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C. Hou, W.V. Good, A.M. Norcia; Validation Study of VEP Vernier Acuity . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4119.
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Purpose: Vernier acuity, the ability to discriminate the relative positions of features, is one of the visual hyperacuities and is strongly affected in amblyopia. Vernier offset thresholds can be measured with the VEP and may therefore be useful in non-verbal subjects. To test whether VEP vernier thresholds are tapping fine-grain position mechanisms we determined whether VEP thresholds are degraded by altering the positions of the elements composing the targets in a way that affects psychophysical performance. Methods: Vernier acuity thresholds were measured in eight normal adults using square-wave gratings containing vernier offsets that appeared and disappeared at 3 Hz (Norcia et al., 1999 Vis. Neurosci.). In the VEP experiments, offset size was swept from 0.25 to 4 arc min and VEP amplitude vs offset functions were measured at the first and second harmonics. Psychophysical thresholds were measured using a 2AFC procedure. In one experiment, gaps ranging from 1 to 20 arc min were introduced at the location of the vernier offset. In the second experiment, lateral offsets between 0.2 and 3.75 min were introduced. Results: VEP and psychophysical thresholds were each 0.5 arcmin with abutted targets that had no gaps, or lateral offsets. VEP thresholds and overall amplitudes at both the first and second harmonic were strongly affected by the introduction gaps or lateral offsets, with the first harmonic being substantially more affected. The introduction of comparable co-axial gaps or lateral offsets produced similar reductions in response amplitude. Psychophysical thresholds were also systematically elevated by gaps and offsets. Conclusions: Vernier acuity thresholds derived from the offset grating technique are tapping mechanisms that are specific for the relative position of stimulus elements.
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