Potier et al. supporting figures and table from High resolution of colour vision, but low contrast sensitivity in a diurnal raptor

Animals are thought to use achromatic signals to detect small (or distant) objects and chromatic signals for large (or nearby) objects. While the spatial resolution of the achromatic channel has been widely studied, the spatial resolution of the chromatic channel has rarely been estimated. Using an operant conditioning method, we determined (i) the achromatic contrast sensitivity function and (ii) the spatial resolution of the chromatic channel of a diurnal raptor, the Harris's hawk Parabuteo unicinctus. The maximal spatial resolution for achromatic gratings was 62.3 c deg−1, but the contrast sensitivity was relatively low (10.8–12.7). The spatial resolution for isoluminant red-green gratings was 21.6 c deg−1—lower than that of the achromatic channel, but the highest found in the animal kingdom to date. Our study reveals that Harris's hawks have high spatial resolving power for both achromatic and chromatic vision, suggesting the importance of colour vision for foraging. By contrast, similar to other bird species, Harris's hawks have low contrast sensitivity possibly suggesting a trade-off with chromatic sensitivity. The result is interesting in the light of the recent finding that double cones—thought to mediate high-resolution vision in birds—are absent in the central fovea of raptors.学界普遍认为，动物会利用消色差信号（achromatic signals）探测小型（或远距离）物体，而利用彩色信号（chromatic signals）识别大型（或近距离）物体。尽管消色差通道（achromatic channel）的空间分辨率已得到广泛研究，但彩色通道（chromatic channel）的空间分辨率却鲜有相关估算。本研究采用操作性条件反射法（operant conditioning method），测定了昼行性猛禽哈里斯鹰（Harris's hawk, *Parabuteo unicinctus*）的两项指标：（i）消色差对比度敏感度函数，以及（ii）其彩色通道的空间分辨率。消色光栅的最大空间分辨率可达62.3周/度（c deg⁻¹），但其对比度敏感度相对较低，仅为10.8–12.7。等亮度红-绿色栅（isoluminant red-green gratings）的空间分辨率为21.6周/度，虽低于消色差通道，但却是迄今动物界已报道的最高值。本研究结果表明，哈里斯鹰在消色差视觉与彩色视觉方面均具备优异的空间分辨能力，这暗示了色彩视觉在其觅食行为中的重要性。与之相对，与其他鸟类类似，哈里斯鹰的对比度敏感度较低，这或许意味着其对比度敏感度与彩色敏感度之间存在权衡关系。鉴于近期的一项发现，该结果颇具研究价值：此前被认为介导鸟类高分辨率视觉的双锥细胞（double cones），在猛禽的中央凹（central fovea）中并不存在。