The structure of our visual system, beginning at the eyes and ending at the primary visual cortex at the back of the brain, is a little like a maze, intricately constructed to send visual signals through myriad portals and passageways to reach just the right neurons at the end of the path. In the 1950s, H. Keffer Hartline, a member of The Rockefeller Institute for Medical Research, charted the first avenues of that maze when he revealed how the visual stimulus received by the retina is divided, altered and sharpened by the optic nerve network in order to send a more useful picture to the brain. Former Rockefeller president Torsten N. Wiesel, along with his colleague David H. Hubel, continued Dr. Hartline’s exploration at their Harvard Medical School laboratory by delving further back, into the brain, and described for the first time how the system develops innately, how experience shapes it further and how it analyzes visual signals. For this work, Drs. Wiesel and Hubel shared the 1981 Nobel Prize in Physiology or Medicine.
The complex array of stimuli in our visual field passes first through several distinct layers of cells known collectively as the retina. Next they are analyzed by the optic nerve and make their way to the lateral geniculate nucleus (LGN), the first visual processing center in the brain, located in the thalamus of each brain hemisphere. From the LGN, the signals are sent to the primary visual cortex, also known as the striate cortex. Working with cats and rhesus macaque monkeys, Drs. Wiesel and Hubel recorded the electrical impulses of cortical cells in response to various patterns flashed before the eyes. They coined the terms “simple” and “complex” for cells that respond to only one type of stimulus and those that respond to multiple and opposite stimuli.
To understand the differentiation, the scientists conducted a series of experiments to observe the brain’s response when one eye is kept closed for different periods of time. They discovered that animals with one eye closed for the first three months of life become blind in that eye. Examinations revealed no change in the eye itself or in the retina; the LGN cells devoted to that eye had shrunken but still responded to stimulation of the deprived eye as efficiently as those for the normal eye. The difference, they concluded, must therefore be in the striate cortex.
The striate cortex is composed of six distinct but interconnected layers of neurons. Using autoradiography, Drs. Wiesel and Hubel traced neuronal connections and discovered that the signals from the two eyes remain separated on their path from the LGN to the striate cortex, but then branch out into terminals, creating a pattern of alternating columns along the grid of cortical cells; the columns alternate between left-eye preference and right-eye preference — what the two men called ocular dominance columns. Recording the responses of cortical cells to stimulation of both the occluded and normal eyes, they then found that the columns linked to the normal eye were expanded beyond their normal size, their geniculate terminals taking over much of the cortical space innately assigned to the deprived eye. Furthermore, the shrinking of the deprived LGN cells, though functionally insignificant at the level of the LGN, was proportional to the narrowing of the corresponding ocular dominance columns.
In subsequent studies, Drs. Wiesel and Hubel determined that there is a critical period during which this plasticity exists in the visual system. In the first six weeks of the macaque’s life, for example, a few days of monocular deprivation is sufficient to produce a severe shift in ocular preference. After that period, the animal’s susceptibility to deprivation begins to decline, disappearing entirely by two years. If deprivation lasts for only part of the critical period, vision is often restored to the deprived eye over time.
The primary visual cortex is now the best-understood part of the brain, largely due to Drs. Wiesel and Hubel, and the implications of their discoveries are far-reaching. They helped to establish the concept of cortical plasticity — that even innate brain functions can be shaped by experience. Clinically, their findings informed better treatment of childhood eye disorders. And in the field of computer science, their neural maps led to the development of scale-invariant feature transform, a computer vision algorithm used for object recognition, robotic mapping and navigation, three-dimensional modeling and image stitching, among other applications. Drs. Wiesel and Hubel shared the 1981 prize with Roger W. Sperry of the California Institute of Technology.
Born in 1924 in Uppsala, Sweden, Dr. Wiesel received his medical degree from the Karolinska Institute in Stockholm in 1954, after which he taught at the institute and worked at the Karolinska Hospital. He began a fellowship in ophthalmology at The Johns Hopkins University Medical School in 1955 and became an assistant professor there in 1958. In 1959 he joined the faculty of Harvard Medical School, where he remained for 24 years and eventually chaired its department of neurobiology. Dr. Wiesel came to Rockefeller in 1983 as Vincent and Brooke Astor Professor and head of the Laboratory of Neurobiology. He was president of Rockefeller from 1991 to 1998. In 2005, he received the National Medal of Science.
Since retiring, Dr. Wiesel has turned his attention to international science and human rights advocacy. From 2000 to 2009, he served as secretary general of the Human Frontier Science Program. He has served has chairman of the board of governors of the New York Academy of Sciences, chair of the scientific advisory committee of the Pew Scholars Program and chair of the committee on human rights of the National Academy of Sciences. He is a founding member of the Israeli-Palestinian Science Organization and of the International Human Rights Network of Academies and Scholarly Societies.