BVP Book 3 CH2

What is one advantage of having two eyes, even without binocular overlap?

Animals can compare light levels between eyes to orient towards or away from light sources.

How do marine animals use two eyes to orient themselves?

They detect which eye is uppermost based on which receives more light, allowing vertical orientation.

What is the benefit of lateral visual fields in prey species?

They provide wide panoramic vision to detect predators.

What is the benefit of overlapping visual fields in predator species?

They provide stereopsis, improving depth perception for hunting.

What does stereopsis do for animals in low light?

It increases the signal-to-noise ratio, helping overcome camouflage.

What is unique about chameleon vision?

They can move their eyes independently to see two objects at once but can switch to stereoscopic vision to judge prey distance.

What is special about hammerhead shark binocular vision?

They have 360° visual fields and overlapping fields that provide stereopsis both in front and behind.

How much of the optic nerve fibers decussate in reptiles and birds?

100% (full decussation).

How much of the optic nerve fibers decussate in primates?

About 50% (partial decussation).

Why is partial decussation important?

It allows integration of information from both eyes into the same side of the visual cortex, enabling binocular vision.

Which visual pathway population also sends input to the superior colliculus for saccadic eye movement control?

Retinal ganglion cells.

Which reflex eye movements are inborn and not learned?

Vestibulo-ocular reflex, optokinetic reflex, and pupillary reflexes.

Which visual reflexes are learned?

Fixation, re-fixation, version, vergence, and fusion reflexes.

What did Hubel & Wiesel (1965) show about kittens raised in darkness?

They behaved as if blind when later exposed to light, proving visual development requires environmental exposure.

Which visual pathway (P or M) is immature until about 11 years?

Parvocellular (P) pathway (high contrast, high spatial frequency).

What is represented by ocular dominance columns?

Alternating cortical cell groups that receive input from right and left eyes in the visual cortex.

What happens to ocular dominance columns when one eye is occluded during the critical period?

Columns receiving input from the occluded eye shrink, and few binocularly driven cells remain.

What is required for normal binocular development?

Detailed form vision (patterned visual input).

What is the critical period for kitten ocular dominance development?

Between 4 weeks and 3 months of age.

When does stereopsis first appear in monkeys?

Between 4–6 weeks of age.

At what age does stereopsis first appear in humans?

About 4 months of age.

Define "critical period" in visual development.

A sharply defined period after which a certain developmental phenomenon can no longer occur.

Define "sensitive period" in visual development.

A gradually beginning and ending period with maximum sensitivity at its peak.

When is the peak of the human sensitive period for most visual skills?

About 2 years old.

When is the general sensitive period largely over?

By 9 years old.

What is the risk of untreated congenital cataract until age 3?

That eye will never develop normal visual acuity.

What is preferential looking (PL) testing used for?

Assessing visual acuity in infants by observing fixation of striped or patterned cards.

What is the approximate visual acuity at birth (by PL)?

6/240 (or ~6/300).

When does visual acuity reach 6/6 in normal children?

By about 3 years of age.

What is the "crowding phenomenon" in visual acuity?

Worse acuity when optotypes are presented with nearby distractors, seen until ~10 years old.

What is contrast sensitivity (CS)?

Ability to detect gratings of various contrasts and spatial frequencies.

At what spatial frequency is adult CS best?

~4 cycles per degree (detected at 1% contrast).

When does infant CS rapidly improve?

By 3 months (better for low spatial frequencies).

By what age do PL psychophysical CS values reach adult levels?

By 9–14 years.

What is the relationship between binocular CS and monocular CS?

Binocular CS is slightly better in adults; difference not seen in children except at 12 months.

When does accommodation become accurate in infants?

By about 7 months (similar to convergence).

When does accommodative convergence develop?

From about 1 month of age.

When does fusional convergence rapidly develop?

Between 2 and 6 months of age.

What is the average refractive error at birth?

+2.00 D hypermetropia (SD ~2 D).

