The horizontal-vertical illusion is a size illusion in which two same-sized objects appear to be different if presented on a horizontal or vertical plane, with the vertical one appearing longer. This illusion represents one of the main evidences of the anisotropy of the perceived space of humans, an asymmetrical perception of the object size presented in the vertical and horizontal space. Although this illusion has been widely investigated in humans, there is an almost complete lack of studies in non-human animals. Here we investigated whether reptiles perceive the horizontal-vertical illusion. We tested two reptile species: bearded dragons (Pogona vitticeps) and red-footed tortoises (Chelonoidis carbonaria). In control trials, two different-sized food strips were presented and animals were expected to choose the longer one. In test trials, animals received two same-sized strips, presented in a spatial arrangement eliciting the illusion. Only bearded dragons significantly preferred the longer strip in control trials; in test trials, bearded dragons selected the strip arranged vertically, suggesting a human-like perception of this pattern, while no clear choice for either array was observed in tortoises. Our results raise the interesting possibility that the anisotropy of perceived space can exists also in a reptile brain.

中文翻译：

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非灵长类动物感知空间的各向异性？胡须龙 (Pogona vitticeps) 和红脚陆龟 (Chelonoidis carbonaria) 的水平-垂直错觉

水平垂直错觉是一种大小错觉，其中两个相同大小的物体如果呈现在水平或垂直平面上，则看起来不同，而垂直的物体看起来更长。这种错觉代表了人类感知空间各向异性的主要证据之一，即对垂直和水平空间中呈现的物体大小的不对称感知。尽管这种错觉已经在人类中得到了广泛的研究，但几乎完全缺乏对非人类动物的研究。在这里，我们调查了爬行动物是否感知到水平垂直错觉。我们测试了两种爬行动物：胡须龙 (Pogona vitticeps) 和红脚陆龟 (Chelonoidis carbonaria)。在对照试验中，提供了两种不同大小的食物条，动物应选择较长的食物条。在测试试验中，动物收到两条相同大小的条带，以空间排列呈现，引发幻觉。在对照试验中，只有胡须龙明显更喜欢较长的条带；在测试试验中，胡须龙选择了垂直排列的条带，表明对这种模式的感知与人类相似，而在乌龟中没有观察到任何一种阵列的明确选择。我们的结果提出了一个有趣的可能性，即感知空间的各向异性也可以存在于爬行动物的大脑中。