2Vernier AcuityThe 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.
3Vernier 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
4Measurement 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.
5Measurement of Vernier Acuity VEPs are recorded as misalignment is sweptSlope is determined for the VEP as it approaches background noise.The point where the slope intercepts background noise is a measure of vernier acuity
6Measurement 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 thisstimulus if they detect it first.
7Measurement 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.
8Development 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.
9Development 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.
10Mechanisms 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.
11Visual System Plasticity Psychology 4051Visual System Plasticity
12Visual System Plasticity The first decade of life represents a period of substantial development of the visual system.This is also a period of plasticity during which connections to the visual cortex are susceptible to environmental insult.This period represents a critical/sensitive period during which normal visual experience is necessary for the formation of connections to the visual cortex.
13Visual System Plasticity Visual deprivation caused by visual insult can impede the formation of these connections.During this time the eyes are in competition to form cortical connections.Monocular visual deprivation can lead severely impair connections from the deprived eye.
14Animal StudiesHubel and Wiesel have conducted numerous studies indicating that early visual deprivation causes severe changes to the visual system.Cats and monkeys generally have an eye sutured shut early in life.Animal undergo single cell recordingsIn animals, the critical period appears to be between eye opening (or birth) and puberty.
15Animal StudiesThe length of the visual deprivation and the timing of the deprivation is very important.Longer deprivation is generally more drastic.But there are periods in which animals are extremely sensitive even to short periods of deprivation.
16Anatomical/Physiological Effects There are no effects on cells in the retina.LGN cells corresponding to the deprived eye are smaller but respond normally.Most of the effects are in the visual cortex.Overall, there is a decline in the number of cortical cells driven by the deprived eye.Found in all cortical layers.
17Anatomical/Physiological Effects Animals who are forced to use the deprived eye later, often behave as though they are blind.Researchers have also investigated the effects of surgically-induced strabismus.Most cells in the visual cortex are monocular.
18TreatmentWhile in this period of plasticity some of the effects of monocular deprivation can be reversed by reverse suturing.Sewing the other eye shut to force the use of the deprived eye.If done during the critical period, the deprived eye regains cortical connections and acuity improves.
19Human StudiesResearchers have investigated the effects of naturally occurring deprivation in humans by looking at children who had cataracts.Opacity on the lens that prevents the formation of a sharp image.These studies confirm that the critical period lasts from birth to 10 years.
20Human StudiesLonger periods produce more severe effects, but timing is also important.Leads to poorer visual acuities.Early deprivation tends to produce drastic effects.The effects of bilateral cataracts are far less severe.Competition between the eyes is not compromised.