Threshold of visual perception of facial asymmetry in a facial paralysis model

To determine the degree of facial asymmetry required to trigger conscious perception in the observer in a simulated model of facial paralysis.

A model of unilateral facial paralysis was created using the face of a participant without facial paralysis. Digital morphing software was used to create progressive asymmetry of the brow, oral commissure, and combined brow and oral commissure based on the typical sequelae seen in facial paralysis. Volunteers naive to the goals of the study repeatedly were shown a series of photographs of faces without facial paralysis, with the manipulated image interspersed within the series.

At least 3 mm of facial asymmetry of the oral commissure, brow, or both was required before participants detected the asymmetry. With longer display intervals, participants tended to detect a smaller degree of asymmetry.

To our knowledge, this is the first study directed at determining the amount of facial asymmetry required to trigger conscious perception of patients' facial paralysis in the naive observer. The pilot data and the discussion herein provide insight into the processes of visual perception of facial asymmetry and may be useful to surgeons for patient counseling and in setting surgical goals.