Colour Vision Threshold Figure 3. The choice frequencies at different light intensities for two horses (A) and six human subjects (B) as a comparison (binominal tests *P,0.05). The data presented for Chap (black bars) and Rosett (grey bars) start from the second day they performed at least 70% correct choices and the number of choices are written above each bar. Chap discriminated colours at 0.02 cd/m2 but chose randomly at 0.007 cd/m2. Six human subjects (B) were presented with 20 combinations per intensity. Initials of the human subjects are listed from left to right as they appear in the graph. All but RF chose the positive stimulus significantly more often at 0.02 cd/m2. At 0.007 cd/m2 all but AB failed to discriminate the colours. 2 AB did not fail until the light intensity was lowered to 0.003 cd/m . doi:10.1371/journal.pone.0003711.g003 to the tapetum the non-absorbed photons have a second chance to be absorbed by the outer segment of the photoreceptors and therefore it doubles the effective length of the outer segment. The human eye yields a sensitivity of approximately 0.08 mm2sr (Table 1). The sensitivity of horse cone vision is thus very similar to that of humans. As a comparison, an animal truly adapted to dim light vision, the toad (Bufo) has a calculated optical sensitivity of 2.41 mm2sr [18]. The optical sensitivity of human rod vision is 0.085 mm2sr and that of horses is 0.13 mm2sr when no signal summation is taken into account (data not shown). Discussion The large eye, and the fact that dichromatic colour vision is superior to trichromatic vision in very dim light [6] led to our initial hypothesis that horses might discern colours at lower light intensities than humans. In behavioural experiments with dogs, Rosengren [21] found that one of her cocker spaniels could discriminate colours at a somewhat lower light intensity than humans. Unfortunately no light intensity measurements were performed during her experiment and ‘‘deepening twilight’’ is the only description of the light level in her experiment. In our experiment the motivation of some horses dropped in low light intensity and the horses were easily distracted. It is possible that more motivated horses are able to discriminate colours at somewhat lower light intensities. However, one horse was motivated throughout the experiments. Our study suggests that horses and humans have similar thresholds of colour vision 2 and discriminate colours in moonlight intensities (0.02 cd/m ). Still, colour seems to be a less salient stimulus for horses than for humans since only one of the three horses in our study performed well at 0.02 cd/m2. We also found some variation in the threshold Figure 4. The distance from retina to the nodal point was 25 mm in the frozen, sagitally sectioned horse eye (A). The outer segment of the bulkier cones (B) has a diameter of 1 mm and a length of 4 mm (arrows). doi:10.1371/journal.pone.0003711.g004 PLoS ONE | www.plosone.org 3 November 2008 | Volume 3 | Issue 11 | e3711