Resolving ambiguity: A psycholinguistic approach to understanding prosody processing in high-functioning autism

Abstract

Individuals with autism exhibit significant impairments in prosody production, yet there is a paucity of research on prosody comprehension in this population. The current study adapted a psycholinguistic paradigm to examine whether individuals with autism are able to use prosody to resolve syntactically ambiguous sentences. Participants were 21 adolescents with high-functioning autism (HFA), and 22 typically developing controls matched on age, IQ, receptive language, and gender. The HFA group was significantly less likely to use prosody to disambiguate syntax, but scored comparably to controls when syntax alone or both prosody and syntax indicated the correct response. These findings indicate that adolescents with HFA have difficulty using prosody to disambiguate syntax in comparison to typically developing controls, even when matched on chronological age, IQ, and receptive language. The implications of these findings for how individuals with autism process language are discussed.

Introduction

Individuals with autism have well-documented impairments in the pragmatic use of language (Young, Diehl, Morris, Hyman, & Bennetto, 2005), including marked atypicalities in expressive prosody (Tager-Flusberg, 2001). Unusual prosody, which can include atypical pitch, rhythm, or stress patterns, has been observed in both high- and low-functioning individuals with autism. These difficulties often persist even when other areas of language improve (McCann & Peppé, 2003), and can become a stigmatizing barrier to social acceptance (Shriberg et al., 2001). Despite the pervasive nature of prosodic impairment, little research has addressed its etiology (McCann & Peppé, 2003). Moreover, the existing research has focused primarily on prosodic expression, with scant attention paid to comprehension. This is an important distinction because language comprehension precedes production in early development (Hirsh-Pasek & Golinkoff, 1991). Knowing how individuals with autism process prosodic information is critical to understanding their social-communicative difficulties in many areas (e.g., understanding affect, breakdowns in conversational discourse). Research in this area may also answer questions about the etiology of communication impairments. Prosody processing has been shown to facilitate early language acquisition (Demuth and Morgan, 1996, Jusczyk, 2003), and very early deficits in prosody comprehension may lead to later impairments in prosody production or even more general pragmatic deficits.

Clinical descriptions of prosody production patterns in individuals with autism have ranged from flat or monotonous to variable, pedantic, and/or having a singsong quality (Amoroso, 1992, Baltaxe, 1984, Fay and Schuler, 1980, Goldfarb et al., 1956, Provonost et al., 1966). Prosodic differences have been found to be significantly related to ratings of social and communicative functioning (Paul, Shriberg, McSweeny, Cicchetti, Klin, & Volkmar, 2005b). Deficits in prosodic production are included as a diagnostic characteristic of the disorder in the Autism Diagnostic Interview, Revised (ADI-R; Rutter, Le Couteur, & Lord, 2003) and the Autism Diagnostic Observation Schedule (ADOS; Lord, Rutter, DiLavore, & Risi, 1999), which are the gold-standard diagnostic tools used with this population. The importance of these clinical accounts of prosodic expression is not the type of prosody used (singsong versus monotone) but the mismatch between the words spoken, the context in which the words were spoken, and the prosody that was used by the speaker.

Despite the numerous descriptive accounts of prosodic impairments in autism, systematic research on prosodic abilities in individuals with autism is limited, especially with regard to those who have normal intellectual abilities (i.e., high-functioning autism; HFA). Participants defined as having HFA are typically described as having IQs above 80 and/or typical expressive language abilities, although there is no established definition, and studies can vary widely in their characterization of HFA. Recent studies found that approximately half of all individuals with HFA have dysfluent or impaired phrasing and inappropriate stress placement (Paul et al., 2005a, Shriberg et al., 2001). More specific analyses of phrasing indicate that individuals with HFA show difficulties using word or sentence stress (Fine, Bartolucci, Ginsberg, & Szatmari, 1991). Research indicates that individuals with autism who also have significant intellectual impairment (i.e., low-functioning autism; LFA) have variable ranges in the pitch and intensity of their expressive prosody (Amoroso, 1992, Baltaxe, 1984).

While there is evidence for impairment in prosodic expression, fewer studies have examined prosodic comprehension in autism. This is striking because recent research indicates that aspects of prosody perception may be impaired in individuals with autism during infancy (Kuhl, Coffey-Corina, Padden, & Dawson, 2005). The majority of rigorous investigations into prosody comprehension in autism focus on understanding mental or affective states from prosody (Hobson, 1986, Hobson et al., 1988, Kleinman et al., 2001, Rutherford et al., 2002). Many of these studies found deficits in these areas and suggested that impairments in prosody comprehension were related to deficits in theory of mind (Baron-Cohen, 1995), or deficits in affective processing (Hobson, 2004).

