Categorical perception in American Sign Language Karen Emmorey and Stephen McCullough The Salk Institute for Biological Studies, La Jolla, CA, USA Diane Brentari Purdue University, West Lafayette, IN, USA Categorical perception (CP) refers to the finding that certain stimuli (particularly speech) are perceived categorically rather than continuously, despite a continuous variation in form. Two experiments investigated whether Deaf signers or hearing nonsigners exhibit CP for hand configura- tion or for place of articulation (the location of articulation on the body) in American Sign Language (ASL). CP performance was measured using discrimination (ABX) and categorisation paradigms with computer-gener- ated images of signs. In the categorisation task, signers and nonsigners exhibited sigmoidal performance and categorised non-identical stimuli together at each end of the perceptual continuum for both hand configuration and place of articulation, regardless of phonological distinc- tiveness in ASL. The finding that signers and nonsigners performed similarly suggests that these categories in ASL have a perceptual as well as a linguistic basis. Results from the discrimination task, however, showed that only ASL signers demonstrated categorical perception, and only for phonologically contrastive hand configuration. Neither group exhibited CP for place of articulation. Lack of a CP effect for place of articulation may be due to more variable category boundaries. A CP effect for contrastive hand configuration suggests that deaf signers develop special abilities for perceiving distinctions that are relevant to American Sign Language. Requests for reprints should be sent to Karen Emmorey, Laboratory for Cognitive Neuroscience, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, CA 92037. Email: emmorey@salk.edu This research was supported by grants from the National Science Foundation (Linguistics Program; SBR 9809002) and from the National Institute for Child Health and Human Development (ROI HD13249) awarded to Karen Emmorey at the Salk Institute for Biological Studies. We would like to thank Sam Hawk, Melissa Herzig, and Amy Hoshina for help testing deaf subjects, and Sharen Kwan and Jamie Park for help testing the hearing subjects.  c 2003 Psychology Press Ltd http://www.tandf.co.uk/journals/pp/01690965.html DOI: 10.1080/01690960143000416 LANGUAGE AND COGNITIVE PROCESSES, 2003, 18 (1), 21–45