Department of Biomedical Engineering, Northwestern University, Evanston, IL 60208, USA.
Department of Neurobiology, Northwestern University, Evanston, IL 60208, USA.
Curr Biol. 2017 Sep 25;27(18):2751-2762.e6. doi: 10.1016/j.cub.2017.08.012. Epub 2017 Sep 7.
All visual animals must decide whether approaching objects are a threat. Our current understanding of this process has identified a proximity-based mechanism where an evasive maneuver is triggered when a looming stimulus passes a subtended visual angle threshold. However, some escape strategies are more costly than others, and so it would be beneficial to additionally encode the level of threat conveyed by the predator's approach rate to select the most appropriate response. Here, using naturalistic rates of looming visual stimuli while simultaneously monitoring escape behavior and the recruitment of multiple reticulospinal neurons, we find that larval zebrafish do indeed perform a calibrated assessment of threat. While all fish generate evasive maneuvers at the same subtended visual angle, lower approach rates evoke slower, more kinematically variable escape responses with relatively long latencies as well as the unilateral recruitment of ventral spinal projecting nuclei (vSPNs) implicated in turning. In contrast, higher approach rates evoke faster, more kinematically stereotyped responses with relatively short latencies, as well as bilateral recruitment of vSPNs and unilateral recruitment of giant fiber neurons in fish and amphibians called Mauthner cells. In addition to the higher proportion of more costly, shorter-latency Mauthner-active responses to greater perceived threats, we observe a higher incidence of freezing behavior at higher approach rates. Our results provide a new framework to understand how behavioral flexibility is grounded in the appropriate balancing of trade-offs between fast and slow movements when deciding to respond to a visually perceived threat.
所有有视觉的动物都必须判断接近的物体是否构成威胁。我们目前对这一过程的理解已经确定了一种基于接近度的机制，当一个逼近的刺激物通过一个下视角阈值时，就会触发回避动作。然而，一些逃避策略比其他策略代价更高，因此，如果能够额外编码捕食者接近速度所传达的威胁程度，以选择最合适的反应，那么这将是有益的。在这里，我们使用自然的逼近视觉刺激的速度，同时监测逃避行为和多个网状脊髓神经元的募集，发现幼鱼确实会对威胁进行校准评估。虽然所有的鱼在相同的下视角都会产生回避动作，但较低的接近速度会引起较慢、运动学变化较大的逃避反应，潜伏期较长，同时单侧募集与转弯有关的腹侧脊髓投射核（vSPN）。相比之下，较高的接近速度会引起更快、运动学更刻板的反应，潜伏期相对较短，同时 vSPN 的双侧募集和鱼类和两栖动物中称为 Mauthner 细胞的巨纤维神经元的单侧募集。除了对感知到的更大威胁，更高比例的、更昂贵的、潜伏期更短的 Mauthner 激活反应外，我们还观察到较高的接近速度与较高的冻结行为发生率相关。我们的研究结果为理解行为灵活性如何在适当平衡快速和慢速运动之间的权衡提供了一个新的框架，以便在决定对视觉感知的威胁做出反应时。
