The tilt of a wrist, the placement of a palm, or the wiggle of a finger can animate and add meaning to spoken language. For those who cannot  hear, these motions—soundless but not wordless—replace spoken language altogether. Chicago linguist Diane Brentari, PhD’90, studies sign language and its relation to spoken language, arguing that the two modes of speech are not as separate as they often seem. In fact, she says, sign languages offer insights into human language as a whole.

After more than 20 years away from campus, Brentari joined Chicago’s faculty last fall to lead the linguistics department’s new Sign Language Linguistics Laboratory. There she and a team of graduate students study the differences and similiarities among sign languages and between sign and spoken language. Among her areas of research—at Chicago and during 15 years as a scholar and professor at Purdue University—is a concept not always applied to sign language: phonology.

Traditionally, phonology is defined as the system of sounds produced by a speaker to form words. Brentari argues that the vocal apparatus—tongue, vocal chords, palate—is only the instrument, not the inventor. Extending the study of phonology to sign language, Brentari, who in 1998 published A Prosodic Model of Sign Language Phonology (MIT Press), says that subtle variations in hand shape, arm position, and hand and body movements are akin to phonological tools such as intonation, emphasis, and accent, which in spoken language can change or amplify meaning.

For instance, she says, a slight change in thumb position, perhaps barely discernible to the nonsigner, can mean the difference between the signs “car” and “which.” In addition, larger or faster movements can draw attention to particular words or signs in a sentence, much like a raised pitch or loud volume can in speech. These expressive processes, she argues, originate in the brain; they are not wholly determined by the mode of physical articulation. Sign language, says Brentari, who has a deaf cousin and learned signing when she was young, is “a great window on the mind.”

Over the past six years, much of her research has explored how sign language evolves in real time, including a study titled “When Does a System Become Phonological? Handshape production in gesturers, signers, and homesigners.” Coauthored with psychology professor Susan Goldin-­Meadow and two others, it was published in the February Natural Language and Linguistic Theory. Examining the hand motions of hearing people, sign-language users, and “home signers”—deaf people who, without access to formal sign language, have developed their own sign systems—Brentari found “a distinct break,” she says, between gesture and the other two. “Sign language and home sign are not just on a continuum with gesture.” Signers, for instance, tend to use more intricate hand shapes than do hearing people to describe the shape of an object and more economical ones to demonstrate human manipulation of the object. And almost all sign languages employ classifiers: signs that identify the physical properties of an object, its orientation in space, and, possibly, its relationship to other objects.

A few spoken languages use similar kinds of classifiers—by, say, placing a prefix in front of an object noun to indicate the specific shape of the object—but the classifiers that signers use are more elaborate. For signers, the noun, verb, and even an adjective can be expressed in one motion.

Even the elements that distinguish sign from spoken languages point to sign languages’ inclination toward consistency and structure. “The signers and home signers are actually working with a linguistic system, but gesturers are not,” Brentari says. “That kind of system shows that there is a higher level of organization in sign than in gesture. We can compare them to young languages, pidgins and creoles.”

This insight, that home signers try to build complex systems of meaning and form, speaks to a basic human impulse toward rationalized, multilayered communication. That tendency precedes any kind of deliberate linguistic formalization. By showing that home signing is much closer to formal sign language than to gesture, Brentari’s work points to the ways in which people naturally seek to create language systems, even  when they are alienated from formal languages.

And in revealing the brain’s ability to adapt to the body’s linguistic limitations, Brentari says, sign languages can provide insight into the biological basis of language. “Mapping the path from gesture to home sign to sign language has become an important research topic as a window on language creation more generally,” Brentari says. “We don’t have access to those precise moments of language creation for speech, but we do for sign.”

Studying sign-language evolution also can help establish commonalities among all languages. “We can begin to understand what language universals truly are,” Brentari says, “if we can find the same phenomena in spoken and sign languages.”

One overarching mission of Brentari’s career has been to uncover those language universals, to rescue the study of sign language from the ghetto of exceptionalism. Helen Keller famously said that “blindness separates us from things, but deafness separates us from people.” Brentari’s research tries to make that statement less true. In the coming months, collaborating with Goldin-Meadow and linguistics professor Anastasia Giannakidou, she will launch the Center for the Study of Gesture, Sign, and Language at Chicago. Among the center’s aims will be to raise the profile of sign-language research. Despite attention by scholars in a broad range of fields, the study of sign languages began only half a century ago, so it has joined the sub-fields of linguistics relatively recently. But “the academy,” Brentari says, “is changing.”