Biological motion perception in deaf signers and hearing non-signers

The ability to discriminate human actions from other types of motion in the visual environment is especially vital to deaf individuals who use a visual-manual language as their primary form of communication. Signers must be sensitive both to non-linguistic human actions and to symbolic, rule-governed motions that have communicative relevance. PURPOSE: In this study we ask whether deaf signers and hearing non-signers with comparable abilities to discriminate non-linguistic biological motion from visual noise differ in their sensitivity to sign language biological motion. METHOD: Deaf fluent signers of American Sign Language (ASL) and hearing, non-signing speakers of English observed 252 randomized trials, on 2/3 of which a human point-light figure performing a single, non-linguistic goal-directed action or signing a single ASL lexical sign appeared embedded in one of seven levels of visual noise. Participants were blind to the type of human action being presented and simply indicated whether they did or did not observe a human figure on a given trial. Data from seven participants from each group who had comparable discriminability indices on the non-linguistic trials were selected for analysis. RESULTS: Hearing non-signers were significantly less sensitive to the presence of ASL signs in noise than to the presence of non-linguistic actions, whereas deaf signers were equally sensitive to both types of motion. This pattern was consistent across six of the seven noise levels tested. DISCUSSION: Differential sensitivity to linguistic and non-linguistic biological motions on the part of hearing participants who are unfamiliar with sign language suggests that differences exist between these two classes of motion that are salient to the sign-uninitiated visual system. Equivalent sensitivity to these motion types in deaf participants further suggests that lifelong exposure to a signed language, and/or lifelong auditory deprivation, can offset these signal differences.