UNIT 2: Sensation and Perception

🔹 Sensation vs. Perception

• Sensation: Detecting stimuli from the environment (bottom-up).

• Perception: Organizing and interpreting sensory information (top-down).

• Bottom-Up Processing: Analysis begins with sensory receptors and works up to brain.

• Top-Down Processing: Constructing perceptions based on experience and expectations.

🔹 Thresholds

• Absolute Threshold: Minimum stimulation needed to detect a stimulus 50% of the time.

• Difference Threshold (Just Noticeable Difference): Smallest detectable difference between two stimuli.

• Weber’s Law: To perceive a difference, stimuli must differ by a constant minimum percentage (not amount).

  • Example: You’ll notice a 10% increase in weight more than a 1 lb increase.

🔹 Signal Detection Theory

• Predicts how and when we detect faint stimuli amid background noise. Depends on:

  • Experience, expectations, motivation, and fatigue.

  • Hit: Stimulus present + detected

  • Miss: Stimulus present + not detected

  • False Alarm: Stimulus absent + reported

  • Correct Rejection: Stimulus absent + not reported

🔹 Sensory Adaptation

• Diminished sensitivity to a constant stimulus over time (e.g., no longer noticing a strong perfume).

• Does NOT occur with vision (our eyes are always moving—saccades).

🔹 Transduction

• Definition: Conversion of one form of energy into another.

• In sensation, it’s converting stimulus energies (light, sound) into neural impulses the brain can interpret.

🔹 Vision

Pathway of Light:

1. Cornea: Protects eye, bends light.

2. Pupil: Small adjustable opening.

3. Iris: Muscle that controls pupil size.

4. Lens: Focuses light on retina (accommodation = lens changing shape).

5. Retina: Contains receptor cells (rods and cones).

6. Fovea: Central focus point; highest concentration of cones.

7. Optic Nerve: Carries neural impulses to brain.

8. Occipital Lobe (Visual Cortex): Interprets visual info.

Photoreceptors:

• Rods: Peripheral vision, black/white, low light.

• Cones: Color, detail, work in bright light.

• Blind Spot: Where optic nerve leaves the eye—no receptors.

• Feature Detectors (Hubel & Wiesel): Specialized neurons in visual cortex for shape, angle, movement, etc.

Theories of Color Vision:

• Young-Helmholtz Trichromatic Theory: 3 cones (red, green, blue) mix to form all colors.

• Opponent-Process Theory: Cells process pairs (red-green, blue-yellow, black-white); explains afterimages.

🔹 Hearing (Audition)

Sound Wave Properties:

• Amplitude = Loudness

• Frequency = Pitch

• Wavelength = Determines pitch

• Decibels: Unit of loudness (above 85 dB = hearing loss risk)

Ear Structures (Sound Pathway):

1. Outer Ear (Pinna): Funnels sound.

2. Auditory Canal →

3. Eardrum (Tympanic Membrane): Vibrates.

4. Middle Ear Bones (Ossicles): Hammer, anvil, stirrup amplify sound.

5. Cochlea (Inner Ear): Fluid-filled, where transduction happens via hair cells on basilar membrane.

6. Auditory Nerve → Thalamus → Temporal Lobe (Auditory Cortex)

Theories of Pitch Perception:

• Place Theory: Different pitches stimulate different parts of cochlea (explains high-pitched sounds).

• Frequency Theory: Firing frequency of auditory nerve matches sound frequency (explains low-pitched sounds).

• Volley Principle: Neurons alternate firing to account for high-frequency sounds.

🔹 Touch

• Skin Senses: Pressure, warmth, cold, pain.

• Only Pressure has its own dedicated receptors.

• Pain:

  • Gate-Control Theory (Melzack & Wall):

    • Spinal cord has a gate to block/allows pain.

    • Large fibers close gate; small fibers open it.

  • Endorphins: Natural painkillers.

  • Phantom Limb Pain: Brain misinterprets signals from missing limb.

🔹 Other Senses

Kinesthesia:

• Sense of body part movement and position.

• Receptors in muscles, joints, tendons.

Vestibular Sense:

• Sense of balance, head position.

• Controlled by fluid movement in semicircular canals and vestibular sacs in the inner ear.

Taste (Gustation):

• 5 basic tastes: Sweet, salty, sour, bitter, umami.

• Taste buds regenerate every ~2 weeks.

• Influenced by smell (80% of flavor = smell).

Smell (Olfaction):

• Only sense that bypasses thalamus.

• Receptors in nose → Olfactory bulb → Olfactory cortex (limbic system – emotion/memory connection).

🔹 Perceptual Organization (Gestalt)

• Figure-Ground: Object = figure, background = ground.

• Grouping Principles:

  • Proximity: Group close objects together.

  • Similarity: Group similar things.

  • Continuity: Perceive smooth patterns.

  • Closure: Fill in gaps.

  • Connectedness: Uniform/linking = grouped.

🔹 Depth Perception

• Visual Cliff: Used to test depth perception in infants (Gibson & Walk).

• Binocular Cues (2 eyes):

  • Retinal Disparity: Brain compares images from both eyes; greater disparity = closer object.

  • Convergence: Eyes turning inward = object is close.

• Monocular Cues (1 eye):

  • Relative size: Smaller = farther.

  • Interposition: Closer object blocks another.

  • Relative height: Higher = farther.

  • Linear perspective: Lines converge in distance.

  • Texture gradient: Detail = closer.

  • Relative motion: Things closer move faster across vision.

🔹 Perceptual Constancy

• Shape Constancy: Same shape despite angle.

• Size Constancy: Same size despite distance.

• Color Constancy: Perceive same color in different lighting.

🔹 Extrasensory Perception (ESP)

• Telepathy: Mind-to-mind communication.

• Clairvoyance: Perceiving remote events.

• Precognition: Seeing the future.

• Psychokinesis: Mind over matter (e.g., moving objects).

• No solid scientific evidence supports ESP.