Part X: H. Keffer Hartline, 1967 Prize in Physiology or Medicine
By Joseph Luna
While strolling along a beach one day in the summer of 1926, a young physiologist named Haldan Keffer Hartline came across a living fossil. Before him was a horseshoe crab, Limulus polyphemus, with its domed carapace shell, spiked rudder tail and pedipalp legs. Barely changed after over 450 million years of evolution, this mysterious ancient mariner must’ve been a startling and alien sight. We don’t know what Hartline thought of the creature’s primitive book gills, its belly filled with shellfish or its eerie blue blood. But something did enthrall him: the crab’s large compound eyes.
Though he was a medical student, Hartline had no interest in practicing medicine, but was fascinated by research, particularly the physiology of vision. How does seeing work? This question first riveted Hartline while an undergraduate, where he worked on the light-sensing abilities of pill bugs. Moving on to medical school at Johns Hopkins, Hartline attempted to study vision in frogs by using neurophysiological instruments to record activity from their optic nerves, but it proved more difficult and complex than he imagined. What he needed was a simpler model organism, if there was one. He made his way to the Marine Biological Laboratory on the southern coast of Massachusetts, frustrated by past failures, but on a mission to find the right organism to study.
It was a conceptual leap to propose that studying vision in a weird creature like Limulus would yield insight on how animals, including humans, see generally, but the idea wasn’t out of place among biologists in the 1920s. By decade’s end, the Nobel Prize winning Danish physiologist August Krogh laid the case for studying diverse organisms for general biological insight, predicting for the field in 1929: “for such a large number of problems there will be some animal of choice or a few such animals on which it can be most conveniently studied.”
The year before, Hartline published a descriptive study of arthropod compound eyes, where he succeeded in recording nerve impulses after light stimulation in Limulus along with grasshoppers and two species of butterfly. This comparative work revealed that light stimulation could induce characteristic minute electrical spikes that could be measured among arthropods. And whereas the grasshopper and butterfly were difficult to handle and gave complex recordings, those of Limulus were simple waves and could be studied for extended periods of time when bathed in seawater. But what really set Limulus apart was the size of its compound eye as it opened the possibility of studying its single facets.
As the name suggests, a compound eye can be thought of as a closely spaced array of simpler eyes. Each “eye”, called an ommatidium, individually acts as a receptor for light directly above it and is composed of a cornea that directs light to a bundle of photoreceptor cells that are in turn connected to a single optic nerve. In small insect eyes, individual ommatidia number in the thousands and can really only be seen under a microscope; the same is true for analogous rods and cones in vertebrate retinas. The ommatidia of Limulus by comparison are fewer in number but comparatively gargantuan: each is about 1mm across, making them among the largest light receptors in the animal kingdom. Based on their large size, Hartline reasoned that it might be possible to take neurophysiological measurements from single optic nerve fibers in the horseshoe crab. Working with Clarence Graham in the summer of 1931, Hartline succeeded in doing just that. Graham and Hartline dissected single ommatidia, and devised methods to illuminate their photoreceptive cells while recording from the optic nerve. In went light they could control, out went neural signals to the brain that they could measure. These were some of the first measurements of the most fundamental unit of vision.
In the course of this foundational work, Graham and Hartline made many observations that later came to underlie a basic neural code for vision. They found that the firing rate of the optic nerve from single ommatidia varied according to the duration of light and its intensity: under intense light the nerve fired rapidly, under low light it fired less frequently. They also witnessed a form of light and dark adaptation, whereby sudden illumination caused rapid nerve firing that subsided despite no change in light intensity. All of these results helped define how light was predictably converted by photoreceptor cells into electrical signals that the brain could understand.
But it didn’t end there. Since the late 1930s, Hartline had begun to notice that shining light on one area of the crab’s eye would occasionally have the effect of decreasing the firing rate in the area he was recording. It was a strange observation, not really followed up, but it percolated in Hartline’s mind for nearly two decades until the right instrumentation came in the 1950s. Now at Rockefeller, Hartline, working with Henry Wagner and Floyd Ratliff, recorded from two single adjacent ommatidia but only illuminated one of them. They observed that the rate of firing on the ommatidium not being illuminated was inhibited by its illuminated neighbor. This “lateral inhibition” as they named it, formed the mechanistic basis of contrast determination, that is, how eyes are able to distinguish borders. But more generally, it pointed to an astonishing realization: far from passively transforming light to electrical signals, the interconnected ommatidia in the Limulus eye were actively involved in computing. These sense organs in crabs weren’t just passing on the “raw data” to the brain where it could be processed into an image, but were actually doing the first steps of data processing. Eyes could think.
By the time he visited Stockholm in 1967 for discovering lateral inhibition, H. Keffer Hartline had pioneered micro-dissection and instrumentation techniques to study individual photoreceptor cells, worked out a series of equations that formed the first mathematical description of a neural network, and brought the first computer into neurophysiological research (and to the RU campus, a CDC 160A). The tools had certainly changed, but the ancient muse, the lowly horseshoe crab, remained the same.