title: Dissociated vertical deviation: an acquired nystagmus-blockage phenomenon.
abstract: BACKGROUND
  The mechanism of dissociated vertical deviation (DVD) is beginning to be understood, but does DVD serve a useful purpose? This study explores the hypothesis that DVD improves visual acuity in the fixing eye by helping to damp latent nystagmus, thus functioning as a nystagmus-blockage mechanism.
  
  
  METHODS
  Scleral search coil eye movement recordings of ten patients with dissociated vertical deviation were obtained and analyzed.
  
  
  RESULTS
  Latent nystagmus-horizontal, vertical, and torsional-practically always appeared initially, when one eye was occluded, and became damped as DVD developed. The damping occurred over 0.3 to 3 seconds and was often only partial, identified as a decreasing slope of the slow phases of the nystagmus. Occasionally, if the DVD response diminished, the latent nystagmus reappeared. A DVD response could be recorded in total darkness in those individuals who could voluntarily imagine switching "fixation" (attention) from one eye to the other. A head tilt also damped latent nystagmus in one patient, and appeared to decrease the need for DVD.
  
  
  CONCLUSIONS
  This evidence supports the view that DVD is an acquired (learned), often anticipatory, vergence response, occurring upon taking up unilateral fixation, serving to improve vision by damping or blocking latent nystagmus.
authors.forename: David L
authors.initials: DL
authors.lastname: Guyton