1967 Nobel Prize in Physiology or Medicine — Granit, Hartline, and Wald: How Does the Eye Translate Light into Nerve Signals?

What You’ll Learn in This Article

Starting from the 1930s, when the anatomy of the camera-type eye was already known, but it was still unclear how the rod and cone cells within it translate light into nerve signals, this article explains the chemical and electrophysiological mechanisms of visual perception, which were elucidated by Granit at the Karolinska Institute, Hartline at Rockefeller Institute, and Wald at Harvard. It also covers Wald’s story as a visual researcher and anti-Vietnam War activist, and how he ended up on President Nixon's "enemies list."

Something Beyond Common Sense — We Know the Anatomy of the Eye, but Not What’s Inside

Let's map out the state of visual research in the 1930s. The structure of the eye in humans and other higher vertebrates was already known in exhaustive anatomical detail by the 1930s. Our eyes are the so-called camera-type eye — light passes through the pupil, is refracted by the lens, and lands on the retina at the back of the eyeball. It was likewise already established that the retina is densely lined with rod-shaped light-detecting cells and pointed cone cells, and that these cells are connected to the optic nerve. What remained unsolved was the microscopic mechanism — how light actually stimulates the optic nerve, and how we distinguish colors.

This summary is crucial. While the hardware structure was known, the signal transduction process inside was still a mystery. The fact that there were physical lenses and sensory cells had already been observed, but the microscopic principles of how photons become electrical signals in cells and how colors are distinguished were all unknown.

In the language of computer science, this problem is reverse engineering the sensor hardware stack. We can see the external case and lens of the camera, but we need to understand how each layer of the pipeline, from photons → photosensor → analog signal → digital signal → processor, works. The three laureates each tackled one layer of this pipeline.

The joint-award citation for these three scientists, compressed into a single sentence, reads like this — they used direct measurement to reveal the chemical and electrical changes that occur in the retina and optic nerve when light enters an animal's eyes, thereby explaining the entire process by which animal vision is formed.

The Zeitgeist — The Cold War and Social Transformation

1967 was a year when the Cold War and social transformation were happening simultaneously.

In world history, the Six-Day War lasted for six days, with Israel achieving a decisive victory against Egypt, Syria, and Jordan, and occupying the Sinai Peninsula, the Golan Heights, the West Bank of the Jordan River, and the Gaza Strip. This marked the beginning of a fundamental reshaping of the political landscape of the Middle East. In December, Christian Barnard performed the world's first heart transplant in Cape Town, South Africa, a pivotal milestone in modern transplant surgery. In October, Che Guevara was killed by Bolivian government forces, with the support of the U.S. CIA in Bolivia. In July, racial riots broke out in Detroit, USA.

In Korean history, the June 8 general election was held, which intensified the controversy over the Park Chung-hee regime's fraudulent election. This later led to the Third Republic and subsequent political developments.

In this tumultuous year, the Nobel Committee recognized visual research. It was the year when the mechanism by which the eye perceives the world in the midst of global conflict was revealed.

The Three Laureates — Karolinska, Rockefeller, and Harvard

Ragnar A. Granit (1900-1991) was a Swedish physiologist with an interesting background. He was born in Helsinki, Finland, but held Swedish citizenship. After studying medicine at the University of Helsinki, he pursued further study at the University of Pennsylvania in the United States and the University of Oxford in the United Kingdom, and then took up a professorship of physiology at the University of Helsinki in 1937. When Finland was invaded by the Soviet Union in 1940, he crossed into his home country of Sweden, took up a professorship at the Karolinska Institute, and continued his research there until his retirement in 1967.

The Soviet invasion of Finland (Winter War of 1939) profoundly affected his life. He moved from Helsinki to Stockholm and served at the Karolinska (Sweden's leading medical research institute) for 27 years. His later interests branched out in several directions. Late in his career he became interested in the mechanism of muscle movement, studying the muscle spindle — the sensory organ that detects the degree of muscle contraction and relaxation — and the role of tendons. From there he turned to the function of the spinal cord and the study of pain, working across a broad range of fields. He was also an intellectual with a serious interest in the philosophy of science.

