Echolocating bats evolved a sophisticated biosonar imaging system that allows for a life in dim-light habitats. However, especially for far-range operations such as homing, bats can support biosonar by vision. Large eyes and a retina that mainly consists of rods are assumed to be the optical adjustments that enable bats to use visual information at low light levels. In addition to optical mechanisms, many nocturnal animals evolved neural adaptations such as elongated integration times or enlarged spatial sampling areas to further increase the sensitivity of their visual system by temporal or spatial summation of visual information. The neural mechanisms that underlie the visual capabilities of echolocating bats have, however, so far not been investigated. To shed light on spatial and temporal response characteristics of visual neurons in an echolocating bat, Phyllostomus discolor, we recorded extracellular multiunit activity in the retino-recipient superficial layers of the superior colliculus (SC). We discovered that response latencies of these neurons were generally in the mammalian range, whereas neural spatial sampling areas were unusually large compared to those measured in the SC of other mammals. From this we suggest that echolocating bats likely use spatial but not temporal summation of visual input to improve visual performance under dim-light conditions. Furthermore, we hypothesize that bats compensate for the loss of visual spatial precision, which is a byproduct of spatial summation, by integration of spatial information provided by both the visual and the biosonar systems. Given that knowledge about neural adaptations to dim-light vision is mainly based on studies done in non-mammalian species, our novel data provide a valuable contribution to the field and demonstrate the suitability of echolocating bats as a nocturnal animal model to study the neurophysiological aspects of dim-light vision.

中文翻译：

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蝙蝠的弱光视觉的神经基础。

蝙蝠的回声进化出一种先进的生物声纳成像系统，可在昏暗的栖息地中生活。但是，特别是对于归巢等远程操作，蝙蝠可以通过视觉支持生物声纳。假定大眼睛和主要由视杆组成的视网膜是一种光学调节装置，可使蝙蝠在弱光条件下使用视觉信息。除光学机制外，许多夜行动物进化出神经适应性，例如延长整合时间或扩大空间采样面积，以通过视觉信息的时间或空间总和进一步提高其视觉系统的灵敏度。然而，迄今为止，尚未对作为回声定位蝙蝠的视觉能力基础的神经机制进行研究。为了阐明回声定位蝙蝠毛竹变色中视觉神经元的时空响应特征，我们记录了上丘（SC）视网膜-受体浅表层的细胞外多单位活性。我们发现，这些神经元的反应潜伏期通常在哺乳动物的范围内，而与其他哺乳动物的SC中所测量的相比，神经空间的采样面积却异常大。因此，我们认为回声定位的蝙蝠可能会使用视觉输入的空间而不是时间的总和来改善昏暗条件下的视觉性能。此外，我们假设蝙蝠通过整合视觉和生物声纳系统提供的空间信息来补偿视觉空间精度的损失，视觉空间精度是空间求和的副产品。