Vision and hearing in horses

Abstract

no accommodative ability and that objects at various distances may be brought into focus by the horse moving its head up and down to allow images to fall onto different parts of the retina. This hypothesis has been repeated in several veterinary textbooks and, until recently, was generally accepted. However, Sivak and Allen 3 made careful measurements in 2 eyes and found no indication that the retina was ramped the way Nicholas 13 had described. Sivak and Allen 3 also reported that repeated measures of refractive status revealed some variability, suggesting that horses may have some limited accommodation. More recently, Harman et al 8 performed a more extensive investigation. Although they did report some small variation in the lens-to-retina distance, the slope of the ramp was in the opposite direction from that predicted by the ramp retina hypothesis (ie, there is a greater lens-toretina distance in the lower part of the eye). This would not easily serve as an accommodative mechanism.