Cortical Visual Processing in Axial Myopia Essay

Cortical Visual Processing in Axial Myopia Essay

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Myopia is a refractive defect in which parallel rays of light are brought to focus in front of the retina when no accommodation is used (Grosvenor and Goss, 1999; Saw, et al., 1996). This causes distant objects to appear blurred while near objects can still be seen clearly.

Myopia can be classified in various ways: cause (axial or refractive) (Emsley, 1953), age of onset (congenital, youth onset, early adult-onset, or late adult-onset) or degree (low, intermediate or high) (Grosvener, 1987). When classifying myopia by degree, high myopia is indicated where myopia is greater than 6.00 dioptres, intermediate between 3 and 6D and low 0.50 to 3.00D (Fitzgerald, Chung, and Krumholtz, 2005).

Axial myopia is due to the axial elongation of the vitreous chamber meaning that the eye becomes too long for its refractive power. (Benjamin, 1998) Refractive myopia is caused by an error in the dioptric power of the eye either due to variation in the refractive index of lens (index myopia), increase in curvature of the refractive surface of cornea (curvature myopia) or abnormality in anterior chamber depth (anterior chamber myopia) (Borish, 1970). Further research (Strang, Winn and Bradley, 1998) showed that most cases of significant myopia are mainly due to axial myopia.

Theories about the cause of myopia are currently incomplete. It is thought there is some genetic aspect but it can also be induced by an excessive amount of near work stimulating accommodation. (ref) Systemic conditions may also have an influence in myopia and its progression. (ref)These include: Marfans syndrome, foetal alcohol syndrome, Crouzoan syndrome and cerebral palsy. Another influence proven to be associated with myopia and its progression is birth weight and ...

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...y A. (1998). The role of neural and optical factors in limiting visual resolution in myopia. Vision research, 38(11), 1713-1721.
30. Stuart J A, Burian H M. (1962). A study of separation difficulty. Its relationship to visual acuity in normal and amblyopic eyes. American Journal Ophthalmology 53,471-477.
31. Simmers A J, Ledgeway T, Hess R F. (2004) The influences of visibility and anomalous intergration processes on the perception of global spatial form versus motion in human amblyopia. Vision research 45, 449-460.
32. Simmers A J, Ledgeway, T Hess R F & McGraw P V. (2003). Deficits to global motion processing in human amblyopia. Vision Research 43, 729-738.
33. American Optometric Association. 2006. Optometric clinical practise guidelines care of the patient with myopia. [ONLINE] Available at: [Accessed 20 January 12].

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