title: Visual perception of upright: Head tilt, visual errors and viewing eye.
abstract: BACKGROUND
  Perception of upright is often assessed by aligning a luminous line to the subjective visual vertical (SVV).
  
  
  OBJECTIVE
  Here we investigated the effects of visual line rotation and viewing eye on SVV responses and whether there was any change with head tilt.
  
  
  METHODS
  SVV was measured using a forced-choice paradigm and by combining the following conditions in 22 healthy subjects: head position (20° left tilt, upright and 20° right tilt), viewing eye (left eye, both eyes and right eye) and direction of visual line rotation (clockwise [CW] and counter clockwise [CCW]).
  
  
  RESULTS
  The accuracy and precision of SVV responses were not different between the viewing eye conditions in all head positions (P> 0.05, Kruskal-Wallis test). The accuracy of SVV responses was % significantly different between the CW and CCW line rotations (p ≈ 0.0001; Kruskal-Wallis test) and SVV was tilted in the same direction as the line rotation. This effect of line rotation was however not consistent across head tilts and was only present in the upright and right tilt head positions. The accuracy of SVV responses showed a higher variability among subjects in the left head tilt position with no significant difference between the CW and CCW line rotations (P> 0.05; post-hoc Dunn's test).
  
  
  CONCLUSIONS
  In spite of the challenges to the estimate of upright with head tilt, normal subjects did remarkably well irrespective of the viewing eye. The physiological significance of the asymmetry in the effect of line rotation between the head tilt positions is unclear but it %may suggest suggests a lateralizing effect of head tilt on the visual perception of upright.
authors.affiliation: Department of Neurology, The Johns Hopkins University School of Medicine, Baltimore, MD, USA.|Department of Otolaryngology-Head and Neck Surgery, The Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Amir
authors.initials: A
authors.lastname: Kheradmand
authors.affiliation: Department of Ophthalmology, National Institute of Neurology and Neurosurgery, National Autonomous University of México, México City, México.
authors.forename: Grisel
authors.initials: G
authors.lastname: Gonzalez
authors.affiliation: Department of Neurology, The Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Jorge
authors.initials: J
authors.lastname: Otero-Millan
authors.affiliation: Department of Neurology, The Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Adrian
authors.initials: A
authors.lastname: Lasker
mesh_terms: D000328:Adult; D005260:Female; D019416:Head Movements; D006801:Humans; D008297:Male; D008875:Middle Aged; D009799:Ocular Physiological Phenomena; D009949:Orientation; D015203:Reproducibility of Results; D012399:Rotation; D014716:Vertical Dimension; D015348:Vision, Binocular; D015349:Vision, Monocular; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult
keywords: Subjective visual vertical; binocular; head tilt; line rotation; monocular