Psychology 4051 Vernier Acuity

Vernier Acuity The smallest amount of misalignment that can be detected within a stimulus. Usually measured by determining the smallest offset that can be detected within a single line, or series of lines.

Vernier Acuity Human vernier acuity is exceptional. Often measures -0.2 to -0.4 logMAR. 20/12 to 20/8. This is superior to both optotype acuity and to grating acuity. Superior to what would be predicted based on photoreceptor properties. Referred to as a hyperacuity

Measurement of Vernier Acuity In adults, it’s simply measured as the smallest offset that can be detect. In young children, it can be measured using sweep VEP or FPL techniques. In sweep VEPs, the participant is presented with a square wave grating or checkerboard pattern in which the amount of misalignment is swept.

Measurement of Vernier Acuity VEPs are recorded as misalignment is swept Slope is determined for the VEP as it approaches background noise. The point where the slope intercepts background noise is a measure of vernier acuity

Measurement of Vernier Acuity In FPL, a misaligned square wave grating can be paired with normal square wave grating. The square grating is very salient. Subjects may prefer this stimulus if they detect it first.

Measurement of Vernier Acuity This can be fixed using vernier acuity cards in which the entire card is covered with a square wave grating. Part of the square wave grating is misaligned in the form of a shape that is familiar to toddlers.

Development of Vernier Acuity Initially, vernier acuity was thought to be poorer than grating acuity until 3-4 months of age. Then it improved rapidly until it reached adult level at approximately 4 years of age. These early studies were flawed in that the misaligned stimulus was shifted from an aligned to a misaligned position to maintain the attention of the infants.

Development of Vernier Acuity Skoczenski and Norcia (2002) used VEPs to show that vernier acuity is poorer than grating acuity until about 4 years of age. Vernier acuity does not reach adult level until adolescence. This course of development has been confirmed using FPL.

Mechanisms Underlying Development Given its slow maturation compared to grating acuity and the fact it’s a hyperacuity, vernier acuity is not likely mediated by photoreceptor properties. Appears to cortically mediated. Children with cortical impairment show selective deficits in vernier acuity. Achievement of adult level corresponds with synaptic pruning.