When is emmetropisation mostly complete?

By 12 months (reduces ametropia to +0.50 D by age 6).

What is the risk of not correcting hypermetropia of +3.50 D or more after age 1?

Increased risk of amblyopia and strabismus.

How does anisometropia relate to risk of heterotropia?

Anisometropia of ≥1 D with hypermetropia of 2–3 D increases risk.

Name four contributors to head stabilisation in infants.

Cervico-collic reflex, vestibulo-collic reflex, voluntary neck muscle control, inertial forces.

What is the function of the cervico-ocular reflex?

Initiates eye movements to compensate for head movement but is secondary to vestibular system.

How do adults shift fixation to a peripheral target?

With a single saccade landing within 10% of target position, then a small corrective saccade if needed.

How do infants shift fixation before 5 months?

By a series of small saccades with slower initiation.

At what age do vertical saccades become possible?

About 5–6 weeks old.

When is smooth pursuit eye movement first possible in infants?

~4 weeks old for large slow targets.

When do infants achieve near-adult smooth pursuit speeds?

After infancy, reaching ~100°/s in early adulthood.

What is optic flow?

The movement of the visual field across the retina during self-motion, aiding spatial awareness.

What is optokinetic nystagmus (OKN)?

Combination of smooth pursuit and fast saccadic recovery eye movement to follow moving objects.

When does OKN first appear?

Within a few hours after birth.

What is naso-temporal OKN asymmetry?

Immature monocular response where temporalward OKN is weaker or absent until ~5 months.

What is the vestibulo-ocular reflex (VOR)?

Reflex eye rotation opposite to head movement to maintain stable fixation.

When does VOR appear?

Present from birth.

When is fusional vergence reflex regularly present?

At 4–6 months of age.

When does fixation disparity detection first occur?

About 3.5 months (50–100 arcmin stimulus).

When does fusion and stereopsis first develop?

From about 2 months of age.

When does stereopsis for crossed disparity first appear?

~12 weeks.

When does stereopsis for uncrossed disparity first appear?

~17 weeks.

When does fine stereopsis (60 sec arc) become measurable?

Between 6 months and 3 years depending on test method.

What is dynamic stereopsis?

Ability to judge depth of moving targets relative to others.

What is motion-in-depth perception?

Detection of change in depth, better for looming (approaching) objects than withdrawing.

At what age do VEPs first show motion detection?

~8 weeks, improving rapidly by 13 weeks.

What is ocular dominance?

The tendency for one eye to provide stronger input or fixation preference than the other.

What is the cyclopean eye position relative to ocular dominance?

Usually deviated slightly toward the dominant eye.

How can eye dominance be tested subjectively?

By pointing to a distant target with both eyes open, then closing one eye at a time to see which eye aligns.

List three categories of hazards to binocular vision development.

Anatomical anomalies, sensory/neurophysiological anomalies, cortical adaptations (e.g., suppression).

Give an example of cortical adaptation that may affect binocular vision.

Anomalous retinal correspondence or facultative suppression.

What is a visual cliff experiment used to assess?

Depth perception development in infants (heart rate increases at apparent drop).

At what age does visual cliff response begin?

6–8 weeks of age.

What does the appearance of ocular dominance columns indicate in infants?

The onset of cortical pathways for binocular integration.

Most reptiles and birds have f_ decussation.

Full

In primates the proportion of optic nerve fibres that decussate is about __ per cent.

50

In primates the visual information from both eyes is received together in the __ cortex.

Striate

This information is taken to binocular cells in the columns.

Ocular dominance

The columns may receive predominant input from __ eye, or else equally from both.

Either

Visual skills such as acuity, contrast sensitivity, accommodation and convergence have a stage called the .

Sensitive period

Before the sensitive period, development proceeds by genetic control rather than response to the __.

Environment

Other physiological attributes such as vestibulo-ocular reflex are not acquired but are __.

Innate