Ursula Bellugi, a pioneer in the study of the biological basis of language and among the first to demonstrate that sign language was just as complex, abstract and systematic as spoken language, died Sunday in San Diego. She was 91 years old.

His death, in an assisted living facility, was confirmed by his son Rob Klima.

Dr. Bellugi was a top researcher at the Salk Institute for Biological Studies in San Diego for nearly five decades, and for much of that time served as director of its cognitive neuroscience laboratory. She has made significant contributions in three main areas: language development in children; the linguistic structure and neurological basis of American Sign Language; and the social behavior and language skills of people with a rare genetic disease, Williams syndrome.

“She leaves an indelible legacy in shedding light on how humans communicate and socialize with each other,” Rusty Gage, president of the Salk Institute, said in a statement.

Dr. Bellugi’s work, largely done in collaboration with her husband, Edward S. Klima, has advanced understanding of the brain and the origins of language, both signed and spoken.

American Sign Language was first described as a real language in 1960 by William C. Stokoe Jr., a professor at Gallaudet University, the only liberal arts college in the world devoted to deaf people. But he was ridiculed and attacked for this claim.

Dr Bellugi and Dr Klima, who died in 2008, demonstrated conclusively that the world’s sign languages ​​- there are over 100 of them – are real languages ​​in their own right, not just translations of spoken languages.

Dr. Bellugi, who focused on American Sign Language, established that these language systems have been passed down, in all their complexity, from one generation of deaf people to the next. For this reason, the scientific community considers her the founder of the neurobiology of American Sign Language.

The pair’s work led to a major discovery at the Salk lab: that the left hemisphere of the brain has an innate predisposition for language, whether spoken or signed. This discovery has given scientists new insight into how the brain learns, interprets and forgets language.

“This was a pivotal finding for deaf people because it verified that our language is treated equally by the brain – just as we should be treated equally by society,” said Roberta J. Cordano, president of Gallaudet, in a press release.

Until then, sign languages ​​were viewed with contempt as either crude, no-rules pantomime or broken English, and deaf children were discouraged from learning to sign. The couple’s work contributed to wider acceptance of ASL as the language of instruction and helped empower deaf people as the Deaf Pride movement grew in the 1980s.

Another topic studied by Dr. Bellugi and her husband was Williams syndrome. She sought to understand how the disorder, in which a set of about 20 genes are missing from one copy of a chromosome, altered the brain and ultimately shaped behavior.

All of his work, the Salk Institute said in a profile by Dr. Bellugi, “has helped paint a picture of the biology humans use to interact with the world around us.”

Ursula Herzberger was born on February 21, 1931 in Jena, central Germany, a center of science and technology. With Hitler on the rise, his family fled Germany in 1934 and eventually settled in Rochester, NY. There his father, Max Herzberger, mathematician and physicist, became head of the optical research laboratories of Eastman Kodak, a job organized for him by Albert Einstein, his friend and former professor in Berlin.

Mr. Herzberger then developed a special lens that solved the color distortion in the glass. Ursula’s mother, Edith (Kaufmann) Herzberger, was an artist.

Ursula attended Antioch College in Ohio, where she majored in psychology and graduated in 1952. She married Piero Bellugi, Italian composer and conductor, in 1953; they had two sons before divorcing in 1959.

Interested in psychology and language, she moved to Cambridge, Mass., where she became a research assistant for Roger Brown, a leading psychologist at Harvard and the Massachusetts Institute of Technology, who studied how young children acquire language. Soon she was studying at Harvard, where she earned a doctorate in education in 1967 while raising her sons as a single mother. She also took classes at MIT, where one of her professors was Dr. Klima.

When they got married, she changed her name legally to Bellugi-Klima but continued to use Bellugi professionally. They moved west when he started teaching at the University of California, San Diego. She started in 1968 at the Salk Institute, a 10-minute walk from her husband’s campus, where she also taught. She then taught at San Diego State University.

At the time, San Diego was a hotbed of linguistic research, largely revolving around Dr. Bellugi and Dr. Klima, as well as colleagues from Harvard and MIT. She drew a parade of research assistants and made a point of hiring many who were deaf.

Over the years, Dr. Bellugi has received numerous awards. She was elected to the National Academy of Sciences in 2007. She retired from Salk in 2017 at age 86.

She has co-authored hundreds of articles and several books, some with her husband. Their best-known book was “The Signs of Language” (1979), written with 10 associates. It was the first comprehensive study of the grammar and psychology of sign languages ​​and was hailed by the Association of American Publishers as “the most outstanding book of the year in the behavioral sciences”.

In addition to his son Rob, Dr. Bellugi is survived by his sister, Ruth Rosenberg; his brother, Hans Herzberger; four grandchildren; and five great-grandchildren. Another son, David Bellugi, died in 2017.