Spectral and Temporal Auditory Processing in Autism: An fMRI Study

Background: Atypical perceptual processing is one of the mechanisms underlying the superior and inferior abilities of autistic individuals (Mottron et al., 2006). In vision, a performance dissociation in low-level perceptual capabilities led to the formulation of the complexity hypothesis (Bertone et al., 2005), predicting that autistics’ sensory processing efficiency is inversely related to the level of neural resources required, with elementary perceptual task performance higher and complex task performance lower. In the auditory modality, evidence of enhanced simple and diminished complex information processing (Samson et al., 2006), support extending the complexity hypothesis to audition.

Objectives: To investigate the regional functional specialization of the neural responses to sounds varying in spectral and temporal complexity and to test if atypical allocation of neural resources in autism extends to audition.

Methods: Using a 3T MRI system and echo-planar imaging, we studied autistics and controls, matched on FSIQ, sex and age, while they listened to auditory stimuli that varied parametrically in spectral ( two harmonic modulation levels) or temporal (four frequency modulation levels) complexity. Participants were asked to detect the presence or absence of the modulations. We used ANOVA to identify group, task and group by task interaction effects.

Results: We observed between-group differences in sensitivity to temporal complexity in bilateral non-primary auditory areas, with a stronger linear effect of increasing frequency modulation in controls. In contrast, we did not observe between-group differential sensitivity to spectral complexity, as assessed by harmonic stimulus content.

Conclusions: Consistent with predictions of the complexity hypothesis, increasing temporal complexity results in stronger modulation of non-primary auditory areas in controls than in autistics. In contrast, harmonic modulation does not. These results suggest that the complexity hypothesis holds for temporal, but not spectral, domains of auditory perception in autistics.