For those reading this who wear corrective lenses, have you noticed that when you get new prescription eyeglasses or contact lenses, your vision is sharper and you are able to see more details than you were before. Of course, there is more to this than merely having better lenses. Vision is a function of both the eyes and the brain. So, what is going on that allows this better vision to work?
Researchers at the National Institutes of Health (NIH) have identified the brain circuits needed for visual acuity and how they are affected by damaged retinal cells. Their findings were published in The Journal of Neuroscience. While there has been progress in developing treatments that repair the eye, not much attention has been given to what happen beyond the eye. Namely, the downstream brain circuits and the dying retinal cells that lose function after changes to retinal inputs.
The brain changes in order to adapt to retinal injury or disease. This is known as neuroplasticity. For example, a person experiencing vision loss may have a blind spot in a part of the field of vision. Current treatments for vision loss target retinal cells, the problem is that they are just the first stage in a multistep pathway that turns light into images.
Scientists wanted to understand how neurons downstream of the retina are affected by damage to the retinal ganglion cells (RGCs). These cells receive signals from other retinal cells and transfer them to the brain. The RGCs connect to neurons in a relay center in the brain, called the lateral geniculate nucleus (LGN).The LGNs transmit signals to the visual cortex, where they are processed into images. Researchers looked at two types of LGN cells that respond to different visual information and form parallel processing pathways. They are the X-LGN neurons, which lead to visual acuity, and the Y-LGN neurons, which are involved in motion perception.
Researchers studied the effects of retinal cell loss on the X and Y visual processing pathways in ferrets. After an injury to the RGCs in the retina, recordings of LGN neuronal responses were done to assess the impact of the injury to the X and Y pathways. They learned that the X-LGN neurons didn’t respond correctly to visual stimuli. As for the Y-LGN neurons, their responses remained intact. This shows that the retinal cell loss affects downstream visual pathways in different ways. Namely, the X pathway was impacted, while the Y pathway wasn’t. This indicates that the visual pathway is more sensitive to degeneration of the retina.
In the future, vision restoration therapies may target both the retina and circuits responsible for visual acuity. These can include training therapies, like computer games that provide interactive feedback or other vision behavioral therapies. Also, future studies could use the model of RGC loss to learn more about retinal degeneration and visual deficits in psychiatric disorders like schizophrenia.
Of course, there’s more. Another study, this one at the University of Alabama Birmingham looked at how the eye send precise visual signals to the brain.
This study showed that a human’s sharpest vision comes from signals from the individual cone photo receptors. These cells are in the retina, where they sense light and are concentrated in the fovea, which is a depression in the retina where eyesight is the sharpest. The signals from the fovea are sent along a dedicated pathway in the brain that preserves visual detail.
Many anatomical studies suggested that signals from a single cone cell can travel along a private line to the brain. Conversely, there were reasons to believe those signals mixed with nearby cells, which would reduce the ability to detect sharp details. In fact, prior physiological studies showed that neurons involved in this brain pathway collected signals from many cones. This didn’t match with the anatomical evidence or perceptual studies that demonstrated a concept known as hyperacuity. Hyperacuity is when people can detect details smaller than the individual cells in the eye that detect light.
This leads to the question: Once the optics are corrected, is visual acuity limited by the retina, the brain or a combination of the two? That’s where research comes in. Scientists found that when the eye’s optics are optimally corrected, the retina can deliver spatially precise information, limited only by the set of cone cells, and the brain can use this information. This means that the visual pathway that starts in the retina is prepared to transmit details set at the level of cone spacing. This why patients notice fine details so clearly when they get a new pair of glasses or contacts.
Once again research provides answers as to why something works the way it does. These two studies will help inform future vision research, which increases our understanding of vision and can lead to therapies that improve outcomes.
https://www.aao.org/eye-health/anatomy/fovea”