The study of the cortical control of movement experienced a conceptual shift over recent decades, as the basic currency of understanding shifted from single-neuron tuning towards population-level factors and their dynamics. This transition was informed by a maturing understanding of recurrent networks, where mechanism is often characterized in terms of population-level factors. By estimating factors from data, experimenters could test network-inspired hypotheses. Central to such hypotheses are ‘output-null’ factors that do not directly drive motor outputs yet are essential to the overall computation. In this Review, we highlight how the hypothesis of output-null factors was motivated by the venerable observation that motor-cortex neurons are active during movement preparation, well before movement begins. We discuss how output-null factors then became similarly central to understanding neural activity during movement. We discuss how this conceptual framework provided key analysis tools, making it possible for experimenters to address long-standing questions regarding motor control. We highlight an intriguing trend: as experimental and theoretical discoveries accumulate, the range of computational roles hypothesized to be subserved by output-null factors continues to expand.

近几十年来，运动皮质控制的研究经历了概念上的转变，基本的理解方式从单神经元调节转向群体水平因素及其动态。这种转变是通过对循环网络的成熟理解而实现的，其中机制通常以群体层面的因素为特征。通过根据数据估计因素，实验者可以测试网络启发的假设。这种假设的核心是“输出空”因素，这些因素不直接驱动电机输出，但对整体计算至关重要。在这篇综述中，我们强调了输出无效因素的假设是如何由一项古老的观察所激发的，即运动皮层神经元在运动准备期间（在运动开始之前）就处于活跃状态。我们讨论了输出无效因素如何成为理解运动过程中神经活动的类似核心。我们讨论这个概念框架如何提供关键的分析工具，使实验者能够解决有关运动控制的长期存在的问题。我们强调了一个有趣的趋势：随着实验和理论发现的积累，假设由输出零因素促进的计算角色的范围不断扩大。