Chen-Harris Haiyin, Joiner Wilsaan M, Ethier Vincent, Zee David S, Shadmehr Reza
Departments of Biomedical Engineering and Neurology, Johns Hopkins School of Medicine, Baltimore, Maryland 21205, USA.
J Neurosci. 2008 Mar 12;28(11):2804-13. doi: 10.1523/JNEUROSCI.5300-07.2008.
Ballistic movements like saccades require the brain to generate motor commands without the benefit of sensory feedback. Despite this, saccades are remarkably accurate. Theory suggests that this accuracy arises because the brain relies on an internal forward model that monitors the motor commands, predicts their sensory consequences, and corrects eye trajectory midflight. If control of saccades relies on a forward model, then the forward model should adapt whenever its predictions fail to match sensory feedback at the end of the movement. Using optimal feedback control theory, we predicted how this adaptation should alter saccade trajectories. We trained subjects on a paradigm in which the horizontal target jumped vertically during the saccade. With training, the final position of the saccade moved toward the second target. However, saccades became increasingly curved, i.e., suboptimal, as oculomotor commands were corrected on-line to steer the eye toward the second target. The adaptive response had two components: (1) the motor commands that initiated the saccades changed slowly, aiming the saccade closer to the jumped target. The adaptation of these earliest motor commands displayed little forgetting during the rest periods. (2) Late in saccade trajectory, another adaptive response steered it still closer to the jumped target, producing curvature. Adaptation of these late motor commands showed near-complete forgetting during the rest periods. The two components adapted at different timescales, with the late-acting component displaying much faster rates. It appears that in controlling saccades, the brain relies on an internal feedback that has the characteristics of a fast-adapting forward model.
像扫视这样的弹道运动需要大脑在没有感觉反馈的情况下生成运动指令。尽管如此，扫视却非常精确。理论表明，这种精确性的产生是因为大脑依赖于一种内部前向模型，该模型监测运动指令，预测其感觉后果，并在飞行过程中校正眼球轨迹。如果扫视的控制依赖于前向模型，那么每当其预测在运动结束时与感觉反馈不匹配时，前向模型就应该进行调整。我们使用最优反馈控制理论预测了这种调整应如何改变扫视轨迹。我们让受试者在一种范式下进行训练，在扫视过程中水平目标垂直跳跃。经过训练，扫视的最终位置朝着第二个目标移动。然而，随着眼动指令在线校正以使眼睛转向第二个目标，扫视变得越来越弯曲，即次优。适应性反应有两个组成部分：(1) 引发扫视的运动指令变化缓慢，使扫视更接近跳跃后的目标。这些最早运动指令的调整在休息期间几乎没有遗忘。(2) 在扫视轨迹后期，另一种适应性反应使其更接近跳跃后的目标，产生了弯曲。这些后期运动指令的调整在休息期间显示出几乎完全遗忘。这两个组成部分在不同的时间尺度上进行调整，后期起作用的部分显示出更快的调整速度。看来，在控制扫视时，大脑依赖于一种具有快速适应前向模型特征的内部反馈。
