Session 8 a - Visual Coding

Visual Coding Biopsychology 1

General Principles of Perception

  • The brain interprets signals from the retina based on the type of nerve:

    • Johannes Muller’s Law of Specific Nerve Energies:

      • Visual nerve = vision

      • Auditory nerve = sound

      • Olfactory nerve(sense of smell) = odors

Anatomy of the Eye

-Photoreceptor cells=specialized cells in the retina of the eye that detect light and convert it into electrical signals that can be processed by the brain, allowing us to see(rods & cones)

  • Components of the Eye:

    • Rods=photoreceptor cells in the retina that helps us see in low/dim light but do not detect color

    • Cones=photoreceptor cells in the retina that helps us see colors and detailed vision in bright light

    • Iris (colored part)=muscular ring that controls the size of the pupil(regulates how much light enters the eyes)

    • Fovea=responsible for the sharpest and most focused part of your vision.

    • Blind Spot=area of the retina without any photoreceptor cells thus cant process vision

    • Pupil=lens help focus light on the retina

    • Cornea=Focuses light as it first enters the eye, helping direct it toward the lens

    • Lens=Focuses light helping direct it toward the retina

    • Ciliary Muscle=change the shape of the lens for focusing

    • Retina=captures light and sends signals to the brain to create vision

  • An image is inverted on the retina; the optic nerve exits on the nasal side.

Visual Receptors: Rods and Cones

-periphery=outer edge of something

-retina’s periphery=outer edges of the retina, the light-sensitive layer at the back of your eye. responsible for motion detection and peripheral vision but doesnt have detailed vision that the center of the retina (fovea) does.

  • Rods: Located in the retina's periphery; sensitive to faint light.

  • Cones: Many located in the fovea; essential for color vision in bright light.

  • Rods:Cones ratio = 20:1 in the retina. Cones provide 90% of the brain's visual input.

  • Variation in optic nerve axons and visual cortex size is often genetic.

  • Photopigments release energy when struck by light.

Visual System of the Vertebrate

-bipolar cells=so bipolar cells are neurons in the retina that receive signals from the photoreceptor cells (rods and cones) and sends signals to the ganglion cells.

-Amacrine cells=specialized neurons found in the retina that make the visual signals more clear

  • Process of visual information:

    • Messages travel from receptors to bipolar cells.

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    • Bipolar cells send information to ganglion cells.

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    • Ganglion cells' signals combine and go to the brain.

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    • Amacrine cells contribute to shape, movement, lighting changes, and color detection.

Visual Characteristics

  • Foveal Vision:

    • Receptors: Only cones.

    • Good detail and color vision.

  • Peripheral Vision:

    • Proportion of rods increases; poor detail and color discrimination.

    • Each ganglion cell receives input from multiple cone receptors.

Color Vision

  • Visible light is a type of energy that moves in waves, and our eyes can only see waves within 400 nm to 700 nm range.

  • Trichromatic (Young-Helmholtz) Theory: Color perception based on three kinds of cones sensitive to different light wavelengths:

    1. S-cone(short wave length)=sensitive to blue light

    2. M-cone(medium wave length)=sensitive to green light

    3. L-cone(long wave length)=sensitive to red light

  • Opponent Process Theory: Describes color by organizing it into opposing pairs on a scale:

    • Red vs. Green, Yellow vs. Blue, White vs. Black.

    • “Fatigue response” leads to seeing the opposite color to keep it balance

  • Color Vision Deficiency:

    • Caused by genetic factors affecting cone development.

    • Red-green deficiency: occurs due to when the X chromosome made the pigment, the red cones and green cones are using the same pigment instead of different ones. Thus they cant differenciate between the two

Visual Experiments

  • Eye Pressure Experiment:

    • Rub your eyes and observe seeing spots/flashes of light in darkness.

    • Explanation: Mechanical pressure excites visual receptors, perceived as light.

  • Negative Color Afterimage Experiment:

    • Stare at a point for 1 minute under bright light, then look at a white surface to see the negative color afterimage.

  • Afterimage Experiment:

    • Observe the colors after staring at a colored object and viewing a white page.

  • Retinex Theory:

    • Integrates information from the retina and cortex for brightness and color determination.

    • Highlights color constancy despite lighting changes.

  • Retinex Theory Experiment:

    • Close focus on the center of an object and observe changes.

Additional Information

  • Blind Spot:

    • Definition: Point where the optic nerve exits, having no receptors.

  • Fovea and Retinal Periphery:

    • Fovea: Specialized for acute and detailed vision. No blood vessels or ganglion axons interfere. Has many receptors connected to a single bipolar cell.

    • Peripheral Retina: More receptors converge to bipolar and ganglion cells. Increased sensitivity to dim light.