Speech-like cerebral activity in profoundly deaf people processing signed languages: Implications for the neural basis of human language

TLDR

Language processing is primarily localized in the left hemisphere, but the reason for this lateralization remains unclear. The study tests whether language‑processing neural tissue is modality‑independent and instead specialized for linguistic patterning. PET revealed that both speech‑assumed regions—the left inferior frontal cortex and the planum temporale—are activated during sign language, indicating that these areas process abstract linguistic features rather than modality‑specific sounds, and that such specialization is consistent across modalities while expressive and perceptual pathways remain flexible.

Abstract

For more than a century we have understood that our brain's left hemisphere is the primary site for processing language, yet why this is so has remained more elusive. Using positron emission tomography, we report cerebral blood flow activity in profoundly deaf signers processing specific aspects of sign language in key brain sites widely assumed to be unimodal speech or sound processing areas: the left inferior frontal cortex when signers produced meaningful signs, and the planum temporale bilaterally when they viewed signs or meaningless parts of signs (sign-phonetic and syllabic units). Contrary to prevailing wisdom, the planum temporale may not be exclusively dedicated to processing speech sounds, but may be specialized for processing more abstract properties essential to language that can engage multiple modalities. We hypothesize that the neural tissue involved in language processing may not be prespecified exclusively by sensory modality (such as sound) but may entail polymodal neural tissue that has evolved unique sensitivity to aspects of the patterning of natural language. Such neural specialization for aspects of language patterning appears to be neurally unmodifiable in so far as languages with radically different sensory modalities such as speech and sign are processed at similar brain sites, while, at the same time, the neural pathways for expressing and perceiving natural language appear to be neurally highly modifiable.