Color discrimination and dichromacy in the horse (Equus caballus)
C.A. Hall1, H.J. Cassaday2, C.J. Vincent3 and A.M. Derrington3
1School of Animal, Rural and Environmental Sciences, Nottingham
Trent University, Nottingham, UK
2School of Psychology, University of Nottingham, Nottingham, UK
3Psychology, Brain and Behavior, University of Newcastle upon Tyne, Newcastle
upon Tyne, UK
The findings of this paper demonstrate a correlation between the results
of color discrimination training and cone excitation ratios in the horse.
Six horses were trained to discriminate fifteen different colors from
shades of gray. The behavioral data obtained was compared with cone excitation
ratios that were calculated for each of the colors, based on spectral
peak values of 429 nm and 545 nm. The horses were first trained to discriminate
each color from the grays to a criterion of ten consecutive correct choices,
with the order of presentation being determined by the spectral appearance
of the colors to the trichromatic human. In order to make lightness cues
irrelevant, the chromatic stimuli were randomly located within a chequerboard
of shades of gray. All six horses successfully completed the training,
reaching the learning criterion for all fifteen colors.
The relationship between the behavioral data obtained, and the spectral
sensitivity of the cone photopigments of the horse, was then investigated.
The reflectance spectrum of each of the colors was measured and cone excitations
were calculated. Cone excitation ratios for the colors and the grays were
compared and the difference between them used as an indication of the
effect that each color had on the visual system of the horse. Significant
correlations between the behavioral data (accuracy, repeated errors and
latency of approach) and the cone excitation data were found for all colors.
In addition to providing further evidence for dichromacy, the results
of this study
demonstrate that cone excitation calculations can be used to predict the
ease with which
the horse can discriminate colors. Such information will facilitate further
investigation into
color perception in the horse as well as offering greater insight into
the ways in which color
vision in the horse differs from that of the trichromatic human.
Paper presented
at Measuring Behavior 2005
, 5th International Conference on Methods and Techniques
in Behavioral Research, 30 August - 2 September 2005, Wageningen, The
Netherlands.
© 2005 Noldus
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