All About the Special Senses

Ever since people have wondered about where their thoughts came from, they have tried to understand the human senses. Much was learned from observing the results of head injuries, as well as by dissecting postmortem human brains and the brains of animals.

In the 1930s and 1940s, scientists applied electrodes to the surface of the brain or placed them on the skull of humans to study "evoked responses," the changing rhythms of electrical signals in the brain in response to specific stimuli such as light or sound. Unfortunately, these signals from billions of brain cells proved almost impossible to unscramble.

When extremely thin microelectrodes became available in the late 1950s, researchers implanted them into the brains of living animals to spy on the activity of individual cells. Sharp popping sounds could be heard as specific neurons fired, and the scientists tried to find out what provoked these electrical discharges.

This is how David Hubel and Torsten Wiesel, who were then at Johns Hopkins University, began the groundbreaking experiments on the visual cortex of cats and monkeys for which they later won a Nobel prize.

They discovered that one neuron in the primary visual cortex at the back of a cat's brain might fire only when the animal's eye was exposed to a bright line at a particular location and angle, while another next to it would fire only in response to a line at a slightly different location and angle. No one had suspected that these neurons would dissect a scene—and respond to particular elements of it—with such amazing specificity. Hubel and Wiesel's success led to a general focus on the abilities of single neurons, especially in the visual system.

The past decade has seen an explosion of research on all the senses, partly because of the new tools supplied by molecular biology. Scientists can now focus ever more precisely on the work of sensory neurons, down to the level of specific genes and proteins within these neurons.

This publication will describe some recent research on three of our senses—vision, hearing, and smell—in which there have been particularly interesting developments. It shows how the eye sees, how the ear hears, and how the nose smells.

The visual system, whose activity involves roughly a quarter of the cells in the human cerebral cortex, has attracted more research than all the other sensory systems combined. It is also the most accessible of our senses.

The retina, a sheet of neurons at the back of the eye that any physician can see through an ophthalmoscope, is the only part of the brain that is visible from outside the skull. Research on the visual system has taught scientists much of what they know about the brain, and it remains at the forefront of progress in the neurosciences.

Research on hearing is also gathering momentum. One group of scientists recently discovered how receptor neurons in the ear—the so called "hair cells"—respond to sounds. Another group explored how animals use sounds to compute an object's location in space. This may be a model of similar operations in the auditory system of humans.

The olfactory system, which was an almost total mystery until a few years ago, has become the source of much excitement. The receptor proteins that make the first contact with odorant molecules have been identified with the help of molecular genetics, and researchers are beginning to examine how information about smells is coded in the brain.

The use of molecular biology has enabled scientists to discover just how receptor neurons respond to light, to vibrations in the air, to odorant molecules, or to other stimuli.

The receptor neurons in each sensory system deal with different kinds of energy—electromagnetic, mechanical, or chemical. They look different from one another, and they exhibit different receptor proteins. But they all do the same job: converting a stimulus from the environment into an electrochemical nerve impulse, which is the common language of the brain.

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