Can't Tone It Down? Auditory Habituation in ASD

Background: Improved understanding of the mechanisms that underlie unique manifestations of anxiety in autism spectrum disorders (ASD) may provide targets for etiological research as well as for better treatment specificity. Recent studies using either psychophysiological measures or fMRI have shown atypical reversal learning and extinction in ASD during fear conditioning tasks. However, it is unclear if these abnormalities in ASD are due to a failure to learn new associations, a failure to extinguish previous associations, or an inability to habituate to novel stimuli.

Objectives: We report findings from an auditory habituation study including adults diagnosed with ASD, compared to typical and to anxious adults. In conjunction with previous studies in our lab regarding fear conditioning and extinction, we hypothesized that pupil dilation—a sensitive index of autonomic nervous system activity--would reflect delayed habituation in ASD and may serve as a mechanism underlying anxiety in ASD.

 Methods: Twenty-eight adults ages 18-29 diagnosed with ASD (ASD group) were compared to 1) age-matched adults with high levels of anxiety in treatment at a university counseling center (ANX group); and 2) healthy adults with no known psychopathology (HC group). The habituation task consisted of three blocks (silence, 60db sine wave tone, 80db sawtooth wave tone) with 10 trials per block in which the tone was played for an average of two seconds with a jittered, ~20 second inter-trial interval. Pupil dilation was measured using SR Research Eyelink 1000 Plus eye tracker. We used Hierarchical Linear Modeling (HLM) to track average percent change in pupil dilation trajectories across all three groups. 

Results: HLM analyses reveal that the ANX group had significantly higher physiological reaction to the tones than the ASD and HC groups. Both the ANX and HC groups follow a similar habituation trajectory, showing a return to baseline around 6 to 10 seconds after sound offset. However, the ASD group did not return to baseline until approximately 15-20 seconds after tone onset (see attached figure). Trial-by-trail analysis showed delayed habituation to the tones in the ASD group, but not in the HC or ANX groups.

Conclusions:  Sensitive pupillometry measures suggest that habituation to even simple sensory stimuli may be delayed in ASD. After multiple exposures the ASD group showed atypically higher arousal during the recovery periods, indicating that they were not able to “calm down” from initial shock of these mild noises. These delays support previous finds of delayed fear extinction in ASD and suggests the possibility that this failure to habituate may be a prominent factor underlying anxiety as well as difficulties with change and transition. Exposure therapy and other treatment approaches for anxiety in ASD will likely take more time and would benefit from a focus on integrating internal and external stimuli.