The Absolute Threshold of Colour Vision in the Horse Lina S. V. Roth1*, Anna Balkenius2, Almut Kelber1 1 Department of Cell and Organism Biology, Vision Group, Lund University, Lund, Sweden, 2 Department of Plant Protection Biology, The Swedish University of Agricultural Sciences, Alnarp, Sweden Abstract Arrhythmic mammals are active both during day and night if they are allowed. The arrhythmic horses are in possession of one of the largest terrestrial animal eyes and the purpose of this study is to reveal whether their eye is sensitive enough to see colours at night. During the day horses are known to have dichromatic colour vision. To disclose whether they can discriminate colours in dim light a behavioural dual choice experiment was performed. We started the training and testing at daylight intensities and the horses continued to choose correctly at a high frequency down to light intensities corresponding to moonlight. One Shetland pony mare, was able to discriminate colours at 0.08 cd/m2, while a half blood gelding, still discriminated colours at 0.02 cd/m2. For comparison, the colour vision limit for several human subjects tested in the very same experiment was also 0.02 cd/m2. Hence, the threshold of colour vision for the horse that performed best was similar to that of the humans. The behavioural results are in line with calculations of the sensitivity of cone vision where the horse eye and human eye again are similar. The advantage of the large eye of the horse lies not in colour vision at night, but probably instead in achromatic tasks where presumably signal summation enhances sensitivity. Citation: Roth LSV, Balkenius A, Kelber A (2008) The Absolute Threshold of Colour Vision in the Horse. PLoS ONE 3(11): e3711. doi:10.1371/journal.pone.0003711 Editor: Hiromu Tanimoto, Max-Planck-Institut fuer Neurobiologie, Germany Received August 11, 2008; Accepted October 27, 2008; Published November 12, 2008 Copyright: 2008 Roth et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: We are grateful for financial support from the Crafoord foundation, Stockholm, for a new spectrometer and from the Swedish Science council (VR) for contribution to half the PhD scholarship of the first author. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: lina.roth@cob.lu.se Introduction Between a sunny summer day and a moonless night there is a 1000 million-times intensity difference [1]. To function over this huge range of intensities puts an eye at very high demands. Therefore, most vertebrates have duplex retinae with multiple types of cones that can contribute to colour vision in daylight intensities and very light-sensitive rods for vision in dim light [2]. At night when light is dim it is vital for the eye to capture as many photons as possible to allow for a strong visual signal. Common optical adaptations in animals active at night are consequently large eyes with large pupils and short focal lengths (thus, a low f-number) to concentrate the sparse photons onto fewer photoreceptors. In addition, the visual signals from neighbouring photoreceptors can be summed in space and in time to generate a higher signal-to-noise ratio at night at the expense of spatial and temporal resolution [1,3]. Colour vision is compromising sensitivity since the colour signal requires a comparison between signals from photoreceptor types with different spectral sensitivities. For vertebrates this usually means a comparison between at least two different types of cones [4,5], even though rods influence colour perception at low light intensities. As the signals from the cones are compared to generate a colour signal, the photoreceptor noise adds up and becomes relatively larger generating a low signal-to-noise ratio. Spectral pooling–summation of signals from all photoreceptors independently of their maximum spectral sensitivity–could be one way of strengthening the signal. However, this means sacrificing colour vision. In dim light a monochromat should, due to lower noise levels, be able to discriminate more shades than a dichromat [6]. For the same reason, dichromats, comparing signals between only two cone types, fare better than trichromats [6]. Trichromatic colour vision, such in humans, demands two comparisons of signals from three cone types thus generating a low signal-to-noise ratio. Therefore, dichromatic vision theoretically generates a stronger signal-to-noise ratio and allows discrimination of colours at dimmer light conditions compared to trichromatic vision (Fig. 1). It was earlier thought that animals sacrifice colour vision in favour of absolute sensitivity at night. However, colour informa- tion is rich and valuable also during the evening and night. When the light intensity and the colour of light change during dusk and dawn, colour information is more reliable than brightness and obviously there are animals taking advantage of this [7]. During the last decade nocturnal hawkmoths and geckos were shown to be able to discriminate colours in dim light [8,9]. Nocturnal geckos, which only have cones in their retinae, have adapted their eye and cones to become sensitive enough to make use of colour information at night. How about vertebrates that possess both rods and cones? To answer this question, we studied colour vision in an arrhythmic mammal, the horse. The eyes of horses are among the largest terrestrial eyes and their pupil is very mobile and can dilate greatly to catch the sparse photons at night [2,10]. In addition, the reflecting tapetum lucidum in the back of the horse eye, gives the non-absorbed photons a second chance to be captured by the photoreceptors and thereby enhances sensitivity further [11]. Grzimek [12] showed behaviourally that horses can see colours during the day, and there have been several studies confirming these results [13–16]. The aim of our study was to behaviourally determine the absolute threshold of colour vision in horses, and to compare it to the colour vision threshold of humans. For this purpose we also collected information on the photoreceptors and PLoS ONE | www.plosone.org 1 November 2008 | Volume 3 | Issue 11 | e3711