Is it possible to fully restore sight? New surgeries and advances in stem cell and gene therapies seem to indicate that this is possible and may happen within the next decade. Recent research in neuroscience, however, demonstrates that it may not yet be possible to restore full vision in persons with long-standing blindness or low vision.
A group of international neuroscientists has discovered, via a single-subject research design, that the rewiring of the senses occurring in the brains of persons with long-term blindness could mean that visual restoration may never be complete or may only be partially reversed by visual restoration in adulthood.
From the Journal of Neurophysiology
This new brain research, entitled Tracking the evolution of crossmodal plasticity and visual functions before and after sight-restoration, has been published online ahead of print in the December 17, 2014 edition of the Journal of Neurophysiology. [Editor’s note: “Crossmodal” refers to perception that involves interactions between two or more different sensory systems, such as vision and hearing or vision and touch. “Plasticity” refers to the brain’s ability to reorganize itself by forming new nerve cell connections to compensate for injury and/or disease.]
The authors are Giulia Dormal, Franco Lepore, Mona Harissi-Dagher, Geneviève Albouy, Armando Bertone, Bruno Rossion, and Olivier Collignon, who represent the following institutions: the Catholic University of Louvain, Belgium and the University of Montreal, Canada.
About the Research
Excerpted from How does the brain adapt to the restoration of eyesight? Surgery cannot completely undo the brain rewiring caused by long term blindness, via EurekAlert:
Recent scientific advances have meant that eyesight can be partially restored to those who previously would have been blind for life. However, scientists … have discovered that the rewiring of the senses that occurs in the brains of long-term blind [persons] means that visual restoration may never be complete.
“We had the opportunity to study the rare case of a woman with very low vision since birth and whose vision was suddenly restored in adulthood following the implantation of a Boston Keratoprosthesis [i.e., KPro, or artificial cornea] in her right eye,” explained Giulia Dormal, who led the study.
“On one hand, our findings reveal that the visual cortex maintains a certain degree of plasticity – the capacity to change as a function of experience – in an adult person with low vision since early life. On the other, we discovered that several months after the surgery, the visual cortex had not regained full normal functioning.” The visual cortex is the part of the brain that processes information from our eyes.
Scientists know that in cases of untreatable blindness, the occipital cortex – the posterior part of the brain that is normally devoted to vision – becomes responsive to sound and touch in order to compensate for the loss of vision. “This important brain reorganization represents a challenge for people encountering eye surgery to recover vision, because the deprived and reorganized occipital cortex may not be capable of seeing anymore…,” Dormal said.
The researchers worked with a 50-year-old Quebec woman. This involved taking MRI images as she completed various visual and auditory tasks and comparing her scans with scans that had been taken from people with normal eyesight and people with untreatable blindness who had performed the same tasks. The study suggests that eye surgery can lead to a positive outcome even when performed in adulthood after a life-time of profound blindness.
There is however an important caveat. “The recovery observed in the visual cortex … is not total,” Dormal explained. “Indeed, auditory-driven responses were still evidenced in certain regions of the visual cortex even seven months after surgery, and these responses overlapped with visually-driven responses. This overlap may be the reason some aspects of vision, despite having improved with time, still remained below normal range seven months after surgery.”
What Does It Mean to See? More about the Eye and How It Works
Diagram of the eye, viewed from the side
To understand this diagram of the eye, try to picture it as being split in two, like an apple that’s been cut in half. Imagine yourself looking into the eye from the cut side.
The cornea is a transparent dome-shaped tissue that forms the front part of the eye. It functions as a window and allows light to enter the eye through the pupil and iris. It also begins the process of focusing light rays that help you see words and images clearly. The cornea does not contain any blood vessels, but instead contains nerve endings that make it extremely sensitive.
Aqueous humor is a clear, watery fluid contained in a chamber behind the cornea that helps bring nutrients to the eye. It flows between, and nourishes, the cornea and the lens. Problems with the flow of the aqueous fluid can lead to problems with pressure inside the eye, including glaucoma.
The sclera is a tough white outer coating of fibrous tissue that covers the entire eyeball (all the way around) except for the cornea. The muscles that move the eye are attached to the sclera. The name sclera comes from the Greek word “skleros,” which means “hard.”
The Iris and the Pupil
The iris is a tissue containing muscles that surround and encircle the opening in the center of the iris. This opening is called the pupil. The iris regulates the amount of light that enters the eye by adjusting the size of the pupil opening.
In bright light, the iris closes (or constricts) and makes the pupil opening smaller to restrict the amount of light entering the eye. In dim light, the iris opens (or dilates) and makes the pupil opening larger to increase the amount of light entering the eye.
The lens is composed of transparent, flexible tissue and is located directly behind the iris and the pupil. It is the second part of the eye, after the cornea, that helps to focus light and images on the retina. Because the lens is flexible and elastic, it can change its curved shape to focus on objects and people that are either nearby or at a distance.
The choroid is a dark brown membrane that is rich with blood vessels, located between the sclera and the retina. It supplies blood and nutrients to the retina and nourishes all of the other structures within the eye.
The vitreous is a jelly-like substance that fills the inside of the back part of the eye and gives it nourishment and shape. Over time, the vitreous becomes more liquid and can detach from the back part of the eye, which can create “floaters.”
The Retina and Optic Nerve
The retina is the light-sensitive tissue that lines the inside surface of the eye, much like wallpaper. Cells in the retina convert incoming light into electrical impulses. These electrical impulses are carried by the optic nerve (which resembles a television cable) to the brain, which finally interprets them as visual images. Thus, the process of “seeing” involves the eye and the brain and, in a healthy eye, occurs automatically.
The Prevent Blindness website provides a detailed view of the inside of the eye and its component parts at The Eye and How We See. You can read more about the eye and vision at Eye Health on the VisionAware website.
More about the Study from the Journal of Neurophysiology
From the article abstract:
Visual deprivation leads to massive reorganization in both the structure and function of the occipital cortex, raising crucial challenges for sight restoration. We tracked the behavioral, structural and [functional] changes occurring in an early and severely visually impaired patient (a) before, (b) 1.5 months, and (c) 7 months after sight restoration, using magnetic resonance imaging.
Robust pre-surgical auditory responses were found in occipital cortex despite residual preoperative vision. In the primary visual cortex, crossmodal auditory responses overlapped with visual responses and remained elevated even 7 months post-surgery.
However, these crossmodal responses decreased in extrastriate occipital regions [i.e., the brain regions located next to the visual cortex] after surgery, together with improved behavioral vision and with increases in both grey matter density [i.e., a major component of the central nervous system] and neural activation [i.e., brain activity] in low-level visual regions.
Selective responses in high-level visual regions involved in motion and face processing were observable even pre-surgery and did not evolve after surgery.
Taken together, these findings demonstrate that structural and functional reorganization of occipital regions are present in an individual with a longstanding history of severe visual impairment, and that such reorganizations can be partially reversed by visual restoration in adulthood.