PSYC201: Fri 21/03 W4/12

Bottom-Up vs. Top-Down Processing

  • Bottom-up processing: Data-driven processing where perception is built from sensory input.
  • Top-down processing: Conceptually driven processing where prior knowledge and expectations influence perception.
  • Lorimer Mosley example: Feeling pain is determined by signals from the body and knowledge/experience.

Demonstration of Bottom-Up and Top-Down Processing

Cow Picture Experiment
  • Initial viewing of a poorly defined picture of a cow requires significant brain work due to limited data.
  • The brain struggles to recognize the image until an "aha" moment occurs.
  • Subsequent viewings are trivially easy because of pre-existing knowledge or top-down knowledge.
Real-World Examples
  • Face recognition: Difficult when context is missing (e.g., recognizing someone from class in a grocery store).
  • Top-down knowledge is essential for everyday perception.
  • These processes occur in memory, attention, and language processing.

Hallucination and Dreams

  • Hallucination: Perception without data.
  • Dreams: Perceptions occur without external data input.

Synesthesia

  • A phenomenon where sensory experiences are blended or crossed (e.g., tasting shapes).
  • Synesthesia is a form of neurodiversity.
  • Example: The man who tasted shapes.
Types of Synesthesia
  • Colors for numbers: Numbers evoke specific colors.
  • Touch from sound: Sound elicits tactile sensations.
  • Cross-modal correspondences: Overlap between senses that exist in most people, such as associating high pitches with looking up.
  • Synesthesia involves creating experiences that do not directly correspond to input.
  • Prevalence: Affects about 1-2% of the population.
  • Common among artists.
Examples of Synesthetes
  • Vladimir Nabokov.
  • A New Zealand winemaker perceives flavors as shapes.
Perception as a Subjective Experience
  • Synesthesia illustrates how the mind creates subjective reality based on experience and knowledge.
Spectrum of Synesthesia
  • Exists along a continuum, with varying degrees of sensory mixing.
  • Number lines: Numbers evoke a sense of spatial location.

Vision: The Eyeball and Retina

  • Vision involves the conversion of light into neural signals.
The Electromagnetic Spectrum and Visible Light
  • Visible light is a form of electromagnetic radiation.
  • Different frequencies correspond to different wavelengths.
  • Gamma rays, X-rays, ultraviolet rays, infrared, radar, FM, TV signals, and alternating current circuits are all electromagnetic energy.
Why Visible Light is Special
  • Photoreceptors respond to light in this range.
  • Small wavelengths (gamma rays, X-rays) pass through objects, while large wavelengths (radio waves) go around objects.
  • Visible light bounces off objects, enabling object detection.
Color as a Creation of the Mind
  • Color does not exist in the physical world.
  • It is an experience created by the brain based on signals from photoreceptors.
How Light Enters the Eye
  • Light passes through the pupil and is focused by the lens onto the retina.
  • The retina contains photoreceptors (neurons).
  • Photoreceptors (rods and cones) are located at the back of the retina.
  • Light passes through other layers of neurons before reaching photoreceptors.
  • Signals are sent from photoreceptors to other neurons, which then transmit signals to the optic nerve.
The Blind Spot
  • The optic nerve creates a blind spot where there are no photoreceptors.
  • Brain fills in the blind spot using top-down knowledge.
Rods and Cones
  • Two types of photoreceptors: rods and cones.
  • Distribution varies across the retina.
  • Fovea: Primarily cones, responsible for high-acuity vision.
  • Periphery: Mostly rods, sensitive to dim light.
    insertgraphondistributionofrodsandconesinsert graph on distribution of rods and cones
Three Types of Cones
  • Each cone type is most sensitive to a particular wavelength of light.
Rods
  • Sensitive to light around 500 nanometers.
Cones

  • Short (S) cones: Most sensitive to blue light (around 420 nm).

  • Medium (M) cones: Most sensitive to green light.

  • Long (L) cones: Most sensitive to yellow-orange light, referred to as red cones.

  • The brain determines color by analyzing the activity of these cones.

Color Blindness
  • Various forms of color blindness result from deficits in cone types.
    Deuteranopia: Red-green color blindness, more common in men (X-chromosome linked).
  • Tritanopia: Blue-yellow color blindness (rare).
  • Monochromacy: No cones at all (extremely rare).
  • Achromatopsia: Damage to the color vision part of the brain (V4), resulting in seeing the world in gray.
Calitrichids (New World Monkeys) and Color Vision
  • Old World apes (gorillas, chimpanzees, bonobos, orangutans) are trichromats, like humans.
  • Most other mammals are dichromats (e.g., dogs, cats).
  • Male calitrichids are dichromats, while females can be heterozygous (dichromats or trichromats).
Evolutionary Advantages of Dichromacy and Trichromacy
  • Trichromats: Better at foraging for food, especially berries (red-green distinctions).
  • Dichromats: Better at spotting camouflage (useful for sentries).
  • Having both dichromats and trichromats provides a societal advantage.
Vision and Sensory Systems
  • Receptors determine the experience; different sensory systems result in different experiences.
Wiring of Rods and Cones
  • Cones: Each cone connects to one or two bipolar cells (one-to-one mapping).
  • Rods: Many rods converge onto a single bipolar cell.
Implication of Wiring
  • Cones: Require more light to activate bipolar cells.
  • Rods: Summation of weak signals allows for sensitivity to dim light.
  • Peripheral vision (rods) is more sensitive to dim light than central vision (cones).
Dark Adaptation
  • Rods become more sensitive in the dark.
    Dim light is easier to see in peripheral vision.
Night Vision and Color
  • Cones do not function well in dim light, resulting in a lack of color vision at night.