Normal vision relies on healthy eyes, retinas, and their proper wiring of the brain structures that process visual information. Light which enters the eye is sensed by photoreceptors on the retina. The information is then transmitted via the optic nerve to the lateral geniculate nucleus and from there to the first stage of cortical visual processing, the primary visual cortex.
Amblyopia refers to the loss or reduction of vision from one eye because it is improperly wired to the brain structures that process visual information. Even an eye with normal optics and retina may be weakly or incorrectly connected to the brain, resulting in substantial vision loss from that eye.
What causes amblyopia? Frontal- eyed animals combine the images of the two eyes into a single image. The process also yields the percept of depth — estimates of the distance of objects from the observer. When then the two eyes receive very different scenes that cannot be fused into a single one, the brain opts to ignore information from one of the eyes. This can happen when the eyes are misaligned and pointing in different directions (strabismus), or when the one eye is much more nearsighted, farsighted, or astigmatic compared to the other.
About 3% of children are affected by the condition and, unless it is treated during a period of high plasticity in the brain that may allow external factors to help the brain rewire, called the critical period, the loss of vision might be permanent as the adult visual system becomes hardwired. In other words, if we do not treat them amblyopic patients would be effectively blind from the input of the affected eye.
It is important to correct amblyopia for the simple reason that we are born with only two eyes. Starting your life with only one good eye means your likelihood of going blind during your lifetime is much higher. Thanks to advances in medical research, we are living 25 years more than our grandparents; thus it makes sense to ensure our children start their lives with a pair of healthy eyes.
Animal rights activists argue that because blindness is not a life-threatening disease using animals in this type of research is not justified. I concede blindness is not life threatening, but I ask you to participate in the following exercise – blindfold yourself for just one week and try to go about your daily activities — helping the kids to school, getting to and from work, shopping at the supermarket, doing the laundry, cooking, washing the dishes, assisting your children with homework, and so on. Please return to the comments section of this blog and share with us what you have learned about blindness and the suffering it can cause. This is the suffering the research is intended to prevent and alleviate.
Some of these points were well expressed by a Cardiff University statement in response to the Mirror’s negative coverage of these experiments which included an on-line poll asking readers to participate. The Mirror article elicited the response of scientific blogger PZ Myers who tried once more to explain the true reasons for such experiments and asked scientists to make their voices heard in the poll. The Mirror, apparently disliking the trends in the results, responded with a re-poll. Aside from the obvious scientific invalidity of such internet polls, it is evident from the comments in the article that those who voted against such experiments fail to understand the impact of severe vision loss on quality of life and the methods of the research. While they appear ready to rule out the use of animals in sight-saving research, the same population appears to think differently when it comes to ruling animals out of their dinner plates.
So let me explain again how animals are involved in these studies. To study the early wiring of the brain scientists have used frontal-eyed mammals that have an early visual cortex organized similarly to that of humans. Kittens have historically been used in many of these developmental experiments because they have frontal eyes and binocular vision, and their visual cortex expresses ocular dominance columns as other higher mammals and humans do. Such “columns” represent the amount of cortical territory that each eye takes during development which changes if one eye is weakened. Animal work has shown how different rearing conditions influence the balance the input of the eyes into the cortex, the timescales involved, and the effects of multiple reverse occlusion procedures on visual acuity. Mice also have a small area of binocular vision where the cortex receives inputs from the two eyes and exhibit similar plastic changes. The study of the molecular pathways and events that lead to the opening and closing of the critical period are now being studied almost entirely in mice.
Finally, studying the normal wiring of the brain during development has potential benefits for many other areas of medicine. Amblyopia serves as a general model of a developmental disorder of brain wiring. Understanding the factors that control the opening and closing of the critical is key not only for vision but other diseases. Specifically, if we understood the molecular events that open and close the critical period we could potentially learn how to open such a window of plasticity in the adult. This would allow us not only to treat amblyopia in the adult, but also to enhance neural repair in many conditions that involve damage to neural tissue, such as in stroke.
We do not currently have non-invasive methods that would allow us to study normal and abnormal brain wiring during development in humans. Animal models allow us to understand the molecular and cellular events that take place during development. The experiments involve artificial closure of one eye and recording form brain structures while animals are fully anesthetized. The anesthetic plane is monitored continuously by measuring the heart rate, electrocardiogram, end-tidal CO2, and core temperature. Such monitoring parallels and even exceeds what one may see in human surgeries. The animals are euthanized at the end of the procedure with an overdose of anesthetics — the same way your pet may be euthanized by your veterinarian.
It is outrageous, ignorant or simply deceptive for Nick Palmer of the BUAV to claim on the BBC during a debate with Tom Holder on the justification for this experiments that “you wake up the animal” during the electrophysiological recording procedures. This is flat wrong. Animals are anesthetized for both the surgical procedures, and continuously during the recordings. They never regain consciousness as they are euthanized at the end of the experiment.
We all benefit from the medical advances of the past. Animal research has undeniably contributed to such advances. Opting out of such research without a viable alternative would cause much human and animal suffering. Inaction needs a moral justification, one that our opponents have yet to spell out on moral and scientific grounds.
Addendum: It’s worth remembering that the Mirror – a British tabloid that is characterised by all the usual vices associated with such publications – has a history of making false allegations against scientists. Back in the 1980′s its Sunday edition was obliged to print a Press Council ruling that an earlier report on amblyopia research performed by the neuroscientist Professor Colin Blakemore was “exaggerated, unbalanced and unfair”. It seems that the Mirror is still living up to its “gutter press” reputation.