Pitch discrimination in autism - links to language delay?

Until recently, research on perception in autism has focused primarily on the visual modality. However, there is now a growing body of research on auditory processing. Of particular note are two recent studies, both published in the journal Neuropsychologia, which report enhanced auditory discrimination abilities in a subgroup of individuals on the autism spectrum.

"Beep"

The first of these studies was conducted by Catherine Jones and colleagues from the Institute of Education in London, who tested 72 adolescents with autism and a control group matched on age and IQ. Participants played a computer game in which they saw two dinosaurs, each of which produced a pure tone (beep) sound. They simply had to decide which dinosaur had made the higher sound. If they got two in a row correct, then the task got slightly more difficult (the two dinosaur sounds were made more similar in pitch). If they were incorrect then the task was made slightly easier. In this way, the researchers could work out each participant's threshold for detecting a pitch difference between two tones. Participants also completed similar tasks that involved discriminating between tones of different amplitude (loudness) and duration.

The performance of adolescents with autism on the tests of duration and amplitude discrimination was fairly unremarkable. But while the group differences in pitch discrimination threshold also failed to reach statistical significance, Jones et al noted that a disproportionate number of the adolescents with autism performed exceptionally well on the task. To be precise, 20% of the autism group (14/72) had thresholds that would have put them in the top 9% of the control group*. Interestingly, these exceptional performers were themselves disproportionately made up of adolescents with a history of language delay (based on parental interview)

Broadly comparable results were reported in a second study, this time from a Canadian group headed by Anna Bonnel from McGill University. Adults with autism performed better than control adults at discriminating between pure tones based on pitch. However, adults with Asperger syndrome failed to show this advantage. Given that participants were allocated to the autism or Asperger group based on parental reports of early language development, this again suggests a link between early language delay and exceptional pitch perception. Notably, differences were again found only for discrimination of pure tones. On other tasks involving complex tones or speech stimuli, performance was comparable across the groups.

Some caution is justified here. In both studies, the key results are hovering around the borders of statistical significance, so it's not clear how robust they are. Nevertheless, the apparent link between enhanced pitch discrimination and delayed language is as intriguing as it is counterintuitive. Not least because studies using the same tasks with non-autistic children with language difficulties have found the opposite pattern or results - a subgroup of these children have very poor pitch discrimination.

Jones and colleagues float the possibility that enhanced pitch perception may actually disrupt language processing. Bonnel et al, on the other hand, suggest that diminished interest in linguistic stimuli might lead to enhanced discrimination skills for non-linguistic stimuli. Both hypotheses are fairly speculative at this point, particularly as they assume that participants' current auditory thresholds, measured in adolescence or adulthood, are a fair reflection of their auditory skills in early childhood.

Looking at the bigger picture, what these two studies highlight is the importance of looking at differences between individuals with autism rather than focusing only on differences between autistic and non-autistic participants. It seems that there are some autistic people with very good pitch discrimination skills. But this isn't true of everyone, and there's no real reason to suppose that it should be. As is often pointed out, autism is a heterogeneous disorder - if you've met one person with autism, you've met one person with autism. So asking whether a particular faculty is impaired / unimpaired / enhanced (delete as applicable) is probably not the best research strategy. Investigating the nature of this variation, whether at the cognitive or neurobiological level, should ultimately lead to a better understanding of the mechanisms that cause different forms of autism


* The cut-off for 'exceptional performance' was 1.65 standard deviations away from the control group mean. 4 of the 48 kids in the control group also met this criterion.


References:

Bonnel A, McAdams S, Smith B, Berthiaume C, Bertone A, Ciocca V, Burack JA, & Mottron L (2010). Enhanced pure-tone pitch discrimination among persons with autism but not Asperger syndrome. Neuropsychologia, 48 (9), 2465-75 PMID: 20433857

Jones CR, Happé F, Baird G, Simonoff E, Marsden AJ, Tregay J, Phillips RJ, Goswami U, Thomson JM, & Charman T (2009). Auditory discrimination and auditory sensory behaviours in autism spectrum disorders. Neuropsychologia, 47 (13), 2850-8 PMID: 19545576