Sensation & Perception: Hearing, Smell, Taste, Touch and Perceptual Organisation

Auditory (Hearing) System

• Physical nature of sound
  • Produced by vibrations → create alternating expansions & contractions of air molecules (sound waves).
  • Travels at a constant speed of 340\;\text{m\,s}^{-1} (≈ speed in air at room temperature).
  • Weakens with distance but, unlike light, can travel through many objects.
• Acoustic energy – three measurable properties
  • Frequency = number of wave cycles per second • unit \text{Hz} (1 Hz = 1 cycle s⁻¹).
    • Psychological correlate → pitch (high/low tone).
    • Human range: \approx15 Hz – 20\,000 Hz; dogs up to 50\,000 Hz.
  • Amplitude = height/depth of wave • psychological correlate → loudness.
  • Complexity = number & mix of frequencies • psychological correlate → timbre (texture/quality of sound).
• Anatomy of the ear
  1. Outer ear – pinna funnels waves → auditory canal (≈ 2.5\;\text{cm}) → amplification (≈ ×2) → eardrum (tympanic membrane).
  2. Middle ear – air-filled cavity containing ossicles (hammer / anvil / stirrup).
     • Function: convert air-pressure waves → mechanical vibration; further amplify.
  3. Inner ear – cochlea (fluid-filled, coiled).
     • Main structures: vestibular canal, cochlear duct, tympanic canal, basilar & tectorial membranes, organ of Corti (hair cells = auditory receptors).
• Transduction sequence
  1. Eardrum vibrates → ossicles move.
  2. Stirrup presses on oval window → pressure waves in cochlear fluid.
  3. Traveling waves flex basilar membrane; hair cells shear against tectorial membrane.
  4. Hair-cell deflection opens ion channels ⇒ receptor potentials ⇒ action potentials in auditory nerve.
  5. Signals relay via thalamus → primary auditory cortex (temporal lobe).
• Pitch–perception theories
  • Place theory (Helmholtz): pitch = place of maximal displacement on basilar membrane (like piano keys).
  • Frequency theory: entire membrane vibrates; firing rate of auditory nerve mirrors sound frequency.
  • Von Békésy’s traveling-wave theory: membrane moves as a wave whose peak location depends on frequency → reconciles both models.
• Central auditory pathway
  • Inner-ear axons → cochlear nucleus (medulla).
  • Most fibres cross to olivary nucleus (some remain ipsilateral).
  • → inferior colliculus (mid-brain) → medial geniculate nucleus (thalamus) → auditory cortex.
• Sound localisation cues
  • Interaural intensity difference (head shadow; useful for high frequencies).
  • Interaural timing difference (up to 10^{-5}\;\text{s} detectability; dominant for low frequencies).
  • Head movements refine localisation.

Olfaction (Smell)

• Functions
  • Detect danger (smoke, gas, spoilage).
  • Discriminate palatable vs. unpalatable food.
  • Social/sexual signalling in many animals (pheromones); subtle effects in humans (menstrual synchrony, gender identification via smell).
• Transduction route
  1. Airborne chemical molecules dissolve in nasal mucus.
  2. Bind to receptors on cilia in olfactory epithelium.
  3. Receptor axons form olfactory nerve → olfactory bulb.
  4. Bulb projects via olfactory tract → primary olfactory cortex → thalamus, amygdala, frontal regions.
     • Close links with emotion & taste ⇒ smells can evoke strong affective memories.
• Humans distinguish ≈ 10\,000 odors yet label them poorly; no universally agreed “primary odor” set.
• Behavioural priming: Subliminal citrus scent ⇒ cleaner tidying behaviour (illustrates unconscious olfactory influences).

Gustation (Taste)

• Evolutionary roles
  • Toxin avoidance (bitterness).
  • Nutrient regulation (salts, sugars).
  • Present even in newborn reflexes.
• Receptors & regeneration
  • Taste buds (mostly on tongue papillae) contain receptor cells replaced every 10–11 days (critical for recovery after burns).
• Neural pathways
  1. Chemicals dissolve in saliva and bind receptor cilia.
  2. Gustatory neurons → medulla & pons.
  3. Split: (a) Cortical pathway → thalamus → primary gustatory cortex (taste identification).
     (b) Limbic pathway → amygdala, hypothalamus (automatic affective responses).
  • Damage to (a) can abolish conscious taste ID but leave reflexive reactions (parallel to blindsight).
• Basic tastes: sweet, sour, salty, bitter (umami & possibly others not covered in transcript).
  • Taste = blend of receptor activations + olfaction + learning & culture.
• Individual differences
  • Supertasters (↑ papillae density) dislike sweets, fats, alcohol, tobacco → lower CVD risk.

