Notes on Sensory Systems: Vision, Hearing, and Smell

Occipital Lobe and Vision

• Function: The occipital lobe is primarily responsible for processing vision.
• Cortical Blindness:
  • Occurs when there is a lesion or damage in the occipital area.
  • The physical eyes are unharmed, but the brain's ability to process visual input is impaired.
  • Results in a "blind spot" or partial blindness, known as cortical blindness.
• Visual Field Processing:
  • Each eye has a left and a right visual field.
  • Information from the right visual field is processed in the left occipital lobe.
  • Information from the left visual field is processed in the right occipital lobe.
• Lesion Example (Left Occipital Lobe):
  • A person with a lesion in their left occipital lobe would be unable to process visual information coming from their right visual field.
  • Hypothetical Scenario (Clock Drawing): If presented with a clock, a person with a left occipital lobe lesion would only be able to draw the left half of the clock (from $12$ through $6$ on the left side, as the right visual field processing is impaired).
  • This individual would perceive only half or a quadrant of their visual world.

Color Vision Theories

The Trichromatic Theory

• Retinal Cones: Our retina contains three types of cones: red, green, and blue.
• Electromagnetic Wave Processing: Each cone type processes a different range of electromagnetic wavelengths:
  • Red: Longest wavelength.
  • Green: Medium wavelength.
  • Blue: Shortest wavelength.
• Color Mixing (Light): Analogous to mixing food coloring, but for light:
  • Red and green light, when mixed in equal parts, produce yellow light.
• Perception of Other Colors: Other colors in the spectrum are perceived through the activation of one cone frequency relative to the frequency of other cones.
• Individual Variability:
  • Cones are not equally distributed across an individual's retina.
  • Some people have more red, green, or blue cones than others.
  • This variability leads to different perceptions of color (e.g., the "what color is the dress?" example).
  • This highlights that each person has their own reality based on how their brain processes visual stimuli.
  • The original dress color discussed was black and blue, although perceived differently by many.

The Opponent Process Theory

• Limitation of Trichromatic Theory: The trichromatic theory explains how we process color but cannot explain negative afterimages.
• Mechanism: This theory proposes the existence of ganglion cells that have opposing color pairs.
  • Three main types of opposing ganglion cells:
    1. Red / Green
    2. Blue / Yellow
    3. Black / White (related to brightness)
• Color Processing Example (Red/Green):
  • To see red, the red cone must be activated significantly more than the green cone (more long wavelengths than medium wavelengths).
  • To see green, the green cone must be activated more than the red cone (more medium wavelengths than long wavelengths).
• Color Processing Example (Yellow/Blue):
  • To see yellow, there must be increased activation of both red and green cones together, with less activation of the blue cone.
  • To see blue, there must be increased activation of the blue cone, with less activation of both red and green cones.
• Negative Afterimages Explanation:
  • Receptor Fatigue: Staring intensely at a specific color (e.g., blue) for an extended period ($30$ seconds) fatigues the receptors for that color.
  • This fatigue depletes the pigmentation associated with that color in the processing pathway, leading to fewer action potentials firing.
  • Rebound Effect: When the gaze shifts to a neutral (e.g., white or grayscale) background:
    • The fatigued receptors (e.g., blue cones) have not yet recovered or repolarized to their homeostatic balance.
    • Because the blue receptors are fatigued (less active), the opposing color pathway (yellow) becomes dominant.
    • Therefore, a person fatigued by blue will see a yellow afterimage on a white background, demonstrating the opponent process (e.g., the bumblebee on a pink flower, originally blue, becomes yellow).
• White Perception: To see white, all cones (blue, red, and green) must be highly activated; they effectively cancel each other out when all are maximally stimulated.

The Auditory System

Basic Sound Properties

• Loudness: Related to the amplitude of a sound wave.
  • Higher amplitude = louder sound.
  • Measured from the baseline to the crest (peak) of the wave.
• Pitch: Related to the frequency of a sound wave.
  • Higher frequency = higher pitch (e.g., a shriek).
  • Lower frequency = lower pitch.
  • Frequency is the amount of sound wave oscillations within a second.

Ear Anatomy and Sound Transmission

• Outer Ear Structures:
  • Pinna (Auricle): The visible outer ear structure.
    • Function: Helps to locate sound and direct sound waves into the ear canal (e.g., turning your head to hear better).
  • Ear Canal (Auditory Canal): A passage that channels sound waves from the pinna to the tympanic membrane.
• Middle Ear Structures:
  • Tympanic Membrane (Eardrum): A thin membrane at the end of the ear canal.
    • Function: Vibrates at the same frequency as the incoming sound wave.
  • Ossicles: Three small bones that transmit and amplify vibrations from the tympanic membrane to the inner ear.
    • Malleus (Hammer): Connected to the eardrum.
    • Incus (Anvil): Connects the malleus to the stapes.
    • Stapes (Stirrup): Pressures the oval window.
    • Function: The ossicles create pressure from the vibrating sound wave and transmit it to the oval window.
• Inner Ear Structures:
  • Oval Window: A membrane-covered opening that receives vibrations from the stapes.
    • Function: Transmits the vibrations into the fluid-filled cochlea of the inner ear.
  • Cochlea: A snail-like, fluid-filled structure.
    • Contains two types of fluid: perilymph and endolymph (not cerebrospinal fluid), which surround the basilar membrane.
  • Basilar Membrane: A membrane within the cochlea.
    • Houses Hair Cells: These are the auditory receptors.
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