What Do Studies of Signed Languages Reveal?
Natural signed languages are a powerful research tool for exploring the underlying basis for the neuroanatomical organization of language in all human brains. Signed languages are real languages that have evolved in the absence of sound, a situation that caused researchers to ask where will signed languages be lateralized: Will they be housed in the left hemisphere because they are real languages? Or, will they be housed in the right hemisphere because they are visual-spatial in nature?
Pioneering neuropsychological studies by Ursula Bellugi (The Salk Institute, La Jolla, California) and her colleagues have revealed that, like spoken languages, signed languages are processed in the left hemisphere, with a handful of recent brain imaging studies corroborating this fundamental observation and also identifying some right hemisphere involvement.
Petitto and her colleagues have moved beyond the fact that signed languages are processed largely in the left hemisphere, to using them as a research tool to understand why this is so in all humans. By examining highly specific cerebral tissue during the processing of highly specific aspects of natural language in both signed and spoken languages, fundamental hypotheses about the neural basis underlying the cerebral organization of all human language have been tested: If specific language functions are processed at specific brain sites because these sites are uniquely dedicated to the processing of sound, then these parts of language in signed languages should utilize different brain tissue in deaf signers — certainly brain tissue different from those classically linked to speech.
On the other hand, if the human brain possesses specialization for aspects of the patterning of natural language, then deaf people processing the same parts of language should engage similar tissue as that which is seen in hearing speakers. This is precisely what Petitto and her colleagues have discovered. In turn, these findings have caused a re-conceptualization about the nature of human language and what makes human language special. The existence of naturally-evolved signed languages has given us a most profound insight. They have taught us that speech is not what makes human language special. Instead, the specific patterns that underlie all human language –be it language on the hands in signed languages or language on the tongue in spoken languages– is indeed the essential, core property of human language that is most “special,” and that makes human language “special” from other forms of animal communication.