Motor Preparation for Compensatory Reach-to-Grasp Responses When Viewing a Wall-Mounted Safety Handle

The present study explored how motor cortical activity was influenced by visual perception of complex environments that either afforded or obstructed arm and leg reactions in young, healthy adults. Most importantly, we focused on compensatory balance reactions where the arms were required to regain stability following unexpected postural perturbation. Our first question was if motor cortical activity from the hand area automatically corresponds to the visual environment. Affordance-based priming of the motor system was assessed using single-pulse Transcranial Magnetic Stimulation (TMS) to determine if visual access to a wall-mounted support handle influenced corticospinal excitability. We evaluated if hand actions were automatically facilitated and/or suppressed by viewing an available handle within graspable range. Our second question was if the requirement for rapid movement to recover balance played a role in modulating any affordance effect in the hands. The goal was to disentangle motor demands related to postural threat from the impact of observation alone. For balance trials, a custom-built, lean and release apparatus was used to impose temporally unpredictable postural perturbations. In all balance trials, perturbations were of sufficient magnitude to evoke a compensatory change-in-support response; therefore, any recovery action needed to carefully take into account the affordances and constraints of the perceived environment to prevent a fall. Consistent with our first hypothesis, activity in an intrinsic hand muscle was increased when participants passively viewed a wall-mounted safety handle, in both seated and standing contexts. Contrary to our second hypothesis, this visual priming was absent when perturbations were imposed and the handle was needed to regain balance. Our results reveal that motor set is influenced by simply viewing objects that afford a grasp. We suggest that such preparation may provide an advantage when generating balance recovery actions that require quickly grasping a supportive handle. This priming effect likely competes with other task-dependent influences that regulate cortical motor output. Future studies should expand from limitations inherent with single-pulse TMS alone, to determine if vision of our surrounding world influences motor set in other contexts (e.g. intensified postural threat) and investigate if this priming corresponds to overt behavior.

本研究探讨了年轻健康成年人的运动皮层活动如何受复杂环境的视觉感知影响——这类环境或可为四肢动作提供支持条件，或对其造成阻碍。尤为关键的是，本研究聚焦于代偿性平衡反应：即受试者遭遇意外姿势扰动后，需通过手臂动作恢复身体稳定的场景。我们的第一个研究问题是：手部区域的运动皮层活动是否会自动与视觉感知到的环境相匹配。本研究采用单脉冲经颅磁刺激（Transcranial Magnetic Stimulation, TMS）评估基于动作可供性的运动系统启动效应，以探究视觉可见的壁挂式支撑握柄是否会影响皮层脊髓兴奋性。我们还评估了：当受试者注视可及范围内的可用握柄时，手部动作是否会被自动易化或抑制。我们的第二个研究问题是：为恢复平衡而需快速完成动作的任务需求，是否会对手部的动作可供性效应产生调节作用。本研究的目标是，将与姿势威胁相关的运动任务需求与单纯视觉观察带来的影响区分开来。在平衡任务试次中，本研究采用定制化的前倾释放装置，施加时间上不可预测的姿势扰动。所有平衡任务试次中的扰动幅度均足以诱发代偿性换支撑动作，因此任何恢复动作都需仔细考量感知环境中的动作可供性与约束条件，以避免摔倒。与我们的第一个研究假设一致：无论受试者处于坐姿还是站姿，当被动观察壁挂式安全握柄时，其手部固有肌的活动均会增强。但与第二个研究假设相悖的是：当施加扰动且需借助握柄恢复平衡时，这类视觉启动效应并未出现。研究结果表明，仅通过观察可供抓握的物体，即可对运动准备状态产生影响。我们认为，这类准备机制可为需要快速抓握支撑握柄的平衡恢复动作提供优势。这类启动效应可能会与其他调控皮层运动输出的任务相关影响因素产生竞争。未来的研究应突破单脉冲TMS单独使用时的固有局限性，探究周围环境的视觉感知是否会在其他场景（如加剧的姿势威胁场景）中影响运动准备状态，并验证这类启动效应是否与外显行为相一致。