title: Measurement of position acuity in strabismus and amblyopia: specificity of the vernier VEP paradigm.
abstract: OBJECTIVE
  An objective measure of positional acuity is desirable in the nonverbal clinical population. This study was conducted to investigate the specificity of the vernier VEP as a measure of positional acuity, evaluating the potential confound of asymmetric motion responses that may be present in some groups of patients. These motion responses could masquerade as position-specific responses, since they occur at the same response frequency as the vernier-related response.
  
  
  METHODS
  Twelve observers with early-onset esotropia (EOE), 30 children with untreated amblyopia, and 15 control children underwent swept vernier VEP acuity testing accompanied by a swept motion control stimulus. The control condition was used to detect the presence of artifactual responses not related to position sensitivity. The patients with EOE were selected for high levels of motion asymmetry as documented with oscillating gratings presented monocularly. As a measure of motion confound (penetration), the proportion of first-harmonic responses recorded in the control condition was determined.
  
  
  RESULTS
  The penetration rate in the vernier condition in each study group (EOE: 0.93%; amblyopes: 4.26%; normal subjects: 2.40%) and the entire group (2.85%) was acceptably low. The level of penetration was not significantly influenced by the presence of amblyopia.
  
  
  CONCLUSIONS
  The vernier VEP paradigm, when applied in the manner described, can be interpreted as a measure of position sensitivity. The presence of motion asymmetry or untreated amblyopia does not affect the validity of vernier measurements made.
authors.affiliation: Smith-Kettlewell Eye Research Institute, San Francisco, California, USA. s.chen@blueyonder.co.uk
authors.forename: Sean I
authors.initials: SI
authors.lastname: Chen
authors.forename: Anthony M
authors.initials: AM
authors.lastname: Norcia
authors.forename: Mark W
authors.initials: MW
authors.lastname: Pettet
authors.forename: Arvind
authors.initials: A
authors.lastname: Chandna
mesh_terms: D000293:Adolescent; D000328:Adult; D000550:Amblyopia / Q000503:physiopathology*; D002648:Child; D002675:Child, Preschool; D004948:Esotropia / Q000503:physiopathology*; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D009039:Motion Perception; D014792:Visual Acuity / Q000502:physiology*