Interaction between the oculomotor and postural systems during a dual-task: compensatory reductions in head sway following visually-induced postural perturbations promote the production of accurate double-step saccades in standing human adults

Humans routinely scan their environment for useful information using saccadic eye movements and/or coordinated movements of the eyes and other body segments such the head and the torso. Most previous eye movement studies were conducted with seated subject and showed that single saccades and sequences of saccades (e.g. double-step saccades) made to briefly flashed stimuli were equally accurate and precise. As one can easily appreciate, most gaze shifts performed daily by a given person are not produced from a seated position, but rather from a standing position either as subject perform an action from an upright stance or as they walk from one place to another. In the experiments presented here, we developed a new dual-task paradigm in order to study the interaction between the gaze control system and the postural system. Healthy adults (n = 12) were required to both maintain balance and produce accurate single-step and double-step eye saccades from a standing position. Visually-induced changes in head sway were evoked using wide-field background stimuli that either moved in the mediolateral direction or in the anteroposterior direction. We found that, as in the seated condition, single- and double-step saccades were very precise and accurate when made from a standing position, but that a tighter control of head sway was necessary in the more complex double-step saccades condition for equivalent results to be obtained. Our perturbation results support the "common goal" hypothesis that state that if necessary, as was the case during the more complex oculomotor task, context-dependent modulations of the postural system can be triggered to reduced instability and therefore support the accomplishment of a suprapostural goal.

人类日常通常通过眼跳（saccade）运动，或是配合眼部与头部、躯干等身体部位的协调运动，对周遭环境进行扫描以获取有用信息。过往绝大多数眼动研究均以坐姿受试者为对象开展，结果显示，针对短暂闪现刺激完成的单步眼跳与连续眼跳序列（如双步眼跳（double-step saccades）），其精准度与准确度均无显著差异。不难理解，人类日常完成的绝大多数视线转移并非在坐姿状态下产生，而是在站姿状态下完成——要么是受试者以直立姿态完成某项动作，要么是在从一处移步至另一处的过程中。本研究开展的实验中，我们设计了全新的双任务范式（dual-task paradigm），以探究视线控制系统（gaze control system）与姿势控制系统（postural system）之间的交互机制。实验招募了12名健康成年人作为受试者，要求他们在站姿状态下同时维持身体平衡，并完成精准的单步与双步眼跳动作。我们采用中外侧方向（mediolateral direction）或前后方向（anteroposterior direction）运动的宽视野背景刺激（wide-field background stimuli），诱发受试者产生视觉诱发的头部晃动。实验结果显示，与坐姿状态下的结果一致，站姿下完成的单步与双步眼跳同样具备极高的精准度与准确度；但在复杂度更高的双步眼跳任务中，受试者需要更严格地控制头部晃动，才能达到与坐姿任务相当的表现。本次实验的扰动结果支持“共同目标”假说（"common goal" hypothesis）：该假说提出，在必要时（如在复杂度更高的动眼任务（oculomotor task）场景中），机体可触发姿势系统的情境依赖性调节（context-dependent modulations），以降低身体的不稳定性，从而辅助完成超姿势目标（suprapostural goal）。