural Muscle vibration Optogenetic stimulation Walking speed Walking speed -1 -1 1 m s 10 mm s Short term 5 s 0.5 s Inverted feedback Genetic ablation Slow walking Walking FeCO 1 0.6 LF Saluting Dragging 0 RF LH RHRF LF 0 -0.6 Flexor 0 1 tendon RH Fast walking Rest Long term DTX DTX 3-6 days 1 0.6 LF Waving Recovered Lumbar spinal cord 0 RF 0 -0.6 0 RH 1 Current Opinion in Physiology Examples of experimental perturbations used to probe the role of proprioceptive feedback in motor control. Top left: Vibration of hamstring muscles in humans stimulated muscle spindles, leading to an increase in walking speed [7]. Bottom left: Inverting sensory feedback from the femoral chordotonal organ of a stick insect front leg by crossing the receptor tendon (arrow) led to saluting or dragging of the leg during walking and waving during rest (front leg in the air, other legs on the ground). The normal rest posture of the front leg (extended in air) recovered after a few days, but saluting and dragging during walking remained unchanged [8]. Top right: Optogenetic stimulation of second-order proprioceptive neurons in tethered Drosophila walking on a treadmill caused a decrease in walking speed. Trace shows mean of multiple animals [9 ]. Bottom right: Genetic ablation of hindlimb muscle spindles in the mouse lumbar spinal cord caused inter-limb coordination deficits only during fast walking. 3D plots show timing (phase) of right hindlimb (RH), right forelimb (RF), and left forelimb (LF) relative to left hindlimb (LH) for gait cycles with (gray) and without (green) muscle spindles [10 ]. both posture and movement, often at the same time. An tibia whenever the leg was unloaded, not just during additional challenge of mechanical perturbations is that walking. Thus, the decrease in walking speed was likely they often lack specificity, leaving it unclear which spe- driven by the fly’s reaction to a perceived extension of her cific proprioceptor neurons underlie the observed tibia, rather than a disruption to walking speed control. behavior. Another recent study used intersectional genetic methods Neural perturbation experiments to test the role of local proprioceptive feedback in rodent With the emergence of genetic tools to label and manip- locomotion. When Takeoka and Arber [10 ] expressed ulate specific cell-types, it has recently become possible Diphtheria toxin (DTX) in muscle spindles that inner- to genetically activate or silence specific proprioceptive vate the front or rear legs of adult mice, they observed neurons to assess their role in behavior. However, such only subtle changes in spontaneous walking gait. The manipulation experiments come with their own chal- inter-limb coordination deficits produced by these abla- lenges [11]. For example, Agrawal et al. [9 ] used opto- tions became apparent only when mice were forced to run genetics to study a population of second-order proprio- at high speeds on a motorized treadmill (Figure 1, bottom ceptive neurons in Drosophila that encode extension of right). the fly’s tibia. Activation of these neurons caused walking flies to slow down, suggesting that they may participate in These examples illustrate how it can be misleading to controlling walking speed (Figure 1, top right). However, infer a direct, causal relationship between neural activity fine-grained kinematic analysis of leg joints revealed that and behavior when the perturbation impacts multiple the same perturbation produced reflexive flexion of the feedback loops at different levels of the nervous system. Current Opinion in Physiology 2019, 1:100426 www.sciencedirect.com Computational models of proprioception Dallmann et al. 3 For example, activating or silencing proprioceptors may [13,14]. Recent studies have begun to map these func- alter postural reflexes that interfere with an ongoing tional subtypes onto neuronal cell-types defined using behavior, such as walking. In other cases, changes to genetic markers [13,15]. These tools now enable targeted motor output produced by genetic perturbations may perturbation experiments to investigate the role of pro- only become visible in specific behavioral contexts. prioceptor subtypes in specific motor tasks. Finally, removing proprioceptive feedback can lead to general deficits to motor coordination that alter an ani- Due to a combination of neural and mechanical properties mal’s ability or desire to perform certain behavioral tasks. [16,17], proprioceptors are tuned to biologically relevant stimuli. These same properties can make proprioceptor How models can help responses vary as a function of stimulus history and Our thesis is that the design and interpretation of such behavioral context. Such nonlinearities make propriocep- perturbation experiments would benefit from the inte- tors
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