Role of Pitch-Discrimination Abilities in Sequential Auditory Stream Segregation by Individuals with Asperger's Syndrome

Background: Sensory reactions and perceptual abilities in individuals with autism spectrum disorder (ASD) seem to differ from those in typically developing individuals. Results from studies assessing perceptual functions in ASD led to the Enhanced Perceptual Functioning model (e.g. Mottron et al., 2006), postulating that ASD is characterized by better-than-normal perception of basic stimulus attributes, but worse-than-normal higher-level processing. Consistent with this model, enhanced discrimination of pitch, i.e., the subjective percept of a pure-tone frequency, was observed in ASD, while speech identification, especially in noisy environments, can be degraded. However, so far, the link between pitch perception and auditory scene analysis has not been assessed behaviourally.

Objectives: To further test the hypothesis of enhanced pitch-discrimination abilities in ASD using a standard psychophysical procedure combined with different tasks, and to investigate the role of this basic auditory function in sequential auditory stream segregation.

Methods: Children with Asperger’s syndrome (AS; age range = 12-16 years), age- and IQ-matched typically developing controls (TDC), and adult controls (AC) were recruited for five experimental sessions. Only participants with audiometrically normal hearing sensitivity were included. Frequency-discrimination thresholds for reference frequencies ranging from 250 to 4000 Hz were obtained in two different adaptive tasks, using a 2-interval, 2-alternative forced-choice procedure with feedback. First, participants had to indicate which of two successively presented pure tones (A vs. B) had the higher frequency (i.e., pitch). Since this task assesses temporal-order judgement in addition to frequency discrimination, participants were also tested on a task requiring to indicate in which of two intervals, each composed of four successively presented pure tones, a change in frequency occurred (A-A-A-A vs. A-B-A-B). Subjective and objective measures of sequential auditory stream segregation were also obtained in the same participants, using pure-tone sequences (A-B-A-B… and A-B-A—A-B-A…, respectively). A-tones had a frequency of either 250 or 1000 Hz, whilst the B-tones had either the same frequency (no streaming; base line) or differed in frequency from the A-tones (measured in terms of the perceptually relevant scale of equivalent-rectangular bandwidths). Prior to data collection, participants received practice on all tasks and conditions during the initial three sessions to stabilize performance.

Results: Preliminary data obtained so far for six AS and three AC indicate that: (i) All participants improved markedly over the initial threshold runs on the first frequency-discrimination task, arguing against its diagnostic use (i.e., without preliminary practice) for the assessment of pitch-processing abilities; (ii) Despite the age difference (and hence a potential maturational difference), AS showed, on average, adult-like frequency-discrimination thresholds; the comparison to data obtained in TDC will reveal if this sensitivity is better-than-normal for that age range; (iii) Under certain frequency separations between A- and B-tones, stronger sequential stream segregation was observed in AS than in AC, indicating that the pitch of pure tones may be a more salient streaming cues in this population.

Conclusions: The final results will be discussed in the light of the predictions of current models of affected perceptual functioning in autism.