Academic journal article Memory & Cognition

The Perception of Handshapes in American Sign Language

Academic journal article Memory & Cognition

The Perception of Handshapes in American Sign Language

Article excerpt

Despite the constantly varying stream of sensory information that surrounds us, we humans can discern the small building blocks of words that constitute language (phonetic forms) and perceive them categorically (categorical perception, CP). Decades of controversy have prevailed regarding what is at the heart of CP, with many arguing that it is due to domain-general perceptual processing and others that it is determined by the existence of domain-specific linguistic processing. What is most key: perceptual or linguistic patterns? Here, we study whether CP occurs with soundless handshapes that are nonetheless phonetic in American Sign Language (ASL), in signers and nonsigners. Using innovative methods and analyses of identification and, crucially, discrimination tasks, we found that both groups separated the soundless handshapes into two classes perceptually but that only the ASL signers exhibited linguistic CP. These findings suggest that CP of linguistic stimuli is based on linguistic categorization, rather than on purely perceptual categorization.

How do we discern the phonetic units that make up the constantly varying linguistic stream around us? One of the most researched answers to this question has been surrounded by controversy since the 1950s, when human adults were found to partition speech sounds from a continuum into discrete categories, a phenomenon known as categorical perception (CP; see Liberman, 1996, for a review). Early findings regarding the CP of speech sounds suggested that CP is specific to spoken language in humans and is the result of linguistic-specific processing. This special mechanisms hypothesis was supported by findings from two lines of research, the first of which was that CP occurred only for contrasts that were phonemic in a speaker's native language. For example, it has been shown that adult speakers of Japanese have difficulty distinguishing between /1/ and /r/ (Miyawaki et al., 1975), and Werker, Gilbert, Humphrey, and Tees (1981) and Werker and Tees (1983) have also shown that English-speaking adults cannot discriminate the Hindi contrasts /t^sup h^a/-/d^sup h^a/ (voiceless aspirated dental stop vs. breathy voiced) or/t'a/-/ta/ (voiceless unaspirated retroflex vs. dental) as well as Hindi adults can.

An earlier finding came from Eimas, Siqueland, Jusczyk, and Vigorito's( 1971) work on 1-month-old infants' speech perception. They showed that infants could discriminate computer-generated sounds that straddled the adult phonetic boundary but failed to discriminate stimuli that were from within the same adult phonetic category. The infants' ability to do this without having had a long period of experience in listening to or producing speech suggested that CP was not learned and was, in fact, a part of the biological makeup of the human species. In addition, it has been well documented in the literature that infants can easily discriminate both native and nonnative oral consonant contrasts without having had relevant experience; however, by 10-12 months of age, infants perform like adults and easily discriminate only native consonant contrasts (see Jusczyk, 1997, for a review). There is a developmental shift in the perception of vowel contrasts as well, from a language-general processing pattern to a language-specific pattern, although this shift seems to occur earlier in development than does that for consonants (Polka & Werker, 1994). This pattern of research findings has led to the hypothesis, first suggested by Eimas (1975), that infants may have a biological predisposition to discriminate the universal set of phonetic contrasts and that there is an apparent reorganization of this universal phonetic sensitivity as a result of their having learned a particular language.

This hypothesis that there is a specialized mode of speech processing in humans has been challenged by research demonstrating that certain nonhuman primates and animals also exhibit CP for human speech (Dooling, Best, & Brown, 1995; Kluender, Diehl, & Killeen, 1987; Kuhl, 1981, 1987; Kuhl & Miller, 1975, 1978; Kuhl & Padden, 1982, 1983; Ramus, Mauser, Miller, Morris, & Mehler, 2000). …

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