Cutaneous (Skin) Senses

• General facts
  • Skin area ≈ 2\;\text{m}^2; ≈ 5\,000\,000 receptors.
  • Functions: protection, object identification, thermoregulation, social contact.
• Qualities & receptor types
  • Pressure / touch
    • Meissner’s corpuscles → brief light touch.
    • Merkel’s discs → steady pressure.
    • Hair-follicle endings → hair movement (makes plucking painful).
  • Temperature – separate warm & cold receptors (thermal energy).
  • Pain – free nerve endings respond to tissue damage or internal states (not direct external energy transduction).
• Neural pathways
  • Receptor → spinal cord (reflex arcs) → medulla (decussation) → thalamus → somatosensory cortex (parietal lobe).
• Clinical note
  • Mis-wiring of receptors to wrong fibres can produce neuropathic pain; re-attachment errors alter perceptual quality.

Pain

• Adaptive value: motivates removal from harm.
• Biopsychosocial modulation
  • Culture: Fijian vs. Indian childbirth expectations alter reported pain.
  • Emotion: anxiety ↑ pain; high stress or focus elsewhere ↓ pain.
  • Personality pattern common in chronic pain: externalising blame, alexithymia, dependency, anxiety/depression.

Proprioceptive Systems

• Vestibular sense (balance, spatial orientation)
  • Semicircular canals (fluid-filled) + otolith organs; hair-cell deflection by fluid inertia & gravity.
• Kinesthesia (body/limb movement & position)
  • Receptors in muscles, tendons, joints signal stretch, tension, angle.
  • Crucial for coordinated movement, tool use, sports.

Perceptual Organization

1. Form Perception – Gestalt Principles

• Figure–ground: segregate focal object vs. background (e.g.
  text on page).
• Laws that group elements into wholes:
  1. Similarity – alike items grouped.
  2. Proximity – near items grouped.
  3. Good continuation – perceive continuous lines.
  4. Simplicity (Prägnanz) – prefer simplest shape.
  5. Closure – fill gaps to complete figures.
• Ambiguous figure example: young-lady / old-woman drawing – global gestalt changes interpretation of each component.
• Recognition-by-components (Biederman)
  • Visual system parses objects into ≈ 20–30 geons (simple 3-D volumes).
  • Object recognised if geon arrangement identifiable; obscuring inter-geon relations impairs ID.

2. Perceptual Illusions

• Reveal normal processing rules.
  • Müller–Lyer: arrowheads create mis-perceived length (linear-perspective cues).
  • "Impossible" trident figure: not an illusion per se – perceived correctly as impossible when whole viewed.

3. Depth / Distance Perception

• Binocular cue
  • Retinal disparity: slight L/R image difference; encoded by binocular cells.
• Monocular static cues
  • Interposition, elevation (height in plane), linear perspective, texture gradient, shading, aerial perspective (atmospheric haze), familiar size, relative size.
  • All visible in Taj Mahal photograph example.
• Monocular motion cue
  • Motion parallax: during self-motion, near objects sweep faster across retina than far ones.

4. Motion Perception

• Retina + cortical motion detectors.
• Two mechanisms: (a) object moves across stationary retina; (b) eye tracks object – retinal image static, background moves; efference copy from eye-movement command signals motion.

5. Perceptual Constancies

• Color – perceived constant despite illumination shifts.
• Shape – remains despite viewpoint changes.
• Size – perceived real size despite retinal image scaling with distance (car example).

Perceptual Interpretation

• Direct perception view (Gibsonian): meaning is immediate & innate (e.g., infants avoid deep side of visual cliff).
• Constructive view: environment input + learning essential; sensory deprivation alters cortical development (kittens reared with only vertical lines cannot later perceive horizontals; adult cataract surgery patients struggle with object recognition).

Processing Directions

• Bottom-up: data-driven, build percept from feature detection upward.
• Top-down: concept-driven, expectations guide feature selection.
  • Interaction illustrated with EMIT / TIME ambiguous design – need both component recognition & global expectation.

Perceptual Set

• Readiness to perceive in a particular way; shaped by schemas (stored knowledge structures) & immediate context.
• Motivation biases perception:
  • Food-deprived participants more readily perceive briefly flashed food-related words.
  • People tend to avoid perceiving unpleasant stimuli when motivated.

Review Quiz Highlights (with correct answers)

• Psychological characteristics of sound: pitch, loudness, timbre.
• Smell in humans is not primarily for communication (it is in many other animals).
• Kinesthesia informs about movement & position of limbs relative to each other.
• Perception differs from sensation by organization & interpretation processes.
• Motion parallax: nearer objects appear to move faster than distant ones during observer motion.
• Kitten vertical-stripe experiment demonstrates brain development depends on environment.

Ethical, Philosophical & Practical Implications

• Sensory deficits (e.g., cochlear damage, anosmia) highlight need for assistive tech & early intervention.
• Cultural framing of pain underscores importance of psychosocial context in medical care.
• Subliminal olfactory effects raise ethical issues about environmental scent marketing.
• Understanding perceptual illusions aids design (architecture, UI) & cautions eyewitness reliability in legal settings.