  Purpose: Facial expression production and perception in autism spectrum
disorder (ASD) suggest potential presence of behavioral biomarkers that may
stratify individuals on the spectrum into prognostic or treatment subgroups.
Construct validity and group discriminability have been recommended as criteria
for identification of candidate stratification biomarkers.
  Methods: In an online pilot study of 11 children and young adults diagnosed
with ASD and 11 age- and gender-matched neurotypical (NT) individuals,
participants recognize and mimic static and dynamic facial expressions of 3D
avatars. Webcam-based eye-tracking (ET) and facial video tracking (VT),
including activation and asymmetry of action units (AUs) from the Facial Action
Coding System (FACS) are collected. We assess validity of constructs for each
dependent variable (DV) based on the expected response in the NT group. Then,
the Boruta statistical method identifies DVs that are significant to group
discriminability (ASD or NT).
  Results: We identify one candidate ET biomarker (percentage gaze duration to
the face while mimicking static 'disgust' expression) and 14 additional DVs of
interest for future study, including 4 ET DVs, 5 DVs related to VT AU
activation, and 4 DVs related to AU asymmetry in VT. Based on a power analysis,
we provide sample size recommendations for future studies.
  Conclusion: This pilot study provides a framework for ASD stratification
biomarker discovery based on perception and production of facial expressions.
