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Chapter Two: Sensation and Perception
🦊 Learning Appetizer
Once upon a time, a couple traveled with their horse. When both rode, people said: “How cruel to overload the animal!” So the husband walked. Then critics said: “Selfish! Why let your wife ride alone?” He switched. Now: “Foolish man—letting his wife ride while he walks!” So both walked. Response: “Stupid! You have a horse but don’t use it!”
Reflection: This story reveals a profound truth: People do not share the same perception of reality. Psychology explores why! 😄
2.1. The Meanings of Sensation and Perception
🤔 Brainstorming Questions
- Have you heard: “You watch but don’t see” or “You hear but don’t listen”?
- Which part refers to sensation? Which to perception?
For example, when light hits your retina, or sound waves vibrate your eardrum, sensation occurs. It’s raw, unprocessed input—like pixels on a camera sensor.
Perception turns light patterns into a face, or sound waves into a melody. It’s where meaning emerges. 🧠
A man with brain damage suffered prosopagnosia—inability to recognize faces. He could see eyes, nose, and mouth (sensation), but couldn’t assemble them into a familiar face (perception). He once mistook his wife’s head for a hat!
→ This proves: Sensation ≠ Perception.
🔍 Reflection
Without perception, the world is just noise and light. With it, you recognize your mother’s voice in a crowd, read this sentence, and feel love. Perception makes reality intelligible. 😊
2.2. The Sensory Laws: Sensory Thresholds and Sensory Adaptation
A. Absolute Threshold
The absolute threshold is the minimum stimulation needed to detect a stimulus 50% of the time.
- **Hearing**: A ticking watch 20 feet away in quiet
- **Taste**: 1 teaspoon of sugar in 2 gallons of water
- **Smell**: 1 drop of perfume in a 3-room apartment
⚠️ But detection depends on response bias. In a dangerous neighborhood at night, you’ll “hear footsteps” even when none exist—your brain is primed to detect threats.
B. Difference Threshold (Just Noticeable Difference – JND)
The difference threshold is the smallest change in stimulation you can detect 50% of the time.
**Weber’s Law**: The JND is a constant proportion of the original stimulus.
→ For weight: JND = ~2%. So if you lift 50 oz, you need +1 oz to notice a difference. For 100 oz, you need +2 oz.
C. Sensory Adaptation
Sensory adaptation is the decreased sensitivity to constant stimulation.
- Jump into cold pool → shiver. Minutes later → “The water is fine!” ❄️
- Enter a smelly room → gag. Minutes later → smell disappears. 👃
**Why?** To free your brain to detect changes (e.g., a new sound, a moving shadow)—critical for survival! But you don’t adapt to intense pain or extreme cold—ignoring those could be fatal. ⚠️
💡 Reflection
Sensory adaptation explains why you stop noticing your watch, your ring, or background music. It’s not that the stimulus is gone—it’s that your brain says: “This is safe. I’ll ignore it.”
2.3. Perception
2.3.1. Selectivity of Perception: Attention
Your senses receive millions of bits of data per second—but you consciously process only a few. Attention is the filter that selects what enters awareness.
Attention divides experience into:
• Focus: What you’re fully aware of (e.g., the ball carrier in a football game)
• Margin: What you vaguely sense (e.g., crowd noise, cold feet)
What grabs attention?
- Size/Intensity: Big, bright signs; loud alarms 🔊
- Repetition: Repeated ads, constant notifications 🔁
- Novelty: A pink elephant in your classroom 🐘
- Movement: A flickering light, a waving hand ✋
But your mind also filters! Two internal factors:
• Set/Expectancy: A waiting father hears a baby cry; a student hears a phone ring.
• Motives/Needs: A hungry person notices food ads; a thirsty person spots water bottles.
Look at this sequence:
A 13 C D E F G
12 13 14 15 16 17 18
→ In the first row, you see the number 13 as the letter B!
→ In the second, it’s clearly 13.
Why? Your brain uses context to interpret sensory input! ✨
2.3.2. Form Perception
To see objects, your brain separates figure (object) from ground (background).
