For over 25 years, Professor Jennifer Steeves has been studying the long-term consequences of losing one eye early in life on behaviour and the brain. She began by asking seemingly simple questions like “how well can someone see when they have only one eye compared to someone with two?” This is an important question for a few reasons. First, when a person views the world with one eye rather than two, they lose about a ¼ of the visual field in front of them so their viewing landscape is essentially slightly smaller. Second, when viewing with one eye a person can’t take advantage of the perception of 3-dimensional depth from “stereopsis” where the brain is able to create 3D relief from the different perspectives that the two eyes have of the world. (This is the “jumping out” depth that you would see at an Imax 3D movie.) Third, viewing with one eye means that you have half of the sensory information coming into your visual system.
The final consideration as to why it’s important to study vision in people with one eye is that the surgical removal of the eye (enucleation) for unilateral (one eye) retinoblastoma typically happens at a very young age since that is often when retinoblastoma is discovered. Why is this important? This is because vision is not mature when we are born and it takes many years (even decades) for different aspects of vision to mature to adult levels. The brain is actually not mature at birth and the parts of the brain that allow for vision are still developing and maturing. So the question becomes, how does vision mature when someone has one eye? Specifically, what happens to all of the brain cells in the visual parts of the brain that should have been receiving signals from the eye that was surgically removed? Do they just lie dormant or perhaps just die off or, are they taken over by the eye that remains in order to give it “superpower”?
Jennifer’s lab started off by examining how good vision is with one eye compared to people with two. She compared vision to people with two eyes when they viewed with both eyes or when they viewed with one eye covered. The short story is that for the most part, vision with one eye is really very good, and in some instances, it is actually better than people who have two eyes, even with both eyes open! This was particularly the case for something called “contrast sensitivity” which essentially measures how well we can see fine detail that is very faint. Jennifer’s lab also examined how well people with one eye see motion in the world. This is where there were small shortfalls in the ability to see. People with one eye sometimes had minor difficulties with seeing how things move, although it doesn’t appear to affect daily life since it is very subtle. Her lab suspects that the parts of the brain that allow us to see motion may be developed on a similar timeline to and linked to the ability to see 3D depth. A lot of this research was done with Dr. Krista Kelly who is now a researcher at the Retina Foundation of the Southwest in Dallas, Texas where she is now studying amblyopia.
This cartoon of something called a “contrast sensitivity functions” shows how well we are able to see things that are big and dark compared to faint and tiny. People with one eye were better able to see the faint things compared to people with two eyes.
Jennifer and Krista then began asking questions like “what happens to all of those brain cells that would have been connected to the eye that was surgically removed?” They performed some brain imaging experiments using magnetic resonance imaging also known as MRI. They took some very high resolution images of the brain and essentially they found that some of the parts of the brain that are dedicated to vision are actually bigger than they should be given that people have only one eye. This indicates that the brains of people with one eye have actually rewired and taken over some of the cells in the visual parts of the brain that were disconnected from the eye that was surgically removed. This is also called “brain plasticity”. Jennifer and Krista think that having these “extra” when a person has one eye probably is what allows them to see so well! See for example Kelly et al., Neuroimage Clinical 2014 .
Another interesting finding that Krista and Jennifer noted was that some of the hearing (auditory) parts of the brain also appeared to be bigger than those in the brains of people with two eyes. These bright blue spots painted on this inflated brain show auditory regions of the brain that are thicker in people with one eye compared to people with two eyes. (from Kelly et al., Neuroimage Clinical 2014, available here)
This led to some new questions, specifically “how well do people with one eye hear?” Working with Dr. Adria Hoover (left below), Jennifer’s lab showed that people with one eye are better able to locate sound in space. In a similar vein, Dr. Stefania Moro (right below) conducted a number of studies in Jennifer’s lab and she found that while people with two eyes often are susceptible to illusions where vision dominates over (or captures) hearing, people with one eye are less susceptible to these audio-visual illusions. One of these illusions is called the “McGurk Effect”. In this example, people look at a computer screen at a person is mouthing the vowel “ga” while the vowel “ba” is being played on a speaker. People with two eyes tend to hear a completely different (intermediate) vowel such as “da”. In the laboratory, Jennifer and Stefania showed that people with one eye are less susceptible to such distortions of hearing and that their hearing appears to be more reliable. An example of the McGurk effect can be seen in this video.
Most recently, Stefania and Jennifer are looking at how the visual and auditory parts of the brain function. They have done a number of functional MRI (fMRI) studies to look at how the brain functions when looking at visual objects and hearing different sounds. These results will be coming out soon so stay tuned!