Fixation disparity

Fixation disparity is a tendency of the eyes to drift in the direction of the heterophoria. While the heterophoria refers to a fusion-free vergence state, the fixation disparity refers to a small misalignment of the visual axes when both eyes are open in an observer with normal fusion and binocular vision.[1] The misalignment may be vertical, horizontal or both. The misalignment (a few minutes of arc) is much smaller than that of strabismus. While strabismus prevents binocular vision, fixation disparity keeps binocular vision, however it may reduce a patient's level of stereopsis. A patient may have a different fixation disparity at distance than near. Observers with a fixation disparity are more likely to report eye strain in demanding visual tasks; therefore, tests of fixation disparity belong to the diagnostic tools used by eye care professionals:[2][3][4][5][6] remediation includes vision therapy, prism eye glasses, or visual ergonomics at the workplace.

Fig.1: Visual axes of the two eyes in optimal binocular vision (blue) and in exo and eso fixation disparity (black and red, respectively).
  1. ^ Howard, Ian P. (2012). Perceiving in Depth: Volume 1 Basic Mechanisms. Oxford University Press. doi:10.1093/acprof:oso/9780199764143.001.0001. ISBN 978-0-19-976414-3.
  2. ^ Eskridge, JB; Amos, JF; Bartlett, JD (1991). Clinical procedures in Optometry. New York: Lippincott Co.
  3. ^ Scheiman, Mitchell (4 October 2019). Clinical management of binocular vision : heterophoric, accommodative, and eye movement disorders. Wick, Bruce (Fifth ed.). Philadelphia. ISBN 978-1-4963-9973-1. OCLC 1098229972.{{cite book}}: CS1 maint: location missing publisher (link)
  4. ^ Evans, Bruce J. W. (2007). Pickwell's binocular vision anomalies. Pickwell, David. (5th ed.). Edinburgh: Elsevier Butterworth Heinemann. ISBN 978-0-7020-3925-6. OCLC 785829294.
  5. ^ Schroth, Volkhard. (2012). Binocular correction : aligning prisms according to the Haase approach (2. ed.). [Heemskerk]: Zijdar Books. ISBN 978-90-78376-09-5. OCLC 835292953.
  6. ^ Jaschinski, Wolfgang (2002). "The Proximity-Fixation-Disparity Curve and the Preferred Viewing Distance at a Visual Display as an Indicator of Near Vision Fatigue". Optometry and Vision Science. 79 (3): 158–169. doi:10.1097/00006324-200203000-00010. ISSN 1040-5488. PMID 11913842. S2CID 2793623.