Vision and the Eye

• Vision and visual processes are described
• Hearing, body senses & balance, and smell & taste will be covered later

Basics of Vision

• Structure of the eye includes the cornea, pupil, iris, lens, retina, and optic nerve
• Cataracts cloud the eye's natural lens, located behind the pupil
  • Proteins break down and clump together, causing blurred or distorted vision
• Astigmatism is a condition where an irregularly shaped cornea causes uneven light focus on the retina
  • This results in blurred or distorted vision
• Color blindness is the difficulty or inability to distinguish certain colors
• Glaucoma damages the optic nerve
  • This damage can lead to vision loss if not treated
• Macular Degeneration (AMD) is a progressive disease that damages the macula
  • The macula is the central part of the retina responsible for sharp, detailed vision, leading to vision loss

Eye Tests

• Astigmatic Dial Test screens for astigmatism, where the cornea's shape is more oval than round
• Ishihara Color Vision Test screens for color blindness
• Color Contrast and Sensitivity test checks sensitivity to slight differences in color
  • It helps detect optic nerve issues like inflammation ("optic neuritis")
• Visual Field Test screens for glaucoma
• Amsler Grid Test screens for macular degeneration

Nature of Sensation and Perception

• Sensation is the registration of physical stimuli from the environment by sensory organs
• Perception is the subjective interpretation of sensations
• Rubin's Vase is a reversible ambiguous image
• Cheetah Perceptual Illusion is where the viewer has to determine which head goes with which cheetah
• Selective Awareness means people are consciously aware of only part of the sensory processing their brain does
• Color Constancy is the visual system's ability to perceive the stable color of an object despite changes in illumination

Predictive Perception

• The brain actively constructs perception by making predictions based on prior knowledge and sensory input
• The brain refines predictions based on incoming information from the environment
• This is an example of “top-down processing”
  • This cognitive process uses existing knowledge, expectations, and context to interpret new sensory information

Common Features of Sensory Systems

• Common Pathway: Receptors relay signals between the receptor and neocortex for sensory coding & representation, leading to Perception
• Sensory Receptors are specialized cells that transduce (convert) specific types of environmental energy into neural activity
  • Visual System receptors (rods and cones) convert light energy into chemical energy, triggering an action potential
  • Auditory System converts air pressure waves into mechanical energy, triggering an action potential
  • Somatosensory System converts mechanical energy (touch, pressure, pain), triggering an action potential
  • Gustatory/Taste System converts chemical energy (molecules in food), triggering an action potential
  • Olfactory System converts chemical energy (molecules in air), triggering an action potential
• Receptive Field (visual field) is the region of sensory space in which a stimulus modifies a receptor's activity
  • Each eye has its own visual field slightly offset from each other
• Density of receptors is related to sensory sensitivity (acuity)
  • Human vision is sharper in the center vs. periphery because there are more visual receptors
• Tactile receptors are more numerous on the fingers compared to the arm, resulting in fingers discriminating touch better
• The human visual system uses different receptors for color vision than for black-white vision
• Simplified Sensory Pathways to the Cerebral Cortex for vision, hearing, and touch are shown
• All mammals have at least 1 primary cortical area for each sensory system
  • In vision, the primary area is called V1
• Visual processing uses the most dedicated brain space
• Synesthesia is the mixing of the senses where you hear in color or identify smells by how the scent sounds
  • It is due to crossing of the signals in the pathway
• Topographic Map is a spatially organized neural representation of the external world
  • Retinotopic Map is a spatial organization in the brain where neurons are arranged according to their receptive fields in the visual field
  • Nearby locations in the visual field are represented by nearby neurons in the brain, contralaterally and inverted
  • Homunculus is a model that reflects the topographic map in the sensorimotor cortex
  • Disproportionately large areas control the body parts used to make the most skilled movements

Visual System's Functional Anatomy

• Structure of the Retina
  • Retina is the light-sensitive surface at the back of the eye consisting of neurons and photoreceptor cells
  • Photoreceptor is a specialized retinal neuron that transduces light into neural activity
  • Humans see a small section of the Electromagnetic Spectrum (400-700 nanometers)
  • There are 2 types of human photoreceptors with light absorbing pigments:
    • Rods are specialized for functioning at low light levels and function mainly in dim light conditions, more numerous, all rods have the same pigment
    • Cones are specialized for color and high visual acuity (fine detail)
      • They do not respond in dim light
      • Densely packed at the fovea (located in the center of the macula)
      • Three different types of cone pigments absorb light across a range of visible frequencies
        • Short Cone (absorbs bluish light)
        • Medium Cone (absorbs greenish light)
        • Long Cone (absorbs reddish light)
    • Optic Disc is the small area of the retina where blood vessels enter and exit the eye and where fibers leading from retinal neurons form the optic nerve heading to the brain
      • There are no photoreceptors in this part of the retina which leads to a "blind spot" in vision
• Optic Nerve is the second cranial nerve responsible for transmitting visual information from the retina to the brain
  • Optic Chiasm is the junction of the optic nerves where axons from the nasal halves of the retinas cross to the brain's opposite side
  • The left half of each optic nerve goes to the left side of the brain, and the right half goes to the brain's right side
• Visual Streams

Advancements in Neuroscience

• Advancements that consider technological advancements, research difficulties, and neuroethics