- Words (figure) on a page (ground)
- A melody (figure) over chords (ground)
Contours (edges created by color/brightness changes) define figures. That’s why:
• Chameleons change color to blend contours with surroundings 🦎
• Soldiers wear green uniforms to disrupt contours in forests 🎖️
Gestalt Laws of Organization (how we group elements):
- Proximity: We group nearby items → ●●● ●●● = two groups
- Similarity: We group similar items → 🔴🔴🔴 🔵🔵🔵
- Continuity: We see smooth lines, not broken ones → ↗️ not ↗️ ↘️
- Closure: We fill gaps → We see a ⬡ as a star, not disconnected lines
- Symmetry/Good Figure: We prefer balanced shapes → A symmetrical face feels “right”
2.3.3. Depth Perception
How do we see 3D from 2D retinal images? Through depth cues:
Binocular Cues (require both eyes):
- Retinal Disparity: Each eye sees a slightly different image. The brain uses the difference to calculate distance. (Try: Hold a finger close—close one eye, then the other. The background “jumps”! 👁️👁️)
- Convergence: Eyes turn inward to focus on close objects. Muscle tension = distance cue.
Monocular Cues (one eye enough):
- Linear Perspective: Parallel lines converge with distance (railroad tracks) 🚂
- Interposition: Closer objects block farther ones (your hand hides the moon) ✋
- Relative Size: Same-sized objects appear smaller when distant 🏙️
- Texture Gradient: Nearby textures are detailed; distant ones are smooth 🌾
- Aerial Perspective: Distant objects look hazy (mountains fade to blue) ⛰️
- Motion Parallax: When moving, close objects zip by; distant ones crawl 🚗
In WWII, pilots crashed into planes ahead because single taillights gave no depth cue. Solution: Two taillights. As distance closed, lights appeared to spread—pilots knew to pull up! ✈️
2.3.4. Perceptual Constancies
Despite changing sensory input, we perceive objects as stable:
• Size Constancy: A car 1 block away isn’t “smaller” than one nearby
• Shape Constancy: A door looks rectangular even when open (casting a trapezoid on your retina)
• Brightness Constancy: A white shirt looks white in dim light or sunlight
⚠️ Alcohol disrupts size constancy—contributing to drunk driving accidents! 🍷
2.3.5. Perceptual Illusions
Illusions reveal how perception *usually* works by showing where it *fails*.
- Moon Illusion: The moon looks huge at the horizon—but it’s the same size as overhead! Why? Horizon cues (trees, buildings) “scale” it as distant → brain inflates its perceived size.
- Müller-Lyer Illusion: Lines with arrowheads (→←) look shorter than lines with feathers (←→)—even when identical! Why? Feathers resemble building corners (distant); arrowheads resemble room corners (close). Distant = larger! 📏
🌌 Final Reflection
Sensation gives you data. Perception gives you your world. But your world is not “reality”—it’s a construction shaped by biology, experience, and expectation. That’s why two people can witness the same event—and swear they saw different things! 🤯
50 Complex Questions – Chapter Two: Sensation and Perception
Deeply Detailed Explanations for Freshman University Students | Based on General Psychology (Psy C 1011)
B) Sensation is hearing a melody; perception is hearing sound waves.
C) Sensation involves memory; perception involves attention.
D) Sensation occurs in the brain; perception occurs in the eyes.
B) Intact perception but impaired sensation
C) Sensory adaptation
D) Absolute threshold failure
B) 50% of the time
C) Only in complete silence
D) Only by trained observers
B) A fixed amount regardless of stimulus intensity
C) A constant proportion of the original stimulus
D) Higher for brighter lights than dim lights
B) You detect a faint sound in a quiet room
C) You see better in dim light after 30 minutes
D) You remember smells from childhood
B) Selective attention
C) Absolute threshold
D) Perceptual constancy
B) A repeated advertisement
C) A stationary object in a familiar environment
D) A sudden movement
B) Set/expectancy
C) Sensory threshold
D) Figure-ground perception
B) Marked differences in brightness or color
C) Smooth gradients
D) Auditory cues
B) Similarity
C) Principle of totality (Gestalt)
D) Closure
B) Close to you
C) Moving slowly
D) Large in size
B) The brain misapplies size constancy to 3D corners
C) Arrowheads create sensory adaptation
D) Feathers trigger proximity grouping
B) Convergence of the eyes
C) Retinal disparity
D) Linear perspective