17: Acuity Limits

What factors affect visual acuity?

Optical blur (refractive error), illumination, location on retina, contrast (and others we don’t care about)

What 2 factors limit our visual acuity?

Blur (refractive error) and optical abberations of the eye

What is the Nyquist limit (concept)?

The limit on the resolution acuity of a system

How far apart are cones spaced in the fovea? (numerical value)

30 arc seconds

How does cone spacing in the fovea limit our acuity?

Since the cones are spaced 30 arc seconds away and we need minimum 2 cones to have resolution, our maximum resolution limit is 60 cy/deg (20/10 Snellen equivalent)

What is the Nyquist limit numerical value?

One stripe per cone or one cycle per 2 cones → 60 cy/deg or 20/10 Snellen equivalent

What are the causes of optical losses within the eye?

Defocus, high order abberations, scatter, and stray light not absorbed by the visual pigments or RPE

Why don’t we normally observe aliasing under natural conditions?

Blur masks it

If our Nyquist limit says that our max visual acuity is 20/10, why is the average acuity 20/20?

Aberrations

What is the best VA of the rods?

Between 20/100 and 20/200

Why are rod VAs so much worse than cones?

Rods have so much more spatial summation—each RGC receives signals from many rods whereas in the fovea each RGC gets a signal from 1 cone only

What is Vernier hyperacuity?

A person’s ability to appreciate the locations of stimuli relative to each other—the minimum detectable offset of 2 nearly-aligned marks

Vernier hyperacuity is NOT limited by what?

Photoreceptor mosaic

Besides Vernier, what are other examples of hyperacuities?

Leveling, centering/bisection, and orientation discrimination

We can distinguish misalignment of 2 very close stimuli when…

When the responses to 2 different stimuli overlap, so the 2 different stimuli share 1 or more receptive field and also have their own unique receptive fields

What makes Vernier acuity possible?

Blur

Blur limits ___ acuity but makes ___ acuity possible

Limits visual acuity

Makes Vernier acuity possible

Why can’t we distinguish 2 stimuli that are on top of each other?

The same cones respond to both dots

What 2 disorders affect Vernier acuity?

Refractive amblyopia and strabismic amblyopia

What is cone morphology like for babies?

Cones are wider and stubbier around the fovea

What is foveal morphology like for babies?

Baby fovea has a single layer of fat cones, no Henle fiber layer, thick INL, and contains nuclei of bipolars, horizontals, amacrines, and ganglion cells

What is foveal morphology like for adults?

Many layers of thin cones, thick Henle fiber layer, thin INL, and cell bodies get shoved off to the side away from the foveal pit

Do we gain or lose cones as our retinas mature? What is the percentage of growth/loss?

Lose (25% of newborn cones die)

The most developed part of the retina at birth is…

The periphery (best vision here at birth)

What happens to light entering the eye of a newborn?

The light waves aren’t well-guided to the retina

Much of the incident light does not reach the cones outer segments

The light that DOES reach the outer segments doesn’t get absorbed well bc outer segments are stubby

How did researchers test average acuity in babies?

Forced choice preferential looking (FPL)

What stimuli are used when testing a baby’s acuity?

Stimuli of different spatial frequencies

How do we test a baby’s visual acuity clinically?

Teller Acuity Cards

Teller Acuity Cards can be used to test…

Resolution acuity, contrast sensitivity, and stereopsis in babies

What psychophysical method is used for scoring Teller Acuity Cards?

Method of limits for scoring

(Preferential looking for measuring)

What are expected VAs for 3, 6, and 24 month olds?

3mo = 3 cy/deg = 20/200

6 mo = 6 cy/deg = 20/100

24 mo = 24 cy/deg = 20/25

What 2 psychophysical methods were used to determine if babies can see color?

2 alternate forced choice

Method of constant stimuli

Do babies have color vision?

Yes but not until 1-3 months

What age do babies see color?

1-3 months