Visual motion detection is one of the most important computations performed by visual circuits. Yet, we perceive vivid illusory motion in stationary, periodic luminance gradients that contain no true motion. This illusion is shared by diverse vertebrate species, but theories proposed to explain this illusion have remained difficult to test. Here, we demonstrate that in the fruit fly Drosophila, the illusory motion percept is generated by unbalanced contributions of direction-selective neurons’ responses to stationary edges. First, we found that flies, like humans, perceive sustained motion in the stationary gradients. The percept was abolished when the elementary motion detector neurons T4 and T5 were silenced. In vivo calcium imaging revealed that T4 and T5 neurons encode the location and polarity of stationary edges. Furthermore, our proposed mechanistic model allowed us to predictably manipulate both the magnitude and direction of the fly’s illusory percept by selectively silencing either T4 or T5 neurons. Interestingly, human brains possess the same mechanistic ingredients that drive our model in flies. When we adapted human observers to moving light edges or dark edges, we could manipulate the magnitude and direction of their percepts as well, suggesting that mechanisms similar to the fly’s may also underlie this illusion in humans. By taking a comparative approach that exploits Drosophila neurogenetics, our results provide a causal, mechanistic account for a long-known visual illusion. These results argue that this illusion arises from architectures for motion detection that are shared across phyla.

视觉运动检测是视觉电路执行的最重要的计算之一。然而，我们在不包含真实运动的静态、周期性亮度梯度中感知到生动的虚幻运动。这种错觉是多种脊椎动物所共有的，但用来解释这种错觉的理论仍然难以检验。在这里，我们证明，在果蝇中，错觉运动感知是由方向选择性神经元对静止边缘的反应的不平衡贡献产生的。首先，我们发现果蝇和人类一样，感知静止梯度中的持续运动。当基本运动检测神经元 T4 和 T5 沉默时，这种感知就被消除了。体内钙成像显示 T4 和 T5 神经元编码静止边缘的位置和极性。此外，我们提出的机械模型使我们能够通过选择性沉默 T4 或 T5 神经元来可预测地操纵果蝇幻觉感知的大小和方向。有趣的是，人类大脑拥有与果蝇模型相同的机械成分。当我们让人类观察者适应移动的光边缘或暗边缘时，我们也可以操纵他们感知的大小和方向，这表明类似于苍蝇的机制也可能是人类这种错觉的基础。通过采用果蝇神经遗传学的比较方法，我们的结果为长期以来已知的视觉错觉提供了因果、机械的解释。这些结果表明，这种错觉源于跨门共享的运动检测架构。