The key to his visual research was measuring the electrical responses of the retina. He recorded the electrical responses of individual cone cells with fine microelectrodes, thereby demonstrating the trichromatic theory of color vision (red/green/blue cone cells) and elucidating how the pattern of nerve firing encodes visual information.

Haldan K. Hartline (1903-1983) was an American physiologist. He obtained his Ph.D. in Medicine from Johns Hopkins University (1927), served as a professor at Johns Hopkins University (1949-1953), and then moved to Rockefeller University (1953-1983), where he served for 30 years.

Hartline’s key contribution was the discovery of lateral inhibition. The principle that adjacent cells in the retina inhibit each other, thereby naturally performing edge detection. This is the biological basis for Sobel filters and Laplacian filters in today's image processing. Hartline demonstrated this principle by measuring the compound eye of the horseshoe crab (Limulus), which has each lens connected to an independent cell, making it a natural experimental material.

George Wald (1906-1997) was an American physiologist. He obtained his Ph.D. from Columbia University (1932) and then served as a lecturer and professor at Harvard University (1934-1977). Of the three laureates, he was by far the most publicly visible — with a life outside academia every bit as substantial as his life inside it.

While at Harvard, alongside his brilliant work in vision, Wald actively led a broad range of political, social, and peace movements — the anti-Vietnam War movement, the global human rights movement, and the anti-nuclear proliferation movement. His activism eventually drew attention from the White House, and he ended up on President Nixon's "enemies list." After his retirement in 1977, as a Harvard emeritus, he remained actively involved in international human rights disputes, and for a time even served as the deputy director of the International Human Rights Tribunal, headquartered in Rome, Italy.

Wald’s key academic contribution was elucidating the chemical mechanism of vision. He showed that retinal, a vitamin A derivative, undergoes photoisomerization (cis → trans) when it absorbs photons. This discovery marked the beginning of visual chemistry.

Key Discoveries — The Three-Layer Pipeline

If we organize the discoveries of the three people in terms of a pipeline, it would look like this.

Layer 1: Photochemistry (Wald) — What happens when photons enter the cells?

• Rod cells contain rhodopsin, a light-sensitive protein, and retinal, a vitamin A derivative, is bound to it.
• When one photon arrives, retinal undergoes isomerization (11-cis → all-trans). This shape change induces a change in the shape of the rhodopsin protein.
• This is the beginning of vision — the moment when photon energy is converted into a chemical reaction.

Layer 2: Electrical Signal Generation (Granit and Hartline) — How does the chemical reaction become an electrical signal?

• The change in the shape of rhodopsin decreases the concentration of cGMP, a signaling molecule, inside the cell.
• As cGMP decreases, the Na+ channels in the cell membrane close, reversing the internal potential of the cell, and hyperpolarization occurs.
• This potential change is the beginning of the visual signal. Surprisingly, visual cells inhibit rather than activate when exposed to light — this is a peculiar case of information processing that was later confirmed.

Layer 3: Neural Processing (Hartline) — How does the signal from a single cell become image information?

• The signals from multiple visual cells are transmitted to other cells in the retina (bipolar cells, ganglion cells), where processing such as lateral inhibition takes place.
• When a cell is exposed to light, it inhibits the signals from adjacent cells. This enhances brightness contrast and edges.
• This signal is transmitted to the brain via the optic nerve.

Color processing — Several types of cone cells (three types sensitive to red, green, and blue) each detect different wavelengths of light. The brain distinguishes colors by the firing rate of these three cells. Granit established this trichromatic principle through electrical measurements.

CS Framework of the Sensor Hardware Stack

Now, let’s organize the three-layer visual pipeline in the language of computer science.

