Visual Perception
🔹 Introduction
Visual perception = Complex integration of:
Retina + Cortex → encode and process visual image.
1️⃣ Light Sense
👉 Awareness of light; ability to detect illumination.
Light Minimum = minimum brightness required to evoke light sensation.
Measured after 20–30 minutes dark adaptation.
Human eye functions in a wide illumination range → Visual adaptation.
a) Visual Adaptation
Eye adjusts to decreasing illumination.
Example: entering a dim room from sunlight.
Dark adaptation curve:
Initial cone adaptation (small curve).
Rod adaptation (main curve).
Retina becomes ~100,000 times more sensitive in dark.
Clinical Note: Delayed dark adaptation in retinitis pigmentosa & Vitamin A deficiency.
From dim → bright light.
Initial discomfort → retina adapts quickly.
Occurs within 5 minutes.
Essentially disappearance of dark adaptation.
2️⃣ Form Sense
👉 Ability to discriminate shapes of objects.
Mediated mainly by cones.
Maximum at fovea, decreases towards periphery.
Clinically measured by Snellen’s chart.
Components of Visual Acuity
Minimum Visible → detect object presence.
Resolution (Ordinary Visual Acuity) → discriminate 2 separate points.
Minimum separation ≈ 0.004 mm (cone diameter).
Requires 1-minute visual angle at nodal point → MAR (Minimum Angle of Resolution).
Tests: Snellen’s letters, Landolt’s rings, gratings, checkerboards.
Recognition → not just discrimination, but identification (e.g. faces, letters).
Minimum Discriminable (Vernier Acuity) → detect alignment/misalignment of lines.
3️⃣ Sense of Contrast
👉 Ability to perceive small luminance differences.
Important for “clarity” of vision.
Reduced contrast sensitivity → fogginess of vision.
Causes of Reduced Contrast
Measurement of Contrast Sensitivity
Pelli-Robson chart (triplets of low-contrast letters)
FACT (Functional Acuity Contrast Test)
4️⃣ Colour Sense
👉 Ability to discriminate between different wavelengths of light.
Function of cones → best in photopic vision.
In dim light → all colours appear grey → Purkinje shift.
Theories of Colour Vision
Trichromatic Theory (Young–Helmholtz)
3 types of cones:
L-cones (Red-sensitive, erythrolabe) → peak at 565 nm.
M-cones (Green-sensitive, chlorolabe) → peak at 535 nm.
S-cones (Blue-sensitive, cyanolabe) → peak at 440 nm.
Colour perception = mixture of these signals.
Genes:
Rhodopsin → Chromosome 3
Blue cone → Chromosome 7
Red & Green cones → X chromosome (q arm).
Opponent Colour Theory (Hering)
Some colours are mutually exclusive (e.g. no reddish-green).
Occurs at ganglion cell level.
Two types of opponent cells:
Red-Green opponent cells.
Blue-Yellow opponent cells.
Integration of Both Theories
Trichromacy at receptor (cone) level.
Colour opponency from ganglion cells onwards.
📝 Conclusion
Visual perception = integrated function of light, form, contrast, and colour senses.
Retina + cortex in brain→ encode, process & interpret visual world.
Clinical tests (Snellen’s, contrast charts, colour vision tests) help diagnose visual pathway disorders