Persons with Autism spectrum disorders (ASD) display atypical perceptual processing in visual and auditory tasks. In vision, Bertone, Mottron, Jelenic, and Faubert (2005) found that enhanced and diminished visual processing is linked to the level of neural complexity required to process stimuli, as proposed in the neural complexity hypothesis. Based on these findings, Samson, Mottron, Jemel, Belin, and Ciocca (2006) proposed to extend the neural complexity hypothesis to the auditory modality. They hypothesized that persons with ASD should display enhanced performance for simple tones that are processed in primary auditory cortical regions, but diminished performance for complex tones that require additional processing in associative auditory regions, in comparison to typically developing individuals. To assess this hypothesis, we designed four auditory discrimination experiments targeting pitch, non-vocal and vocal timbre, and loudness. Stimuli consisted of spectro-temporally simple and complex tones. The participants were adolescents and young adults with autism, Asperger syndrome, and typical developmental histories, all with IQs in the normal range. Consistent with the neural complexity hypothesis and enhanced perceptual functioning model of ASD (Mottron, Dawson, Soulières, Hubert, & Burack, 2006), the participants with autism, but not with Asperger syndrome, displayed enhanced pitch discrimination for simple tones. However, no discrimination-thresholds differences were found between the participants with ASD and the typically developing persons across spectrally and temporally complex conditions. These findings indicate that enhanced pure-tone pitch discrimination may be a cognitive correlate of speech-delay among persons with ASD. However, auditory discrimination among this group does not appear to be directly contingent on the spectro-temporal complexity of the stimuli.
Autism spectrum disorders (ASD) include a range of neuro-developmental variants, including autism and Asperger syndrome, that are characterized by mild to severe atypicalities in communication and social interactions, as well as by restricted behaviors and interests (DSM-IV, American Psychiatric Association, 1994). In addition, idiosyncratic reactions to the auditory environment are often noted with examples of both auditory hypersensitivity and hyposensitivity. The former is evidenced in cases where persons with ASD cover their ears in response to certain sounds that do not bother most others, and the latter in the lack of spontaneous orientation to one's own mother's voice (Grandin, 1997 and Leekam et al., 2007). Consistent with these anecdotal observations, persons with ASD display enhanced and diminished patterns of perceptual processing across a variety of auditory tasks (for a review, see Kellerman et al., 2005, Nieto Del Rincòn, 2008 and Samson et al., 2006).
The various examples of enhanced auditory processing among persons with ASD include the ability to discriminate between and categorize pure tones on the basis of their pitch or height value (Bonnel et al., 2003, Heaton et al., 1998 and O’Riordan and Passetti, 2006), and an increased prevalence of absolute pitch, the rare ability to identify or produce the pitch of a tone without reference to an external standard. Whereas five out of 100 individuals with ASD display absolute pitch (Miller, 1999 and Rimland and Fein, 1988), this skill is found in only 1/10,000 individuals in the general population (Takeuchi & Hulse, 1993). In contrast, persons with ASD appear to display a relative difficulty in more complex auditory verbal tasks involving figure/ground discrimination (Alcantara et al., 2004 and Groen et al., 2008).
Together with specific peaks in visuo-spatial and other perceptually related aspects of cognitive and behavioral functioning, these auditory strengths are the hallmarks of the Enhanced Perceptual Functioning Model (EPF: Mottron and Burack, 2001 and Mottron et al., 2006). This model identifies a short list of “principles” characterizing autistic perception, its overall enhanced performance, and its enhanced role in typically non-perceptual cognitive operations as well as in guiding behaviors in natural settings. As these auditory strengths and difficulties among persons with ASD are identified within the descriptive EPF framework, the next step is to highlight the relevant psychophysical variables and neural mechanisms that are associated with them.
Consistent with the extension of the neural complexity hypothesis to the auditory modality, the current findings indicate that persons with autism display a particular strength in discriminating between pure tones that differ in pitch. Together with the evidence of superior processing of visual information in primary visual cortex, the findings indicate that functions served by primary perceptual areas are enhanced in autism. However, contrary to the extension of the neural complexity hypothesis, the abilities of the participants with ASD to discriminate between elementary tones that vary in pitch, timbre, and loudness were not directly contingent on the spectral and temporal complexity of the processed stimuli. Yet, as the discrimination tasks may be successfully performed by persons with autism through the combination of simple mechanisms, they may not be optimal for assessing the extension of the complexity hypothesis to the auditory modality.
The finding that only persons with autism, and not those with Asperger syndrome, displayed enhanced pure-tone pitch discrimination indicates that perceptual peaks of ability may be a sub-typing tool for ASD. More generally, the current set of findings is additional evidence for the premise of the EPF model that strengths outnumber deficits in perceptual processing among persons with autism. These superior performances and the role of perception may be essential to both sub-typing and explaining the cognitive and behavioural phenotypes of autism.
