LAUSR

Abstract Due to the composition of the cone photoreceptors in the retina of dogs, it has been proposed they might demonstrate human-like red/green colour blindness. However, some assessments have shown that dogs may still be able to distinguish between red and green. This suggests that dogs may be differentiating between the two colours on the basis of their brightness. To further explore this issue, the present investigation tested dogs’ abilities to differentiate between two colours at different physical luminance levels. Seven dogs were trained on a simultaneous size discrimination task in which two circles were presented on a monitor. The colours of the circles were then varied such that dogs would choose one stimulus if they could discriminate between two colours and a different stimulus if they could not. Four experiments were conducted. The first presented dogs with yellow, blue, green, and red stimuli at their maximum red, green, and blue (RGB) values. All seven dogs were equally proficient at discriminating between red and green, yellow and blue, and black and white stimuli as no difference in performance was observed (p =  0.52). In Experiments 2–4, the four colours were presented with equal physical luminance across three different intensities. All seven dogs successfully discriminated between all four colours when the colours were isoluminant at 34.6 cd/m2 as no differences in performance was observed (p =  0.10). When presented with isoluminant stimuli at 41.9 cd/m2, a difference was observed between conditions, with the dogs failing to detect red stimuli presented on a green background compared to the achromatic controls (p =  0.03). When presented with stimuli at 49.3 cd/m2, a difference was again observed between conditions, but this time the dogs were unable to detect blue stimuli presented on a yellow background compared to the achromatic controls (p