Cone excitation ratios correlate with color discrimination performance in the horse (Equus caballus).

Abstract

Six horses (Equus caballus) were trained to discriminate color from grays in a counterbalanced sequence in which lightness cues were irrelevant. Subsequently, the pretrained colors were presented in a different sequence. Two sets of novel colors paired with novel grays were also tested. Performance was just as good in these transfer tests. Once the horse had learned to select the chromatic from the achromatic stimulus, regardless of the specific color, they were immediately able to apply this rule to novel stimuli. In terms of the underlying visual mechanisms, the present study showed for the first time that the spectral sensitivity of horse cone photopigments, measured as cone excitation ratios, was correlated with color discrimination performance, measured as accuracy, repeated errors, and latency of approach.

What Horses and Humans See: A Comparative Review

“…The six horses used in the study were able to use chromatic information across the spectrum, including those wavelengths that would be perceived as green and yellow. The colours orange, yellow, and blue were the colours that the horses most easily discriminated from grey-as was consistent with the predictions made [7]. Moreover as was the case with Grzimek [3], Hall et al [7] also found that the horses actually failed to discriminate the colour red as easily as they discriminated the other colours from the grey control.…”

“…The colours orange, yellow, and blue were the colours that the horses most easily discriminated from grey-as was consistent with the predictions made [7]. Moreover as was the case with Grzimek [3], Hall et al [7] also found that the horses actually failed to discriminate the colour red as easily as they discriminated the other colours from the grey control. In more recent studies with respect to equine colour vision [8,73], the authors have also concluded that horses appear to have dichromatic capability only, much like the red-green colour deficiencies found in some humans.…”

“…One of most recent behavioural studies of colour vision in the horse [7] investigated the correlation between the predicted effect of various colours on the photopigments of the horse and their ability to discriminate the colours from grey colours. Using the values for the peak sensitivities of the cone photopigments of the horse (429 and 545 nm: [72]), the effect of fifteen colours from across the spectrum on the equine visual system was assessed using modelling techniques of Govardovskii et al [85].…”

“…Jameson and Hurvich [86] reported that human subjects, with such dichromatic vision, when asked to arrange colours according to their degrees of similarly would routinely include mixed reds and greens in their chosen sequences. It now seems likely that the horse would experience similar differentiation difficulties, in particular with the colours lime green and orange, and, equally likely, with the colours blue and purple [7], although this phenomenon has yet to be conclusively tested or demonstrated in behavioural experimental work in the horse.…”

“…Further advances in methodology and technology have resulted in additional conclusive findings, but the value of the early work should not be underestimated. For example, despite a series of inconsistent findings [3][4][5][6], the most recent investigation into colour vision in the horse has largely substantiated Grzimek's early work [7,8].…”