How the Visual Abilities of Dogs Change With Age
Research shows that as dogs age, they become more nearsighted.
Posted Apr 07, 2016
My Nova Scotia Duck Tolling Retriever, Dancer, always responded more reliably to hand signals than to voice commands. In fact, he was so attentive to my body movements that often my inadvertent small hand movements (which were not intended to communicate anything to my dog), were read by him as if they were commands, thus producing unintended and unexpected responses. However, when Dancer grew to be 9 or 10 years of age I began to notice that sometimes he would miss, or misread, a hand signal. This problem got worse in successive years. At first I thought that it might be due to a waning of his intellectual abilities with age, or perhaps due to the fact that he had developed arthritis, and so might find it uncomfortable to move when I gave hand signals which required him to change position. I reevaluated this conclusion when I discovered that he was not responding to hand signals when he was at a far distance from me, yet he would still respond with great precision when I was much closer to him.
This behavior puzzled me. One possibility that I considered to explain his actions was that he was becoming more nearsighted (technically myopic) with age. Nearsightedness means that objects which are close are seen clearly while objects which are more distant are blurry. However this did not make any sense in terms of what I knew about the effects that aging has on the human visual system. As humans grow older they become more farsighted (technically hyperopic or presbyopic), meaning that distant objects are seen clearly while near objects are blurry. So I chalked up Dancer's performance as just another mystery of canine behavior that I couldn't explain. Fortunately science has a way of eventually filling in the holes associated with gaps in our knowledge about dogs.
A new report has just appeared in the journal PLoS ONE* which provides an answer. The research was done by a team of researchers headed by Jerome Hernandez of the Nestlé Purina Research Center, in Saint Joseph, Missouri. This investigation used a high tech instrument called an autorefractor to measure the optical status of dogs' eyes in order to see which changes occur as dogs age. The study looked at nine Beagles ranging in age from 1 to 14 years. A total of 20 measurements were taken for each dog (five for each eye under both direct and indirect lighting conditions). Only one set of measures was taken each day, and the measurements proved to be quite stable and reliable from day to day.
The surprising finding to come out of this study is that dogs, contrary to humans, become more nearsighted as they age. These changes are quite large. In order to illustrate this age trend I went back to the original raw data and computed a composite score for the visual status of each dog. I next plotted each dog's refractive error against its age. Finally I plotted the best fitting line to show the age trend. This can be seen in the figure below.
The units of measure are diopters, which is actually a measure of the curvature of a lens. A refractive error is represented by too little or too much curvature. If the refractive error is positive that means that the individual is farsighted while if the refractive error is negative it means that they are nearsighted. As you can see from the graph there is a steady change in the refractive error of dogs with age with an increasingly negative trend indicating that the dogs are becoming steadily more nearsighted as they grow older. The oldest dogs in this study (10 and 14 years of age respectively) had a refractive error around -2.5 diopters. I think that most of you will have some difficulty conceptualizing what this means so I have provided the figure below modified from the original report. What you see in the figure is a typical eye chart and then the degrees of blurring associated with each level of refractive error. If an individual is farsighted these blurring effects occur for near items, while if an individual is nearsighted (as are the dogs) these blurring effects occur for far items.
Since the older dogs have a refractive error in the vicinity of -2.5 diopters, this means that they were viewing distant objects or distant hand signals with a degree of blur between the two and the three diopter images in the figure. I think that you will agree that this involves quite a bit of blur and easily could account for a dog not seeing or misinterpreting a hand signal. So, several years after my well loved Dancer has passed on, I finally have the answer to why my aging dog began to become unreliable in his responses to hand signals as he grew older. He was simply becoming too nearsighted to see my signals when I was at a significant distance away from him.
In aging humans we compensate for the age changes in vision through the use of glasses. Unfortunately optically correcting glasses are not available for dogs, and if they were I doubt that any obedience competition judges would allow a dog into the ring wearing such prosthetics. However for those of you who are still working with dogs that are getting up in years, it is possible to compensate, at least a little bit, for the loss of visual acuity as our own dogs age and become more nearsighted. Using larger hand signals with a greater degree of movement will make the signals somewhat more visible. And of course, in many instances it is possible to resort to voice commands rather than using hand signals.
Stanley Coren is the author of many books including: Gods, Ghosts and Black Dogs; The Wisdom of Dogs; Do Dogs Dream? Born to Bark; The Modern Dog; Why Do Dogs Have Wet Noses? The Pawprints of History; How Dogs Think; How To Speak Dog; Why We Love the Dogs We Do; What Do Dogs Know? The Intelligence of Dogs; Why Does My Dog Act That Way? Understanding Dogs for Dummies; Sleep Thieves; The Left-hander Syndrome
Copyright SC Psychological Enterprises Ltd. May not be reprinted or reposted without permission
* Data from: Hernandez J, Moore C, Si X, Richer S, Jackson J, Wang W (2016) Aging Dogs Manifest Myopia as Measured by Autorefractor. PLoS ONE 11 (2): e0148436. doi:10.1371/journal.pone.0148436