Atypical Brain Responses to Illusory Auditory Pitch in Children with Autism

Background:

Atypical auditory perception is widely reported in association with autism. Previous studies of neurotypical children and adults using electro- and magnetoencephalography (MEG) have identified an event-related brain response, termed the Object Related Negativity (ORN), occurring 200-300 ms after stimulus onset, which is related to the grouping of the acoustic input into separate auditory objects.

Objectives:

In the current study, we recorded the brain responses of children with autism to dichotic pitch stimuli, in which inter-aural timing differences result in the illusory perception of a pitch sound spatially segregated from a carrier white noise. We hypothesized that the ORN would be diminished in autism, reflecting a failure of auditory object formation.

Methods:

Participants were ten 8- to 12-year-old children with autism and ten age-matched typically developing children. Control stimuli were 500 ms, 70 dB white noise presented binaurally. Pitch stimuli were identical apart from a 0.5 ms inter-aural timing difference in a narrow frequency band centred at 600 Hz. Participants viewed a movie while ignoring the auditory stimuli. Brain responses were recorded using 160 channel whole-head MEG and projected onto sources in bilateral auditory cortex.

Results:

Typically developing children demonstrated the expected difference between Control and Pitch stimuli from around 250 milliseconds onwards. While there was a trend in this direction in the ASD group, the effect of stimulus was non-significant, perhaps reflecting somewhat noisier data. Strikingly, however, the ASD group showed a marked difference in responses to Pitch and Control stimuli at around 50 ms, which was not present in the control group.

Conclusions:

Amongst children with autism, brain responses were sensitive to the presence of the illusory pitch from as early as 50 ms. This differential response was absent amongst typically developing children and is also absent in other populations (typical adults, children with dyslexia) we have tested. The increased sensitivity of the auditory cortex response in autism may be related to reports of enhanced pitch discrimination in individuals with autism using behavioural paradigms. Ongoing time-frequency analysis of these data may provide further insights into the neural mechanisms of pitch processing in autism.