Visual development

1. Introduction to Visual Development and Governance

  • Vision for Action vs. Perception: The lecture differentiates between the governance of vision (how we direct our gaze) and the development of vision for action (how visual information guides motor output).

2. Ocular and Neural Structures of the Visual System

  • Optical/Ocular Components:

    • Lens and Cornea: Primary refractive elements. The cornea provides the majority of the refractive power, while the lens allows for accommodation.

    • The Retina: A complex neural tissue layer where phototransduction occurs. It consists of photoreceptors, bipolar cells, and horizontal cells.

    • The Optic Nerve: Formed by the axons of Retinal Ganglion Cells (RGCs) representing the final output of the eye.

  • Neural Pathway Structures:

    • Optic Chiasm: The site of decussation where 50%50\% of fibers (nasal) cross to provide binocular integration in the brain.

    • Lateral Geniculate Nucleus (LGN): A six-layered thalamic relay station. Layers 1−21-2 (Magnocellular) handle motion; layers 3−63-6 (Parvocellular) handle detail/color.

    • Primary Visual Cortex (V1/Striate Cortex): Located in the occipital lobe; the first site of complex cortical processing and binocular neurons.

    • Extrastriate Areas: Further processing in the Ventral Stream (object recognition, "what") and Dorsal Stream (spatial/action, "where/how").

3. Photoreceptor Specialization and Signal Processing

  • Rod Cells: Highly sensitive to low light (scotopic); concentrated in the periphery.

  • Cone Cells: Responsible for photopic vision and acuity. Concentrated in the fovea centralis.

    • Three types: Long (LL, red), Medium (MM, green), and Short (SS, blue).

  • Signal Processing: Light triggers hyperpolarization in photoreceptors, passing signals via bipolar cells to RGCs, which generate the action potentials sent to the brain.

  • Photoreceptors in the eye highly immature at birth and take years to become adult-like (~4 yrs)

  • OS = outer segment. Short OS are inefficient at detecting light

  • Immature (stubby) photoreceptors take up much room so do not resolve visual detail well

  • Spatial resolution (visual acuity) is poor

  • acuity improves drastically with age. Adult vision standards are not suitable for infants and young children.

  • “Normal vision” varies across age groups and is test-dependent

4. Developmental Trajectories of Visual Processes

  • Initial State: At birth, infants are largely subcortically driven with poor foveal development and low acuity (6/1206/120).

  • Structural Trajectory: Cones migrate toward the fovea and become more densely packed over the first year of life.

  • Functional Trajectory:

    • Acuity: Improves rapidly in the first 66 months, reaching adult levels by approximately 4−104-10 years.

    • Stereopsis (3D)/ binocular depth perception: Emerges suddenly between 2.5−52.5-5 months.

      • Stereo vision (as in 3D cinemas) requires comparing slight differences between the image in the left and right eye

      • Many people (~1:10) have poor stereopsis

      • Infants like looking at 3D objects (on right) rather than 2D objects (on left)

      • Development determined by postnatal age: suggests driven by post-natal visual experience rather than biologically pre-determined by gestational age

    • Color perception: Counter to common belief, infants can see color from birth – but only red-green (Adams, 1995).

      • Blue-yellow sensitivity develops around 2 months (Zemach et al., 2007).

      • Color pop-out only emerges around 5 months (Gerhardstein et al., 1999)

    • Colour contrast sensitivity: Develops slowly, continuing to refine into late childhood (age 1111). Color contrast sensitivity develops gradually over the first years.

    • Colour constancy: develops gradually over the first 2 years, and this is linked to language development. less stable to them in different lighting.

    • Object perception: Soon after birth, babies can tell apart different shapes like squares and trapezoids. ~3 months, they start understanding that different shapes can belong to the same category but this is first visually, not conceptually driven.

    • Object permanence: at around 8-9 months infants start understanding that objects persist

    • Clutter: recognizing objects in clutter or hard-to-decode circumstances continues to develop until age 10 years

    • Face perception: Newborn infants show a strong preference for faces from early birth, but this is driven by low-level features. By 4 months infants start showing preferences for familiar faces (caretaker species, gender, race). But: face perception and the brain regions supporting it develop until beyond age 10 years.

5. Methods for Assessing Visual Development

  • Behavioral Measures:

    • Preferential Looking (PL): Exploits the infant's natural tendency to look at patterns rather than plain stimuli. Teller Acuity Cards are commonly used.

    • Optokinetic Nystagmus (OKN): involuntary, jerky eye movement reflex to large, moving visual fields, like watching scenery from a train, involving a slow eye pursuit in the direction of the moving scene (slow phase) and a fast corrective flick back (quick phase)

  • Electrophysiological Measures:

    • Visual Evoked Potentials (VEP): Recording EEG signals from the occipital cortex in response to visual stimuli. VEP often indicates higher acuity levels than behavioral measures because it bypasses motor requirements.

  • Clinical Assessment:

    • Adult acuity test

    • Hiding Heidi: A low-contrast face test for infants and non-verbal children.

    • Cardiff Acuity Cards: Uses "vanishing" pictures to test acuity through vanishing optotypes.

6. Neuroplasticity and Critical Periods

  • Experience-Dependent Development: Requires balanced input from both eyes. Failure to receive input results in cortical reorganization.

  • Critical Period (0−70-7 years): The window where the visual system is most sensitive to environmental input. This is the vital time for treating conditions like Amblyopia.

7. Visual Disorders and Cognitive Milestones

  • Amblyopia (Lazy Eye): Reduced vision due to abnormal development (strabismus or anisometropia). Treated with Patching Therapy to strengthen neural pathways.

  • Object and Face Recognition: Infants show an innate preference for faces, which undergoes a "tuning" process based on environment.

  • Visual Crowding: Children are more susceptible to "clutter." The ability to recognize objects in crowded scenes is a key marker of visual maturity.

8. Research and AI Applications

  • Computer Vision: Modeling human edge, motion, and depth detection to improve AI algorithms.

  • Assisted Technologies: Understanding sensory deprivation helps create better support for those with congenital blindness or low vision.