One way to investigate if there are deficits for prosody processing in autism beyond affective/mental state understanding is to examine the ability of individuals with autism to utilize prosody to determine linguistic (syntactic/semantic) structure from clause boundaries, an approach which is commonly used in psycholinguistic studies (Kraljic and Brennan, 2005, Snedeker and Trueswell, 2003). Prosody can be used in a way that indicates the meaning of a sentence without necessarily reflecting a person’s affective or mental state. Typical adults show sensitivity to how prosody influences sentence structure (Snedeker & Trueswell, 2003). Some studies have concluded that children as old as 11–12 years still have difficulty using prosody to resolve ambiguities in word and sentence meaning (Vogel & Raimy, 2002), although Snedeker and Yuan (2008) have suggested that children as young as 5 years may be better at this than previously thought.

Several studies have begun to examine how individuals with autism use prosody to make linguistic decisions. Paul and colleagues (Paul et al., 2005a) found that adolescents and young adults with HFA (average age = 17 years) were marginally worse at comprehending grammatical stress differences within words (e.g., “PROgress” versus “proGRESS”)¹ than typically developing controls matched on chronological age. The HFA group was not, however, impaired at using prosody to determine phrase structure (e.g., [chocolate cake] [and cookies] versus [chocolate] [cake and cookies]).² Because of the relatively small sample size of the control group (N = 13), however, it was likely difficult to detect medium effect sizes. Additionally, the groups were not matched on language or IQ, so it is possible that group differences were a result of an underlying language or intelligence differences rather than a prosodic deficit. Moreover, the authors noted that stimuli were produced in vivo by the experimenter and therefore they were not standardized across administrations. This is important because even small variations in prosody can affect the interpretation of a phrase structure. Finally, the practice items included training that highlighted the importance of paying attention to prosody.

Similarly, Peppé and colleagues (Peppé, McCann, Gibbon, O’Hare, & Rutherford, 2006) did not find differences in perception of phrasal chunking in children with HFA (ages 6–13 years) when compared to typically developing children (no chronological ages given) who were matched on verbal mental age. The authors used the Profiling Elements of Prosodic Systems—Children (PEPS-C; Peppé & McCann, 2003), a standardized measure of prosodic functioning, one component of which involved resolving syntactic ambiguities in phrases. Participants viewed two pictures (one with a chocolate cake and bread buns, the other with chocolate, cake, and bread buns) and chose the correct picture based on the prosody they heard ([chocolate cake] [and buns] versus [chocolate] [cake and buns]). Although the authors did not find significant group differences, both groups performed roughly at chance on the task, which may have masked group differences. Chance performance is understandable, given that the performance of typical individuals in this mental age range has been inconsistent (see Snedeker & Yuan, 2008, for a brief review).

Several smaller studies looked at aspects of comprehension of prosody as a determinant of linguistic structure. Fosnot & Jun (1999) examined four children with autism (IQ/language levels not specified) and found that deficits in imitating timing and chunking patterns were correlated with severity of autism. Scott, Stamm, Lee, and Dapretto (2005) showed that children with autism (IQ/language levels not specified) did not use prosody to learn words in a novel language task. In contrast, an early study by Frith (1969) of adolescents with LFA found intact use of stress to aid sentence recall. Findings from a brain imaging study suggested intact processing of linguistic–prosodic stimuli (Erwin et al., 1991).

In summary, there has been a paucity of research on prosody in autism, and particularly on prosody comprehension. It is unclear whether observed deficits in prosody processing can be explained by underlying deficits in mental state or affective processing, or whether the impairment encompasses using prosody to make linguistic decisions as well. Research on the use of prosody to determine sentence structure by individuals with autism has produced inconsistent findings across studies. Moreover, it has yet to be determined whether impairments are specific to prosody or related to deficits in general language ability. Additionally, authors of previous studies identified a need for controlled stimuli of appropriate difficulty for higher-functioning individuals (Paul et al., 2005a, Peppé et al., 2007).

The present study adapted a psycholinguistic paradigm to investigate whether individuals with autism have difficulty using prosody to disambiguate sentence structure in order to determine sentence meaning. Deficits in using prosody in this manner would suggest that factors other than mental state or affective processing may be implicated in the etiology of observed prosodic deficits in autism. We hypothesized that individuals with autism would have difficulty using prosody to resolve syntactic ambiguities when compared to typically developing controls matched on chronological age, IQ, and general language abilities.