The image sensor hardware stack consists of the following layers:

• Layer 1: Photodiode/Photodetector — Photons strike the silicon atoms, exciting electrons. The physical reaction of photons → electrons conversion.
• Layer 2: Analog Front-End — Amplifies and filters the tiny electron currents, converting them into analog voltage signals.
• Layer 3: ADC and ISP — Converts analog signals to digital, and the Image Signal Processor (ISP) performs automatic processing such as noise reduction, edge enhancement, color correction, and white balance.

The retina is exactly this pipeline.

• Layer 1: Photochemistry — Photons → Retinal isomerization (Wald’s discovery)
• Layer 2: Electrical Signal Generation — Chemical reaction → Cell membrane potential change (Granit and Hartline’s measurements)
• Layer 3: Neural Processing — Edge detection by lateral inhibition, color decomposition into three channels (Hartline and Granit’s discoveries)

Why this layered structure? If all the processing were done in a single layer, that layer would be too complex. By dividing it into layers, each layer can be composed of optimized components, and the interface between layers can be standardized. Separation of concerns is also found in evolution.

Lateral inhibition = Convolution filter. Hartline’s discovery of lateral inhibition is one of the biological roots of today’s convolutional neural networks (CNNs). The principle that the activation at each location is regulated by the activation at neighboring locations is a fundamental image processing operation. The reason why CNNs in deep learning are good at visual processing is because they approximate this biological principle.

Limitations of the analogy: Of course, the retina’s three-layer pipeline is much more complex than the camera’s hardware pipeline. There are several sub-layers within each layer, and there are also feedback loops in addition to feedforward. However, the fundamental layered structure of “light → chemistry → electricity → neural processing” is strikingly similar.

The Legacy That Continues Today

The door opened by this research can be summarized this way. The three researchers' results provided decisive clues as to how animal vision is formed and how colors are distinguished. This framework was subsequently extended from vision itself to become the standard approach across sensory research, and in the clinic it became the root of the development of ophthalmology.

This backbone leads to the following today:

• Understanding and treatment of eye diseases: The causes of color blindness, night blindness, and retinitis pigmentosa are specifically defects in each layer. Genetic testing and targeted gene therapy are derived from this understanding.
• Artificial retina: For patients with retinal damage due to retinitis pigmentosa, an artificial light-sensitive device is implanted to restore partial vision. Argus II has been clinically approved.
• Diagnosis and treatment of vitamin A deficiency: By understanding that the lack of retinal in rhodopsin is the cause of night blindness, nutritional intervention is possible.
• Computer vision and deep learning: The principles of lateral inhibition and edge detection are the roots of image processing algorithms. The convolutional layer of CNNs is an artificial reproduction of this principle.
• Optogenetics: A technology that expresses light-responsive proteins (e.g., channelrhodopsin) in specific neurons, allowing neurons to be turned on and off with light. This is derived from the understanding of visual photochemistry.

Why It Matters

What the three people left behind is the empirical demonstration that complex signal transduction can be understood with a layered structure.

When we synthesize what they each discovered, we can draw the entire pipeline from photons to brain activity. The details of each layer have been refined over the past half-century, but the layered structure itself has become the standard framework since their discovery.

This principle is today the standard approach in neuroscience — breaking down any sensation or behavior into a layered structure and understanding the detailed mechanisms of each layer. Hearing (1961, Beckesy), smell and taste, pain and touch are all interpreted using this approach.

And Wald’s political involvement is a special human story in this context. A scientist who has reached the pinnacle of academia has dedicated his time and reputation to political and social movements, such as opposing the Vietnam War, promoting human rights, and opposing nuclear proliferation. A Nobel laureate on Nixon’s list of enemies — he remains a representative example of social engagement in American academia in the late 20th century.

1967 Granit, Hartline, and Wald Summary: Granit elucidated the trichromatic cone cells at the Karolinska, Hartline elucidated lateral inhibition at Rockefeller, and Wald elucidated rhodopsin-retinal photochemistry at Harvard. The three-layer pipeline of vision (photochemistry, electrical signal, neural processing) was established. This is the theoretical root of today’s ophthalmology, artificial retina, and computer vision CNNs.

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