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S.I. Chen, A. Chandna, A.M. Norcia; Testing for Motion Artifacts in Vernier VEP Measurements . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4191.
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Purpose: Vernier acuity measured by steady-state sweep visual evoked potential (SSVEP) involves stimulus modulation between alignment and misalignment and yields vernier acuity (first harmonic response) and motion acuity (second harmonic) in normal subjects. The potential exists for patients with motion asymmetry (MA) eg. early onset esotropia, to produce motion responses which masquerade as a vernier response at the first harmonic. The purpose of this study was to determine the extent to which MA is likely to contaminate vernier acuity measurements in patients. Methods: Each eye of 7 subjects with documented early onset esotropia was tested. VEP responses to horizontal and vertical orientations were investigated. The first experiment involved a ‘provocation test’ to detect and measure MA using 80% contrast sine-wave gratings (2 cycles per degree), modulated at 6 and 10 Hz. The second experiment involved a ‘penetration test’ to detect if MA carriers produced a significant first harmonic response to a motion-only version of the vernier acuity paradigm. This motion control consisted of an 80% contrast square-wave grating (2 cycles per degree), the bars alternating at a fixed temporal frequency between symmetric periodic vernier offsets of increasing magnitude. In the third experiment vernier acuity was measured with the bars alternating between collinear and offset states. First harmonic responses in the motion control condition thus serve as an indicator of the potential for MA to contaminate vernier response measurements. Results: All 7 subjects demonstrated MA as calculated by an Asymmetry Index. Four subjects demonstrated MA by the ‘bowtie’ configuration on inter-ocular polar plot comparison. Only one subject demonstrating horizontal MA penetrated to produce a valid first harmonic response in the motion control vernier paradigm. This subject had mild developmental delay. Conclusions: Vernier acuity measured with offset gratings appears to be a reliable method of measuring true vernier acuity in that the potential for asymmetric motion responses contaminating the vernier response is minimal in otherwise healthy subjects who are already MA carriers. The possibility exists that carriers of MA with neurological abnormalities may produce invalid vernier responses and warrants further investigation.
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