Vernier-Derived Position and Motion Deficits in Previously Untreated Anisometropic Amblyopia
Sean I. Chen, PhD; Arvind Chandna, MD; Spero Nicholas; Anthony M. Norcia, PhD
The Galway Clinic
Doughiska, Galway, Ireland
Introduction: Despite a constellation of visual deficits in amblyopia, treatment endpoints typically focus around threshold measurements (eg. visual acuity). We examined neural responses to a stimulus containing both relative position and relative motion cues in children with amblyopia and age-matched controls. The visual responses were recorded across a wide range above and around threshold performance before and after clinical treatment.
Methods: Spectral analysis of swept-parameter Visual Evoked Potential (sVEP) responses were used to isolate relative position (1st harmonic) and motion (2nd harmonic) related responses in children with previously untreated anisometropic amblyopia: n=10, mean age 6.3 years, mean(SD) interocular acuity difference 0.40 (0.20) logMAR. Comparison was made with typically developing children: n=16, mean age 5.14 years. Measurements were repeated after conventional treatment.
Results: Across a range of supra-threshold vernier offsets, children with anisometropic amblyopia displayed super-normal position responses in the fellow eye and markedly subnormal ones in the amblyopic eye (p=0.005). Motion responses on the other hand, showed no difference from normal in the fellow eye and milder losses in the amblyopic eye. After treatment, the position response decreased in the fellow eye and increased in amblyopic eye (p=0.007). After treatment, the motion responses reduced in the fellow eye (p=0.01), but no significant change occurred in the amblyopic eye.
Discussion: The differential effect of early deprivation (amblyopia) on position vs motion evoked potentials and their responses to treatment suggests an unequal level of plasticity in differing neural substrates.
Conclusion: Form versus motion processing mechanisms in developing human visual cortex may have different neural origins and/or plasticity.
References: Chen SI, Norcia AM, Pettet MW, Chandna A. Measurement of Position Acuity in Strabismus and Amblyopia: Specificity of the Vernier VEP Paradigm. Invest. Ophthalmol. Vis. Sci. 2005;46(12):4563-70.