Many thanks to the Mind Science Foundation for the invitation to be part of their Speaker Series in San Antonio! Posted on this website are notes from my talk on May 7, 2007. The topic is the human visual system and how it adapts to blindness and sight recovery. -Melissa Saenz

The Human Visual Sytem

How does the brain receive light information? The retina of the eye detects light and transforms this information into an electrical signal which travels into the brain via the optic nerve. The optic nerve projects to the thalamus (lateral geniculate nucleus), the neurons of which project to the visual cortex (on the convoluted gray matter surface of the brain). The visual cortex has the enormous task of decoding the visual world and generating our vivid perception of vision. As the previous speaker in this series Susan Martinez-Conde elaborated on: our visual perception is an illusion generated by neural activity. If the visual cortex is damaged, partial to complete blindness can result.

Human Brain Response to a Moving Visual Stimulus

Functional magnetic resonance imaging (fMRI) is a non-invasive method of brain imaging that measures hemodynamic responses related to brain activity. In fMRI experiments, brain responses can be measured in response to specific stimuli presented by the experimenter. For an example of brain responses measured with fMRI, play the linked movie. The image on the left is a moving vs. stationary visual stimulus. The center image shows the brain fMRI response to this stimulus on a cross-sectional view (anatomical MRI image). The right image shows the same brain response on a surface representation (left hemispheric view, inflated the so that the convoluted surface appears smooth). Notice that brain respsones are primarily focused on the posterior part of the brain - the visual cortex. Also, notice the delay in the fMRI hemodynamic response relative to the onset of the stimulus (blood flow responses are indirect measures of brain electrical activity and are slow).

Blindness defined

Cross-modal plasticity

Given that much of the human brain is devoted to visual processing, what happens to this valuable brain real estate in people who are blind?

Remarkably, the visual cortex can take on processing of the other senses. This brain reorganization is called cross-modal plasticity, and is most pronounced in people born blind or who became blind at an early age, but has also been documented in people who became blind as adults. Modern brain imaging studies show that the visual cortex of congenitally blind and early-blind individuals responds to a variety of tactile, auditory, and verbal tasks including Braille reading (Sadato, 1995). This cross-modal plasticity is thought to contribute to enhanced non-visual abilities in people who are blind. For example, studies have documented ehanced ability in verbal memory and sound localization and have correlated the extent of these abilities with cross-modal brain responses in blind individuals (Amedi et al., Gougoux et al.)

Some blind people develop extraordianary skills in echo-location, listening to and interpreting echos produced by the tapping of a cane or self-produced vocal clicks. World Access For The Blind is a non-profit orgnanization in the Los Angeles region that teaches echo-location and other skills to enhance non-visual abilities.

Sight restoration. Can an adult learn to see?

Molyneux's Problem: In 1688, Irish scientist William Molyneux sent a letter to English philosopher John Locke asking whether a man who has been born blind and who has learnt to distinguish a sphere and a cube by touch, would be able to distinguish and name these objects by sight alone, once he had been enabled to see.

The Molyneux problem which has long fascinated scientists and philosophers is fundamentally a question of how information from the different senses is represented in the brain. Sight restoration after long-term near-complete blindness is a exceptionally rare phenomemon, however a handful a reported case histories have shed light on this question. As usual, the answer is both "yes" and "no"! The case histories typically involve cataract removal or corneal translplant (Von Senden 1932, Gregory 1963, Sacks 1995, Fine et al. 2001), and the outcomes have a number of key features in common. Sight recovery patients quickly report the ability to detect motion and color as well as some simple form perception. Richard Gregory reported that, shortly after surgery, subject S.B. could recognize by sight capital block letters that he had learned by touch when blind - thus, a partial 'Yes' to the Molyneux question. However, examples such as this are limited and are outweighed by inabilities to interpret complex visual input. Complex object and scene perception (for example face recognition) are highly impaired and remain so. For many aspects of high-level vision, it seems that a critical period for learning and development has already passed - that an adult brain is very limited in its capacity to learn to see.

S.B. never ceased to be "struck by how objects changed their shapes when he walked round them..."

This quote demonstrates the problem of object invariance - recognizing multiple views as all belonging to the same object. This aspect of object recognition seems effortless to a person with normal vision, but remains highly impaired in cases of adult sight recovery after early blindness. It seems that the brain begins to learn object invariance during infancy (think of a baby observing and rotating toys with her hands) in ways that are not completely understood. This complex problem remains a great challenge for the development of computer vision systems that could recognize objects.

Another common feature in these cases is initial euphoria followed by disappointment and disorientation with the restored vision, often leading to severe depression.

A specific case history - Michael May

Michael May of California was born with normal vision and was blinded at the age of three in a chemical accident. At the age of 45 a new development in eye surgery (corneal limbal stem-cell replacement) offered new hope for sight restoration. After much deliberation - he was well aware of the challenges and risks involved with sight restoration - the surgery was performed. See the above video clip documenting Mike May's first exciting moments with new vision as his bandages were removed post-surgery.

Mike May makes use of his partially restored vision which is within the range previously reported in adult sight recovery cases. Because of his realistic expectations and his particular enthusiasm for challenge, Mike May's experience has been remarkably positive, though certainly not easy. He states, "The difference between today and over two years ago (the time before his surgery) is that I can better guess what I am seeing. What is the same is that I am still guessing." More information about Mike May and his forthcoming book can be found on his website.

Artifical Prosthetic Retinas

Several research teams worldwide are working to develop an artificial prosthetic retina that could help people with retinal degenerative diseases such as adult-onset macular degeneration and retinitis pigmentosa. In these diseases, the light detecting photoreceptor cells of the retina are lost. However, other cells in the retina remain and can be artificially stimulated with a tiny electrical device surgically implanted on the retina, leading to the perception of light (again, vision is an illusion!)

One of the leading research teams in this pursuit is at the Doheney Eye Institute at USC and the Second Sight company. Their device uses a small camera mounted on a pair of glasses to capture visual images and transmits information to an array of stimulating electrodes surgically implanted on the retinal surface. Clinical trials are underway - a small number of blind individuals have been implanted with a 16 electrode array. While a sixteen pixel array is a mere fraction compared to the millions of photoreceptor cells on a healthy retina, the first clinical results are quite positive. Retinal implant patients report some newly gained spatial perception of light and motion. In the next round of trials, 60 electrode arrays will be implanted which could allow for a better visual image. The researchers state that the ultimate goal of the device is to achieve enough resolution to improve independent mobility and to allow reading and face recognition.

Researchers at the University of Toronto are pursuing another approach: stem-cell transplants to allow the growth of new photoreceptor cells on the retina. This research is still many years from human clinical trials.

Seeing With Sound

If you google the title of my talk you'll find a website demonstrating The vOICe - a sensory substiution device (SSD). I have no association with that device, but I am happy to share information on what sounds like a great idea. An SSD is another approach to enhancing visual interpretation for the blind by "translating" visual information to another sense. For example, the vOICe translates live camera images into "sound views" via a particular algorithm. These sound views sweep through the visual image about once per second associating horizontal location with time, vertical location with pitch, are brightness with loudness. Learning to interpret this signal might be comparable to learning a new language. By making use of the other senses, an SSD could be useful to someone who has never had vision.

Note: None of the described technologies are a "cure" for blindness. They are partial solutions which may offer some useful visual ability to some individuals.

New brain imaging experiments

More information will be posted in the future about my current functional MRI experiments on cross-modal plasticity (once they get published).