The cause was kidney failure, his son Carl said.

Dr. Hubel (pronounced HUGH-bull) and his collaborator, Dr. Torsten Wiesel, shared the 1981 Nobel in Physiology or Medicine with Roger Sperry for discovering ways that the brain processes information. Dr. Hubel and Dr. Wiesel concentrated on visual perception, initially experimenting on cats; Dr. Sperry described the functions of the brain's left and right hemispheres.

Dr. Hubel's and Dr. Wiesel's work further showed that sensory deprivation early in life can permanently alter the brain's ability to process images. Their findings led to a better understanding of how to treat certain visual birth defects.

Dr. Hubel and Dr. Wiesel collaborated for more than two decades, becoming, as they made their discoveries, one of the best-known partnerships in science.

"Their names became such a brand name that H&W rolled off the tongue as easily in the lab as A&W root beer did at lunch," Robert H. Wurtz, a neuroscientist, wrote in a review article about their work.

Before Dr. Hubel and Dr. Wiesel started their research in the 1950s, scientists had long believed that the brain functioned like a movie screen — projecting images exactly as they were received from the eye. Dr. Hubel and Dr. Wiesel showed that the brain behaves more like a microprocessor, deconstructing and then reassembling details of an image to create a visual scene.

By measuring the electrical impulses of cells in the visual cortex, the scientists discovered that cells respond to straight lines, movement and contrast — features that delineate objects in the environment. They further found that some cells fire rapidly in response to horizontal lines, while other cells prefer vertical lines or angles. Cells with similar functions are organized into columns, they said, tiny computational machines that relay information to a higher region of the brain, where a visual image is formed.

Dr. Hubel and Dr. Wiesel also found that vision does not develop normally if the brain fails to make connections with the eye during a critical window early in life.

In a 1962 experiment, the scientists showed that kittens that had one eye sewn shut after birth became blind in that eye because their brains were deprived of visual stimulation. In a related experiment, the scientists showed that exposure to light did not by itself provide enough stimulation; it was necessary for newborn animals to see the patterns and contours of the world around them.

Doctors now operate on infants born with cataracts early in life to prevent vision loss; before Dr. Hubel's and Dr. Wiesel's research, doctors removed cataracts from infants between ages 6 months and 24 months, with poor results. The research also led to earlier treatment for strabismus, a condition in which the eyes are not properly aligned.

Scientists have since found evidence of similar "critical periods" in hearing and language acquisition.

"David and Torsten did more than open up the study of the primary visual cortex; they laid the basis of all that was to follow in the sensory systems," Dr. Eric R. Kandel, a Nobel laureate, wrote in a recent commentary about their research. "Together this body of work stands as one of the great biological achievements of the 20th century."

David Hunter Hubel was born in Windsor, Ontario, on Feb. 27, 1926, to American parents. His father was a chemical engineer. Dr. Hubel grew up in Montreal, where his boyhood hobbies included chemistry. He once rocked his neighborhood by firing a small cannon in the middle of his street; in another experiment, he launched a hydrogen balloon that was found by a farmer's daughter 100 miles away.

He went to McGill University, where he studied math and physics because he preferred solving problems to memorizing facts. But after earning his bachelor's degree in 1947, he decided against a career in mathematics. "To make it in that field required a virtuosity — like becoming a concert pianist — that I probably lacked," he told an interviewer.