PSYA01 CH.5

Sensation & Perception

5.1 — How Sensation Leads to Perception

Sensation

  • Process of detecting physical energy (light, sound, chemicals, etc.) and sending it to the brain.

  • Transduction: Conversion of physical stimulus → neural signal.

  • Sensory systems evolved to fit species-specific survival needs (e.g., dogs hear higher frequencies, snakes detect infrared).

Perception

  • Brain’s interpretation of sensory information.

  • Influenced by experience, expectations, and context.

  • Synesthesia: Stimulation of one sense automatically triggers another (e.g., seeing colours when hearing sounds).

Attention

  • Selective attention: Focusing on one piece of information while ignoring others.

  • Divided attention: Trying to attend to multiple things at once—limited ability.

  • Sensory adaptation: Decreased response to unchanging stimuli (e.g., smell fading after a few minutes).

Bottom-Up vs Top-Down Processing

  • Bottom-up: Using basic sensory input to build perception (raw data → meaning).

  • Top-down: Using prior knowledge & expectations to interpret stimuli.

    • Example: Reading jumbled sentences because we expect certain words.

Psychophysics

  • Study of links between physical stimuli & psychological experience.

  • Absolute threshold: Smallest detectable stimulus 50% of the time.

    • Vision: candle flame 48 km away

    • Hearing: mosquito 3m away

  • Difference threshold (JND): Smallest detectable difference between two stimuli.

  • Weber–Fechner Law: Larger stimuli require larger differences to be noticed.

Signal Detection Theory

  • Perception = sensory input + decision making.

  • Four outcomes: hit, miss, false alarm, correct rejection.

  • Example: Radiologists reading mammograms.

5.2 — Vision

The Visual Stimulus

  • Wavelength → colour

  • Amplitude → brightness

Parts of the Eye

  • Cornea: Bends light first.

  • Pupil: Opening that lets light in.

  • Iris: Controls pupil size.

  • Lens: Focuses light by changing shape (accommodation).

  • Retina: Contains photoreceptors (rods & cones).

  • Fovea: Centre of retina; high detail.

  • Optic disk: Blind spot; no receptors.

Rods vs Cones

Feature

Rods

Cones

Light sensitivity

High (dim light)

Low (bright light only)

Colour

No

Yes (RGB)

Sharpness

Low

High

Location

Periphery

Fovea

Visual Pathways

  • Retina → Optic nerve → Optic chiasm → Thalamus → Visual cortex (V1)

  • Dorsal stream (“where”)

    • Movement, location, spatial processing

  • Ventral stream (“what”)

    • Object & face recognition

    • Damage → visual agnosia (can see but not identify)

Colour Vision

Trichromatic Theory

  • 3 cone types:

    • short (blue), medium (green), long (red)

  • Colour = comparison of cone activation.

  • Explains colour deficiencies.

Opponent Process Theory

  • Colour channels:

    • Red–green

    • Blue–yellow

    • Black–white

  • Explains colour afterimages.

Both theories are correct but at different levels of visual processing.

Object & Face Recognition

Debunking the "Grandmother Cell"

  • Object recognition uses patterns, not single cells.

  • Top-down processing helps us identify ambiguous images (like the Dalmatian example).

Gratings

  • Patterns of lines differing in:

    • frequency (detail)

    • contrast (intensity difference)

  • Used to study visual limits in humans and animals.

Face Recognition

  • Infants prefer face-like patterns.

  • Fusiform Face Area (FFA): Specialized for processing faces.

  • Debate: Is FFA for faces specifically or expertise in general?

  • Cultural differences:

    • Western observers look at eyes & mouth.

    • East Asian observers look at nose (global processing).

Gestalt Principles of Perception

Rules the brain uses to organize visual information:

  1. Figure-ground: Distinguish object from background.

  2. Proximity: Objects close together are grouped.

  3. Similarity: Similar items are grouped.

  4. Continuity: Smooth, continuous lines are preferred.

  5. Closure: Brain fills in missing information.

  6. Simplicity: Choose the simplest interpretation.

Depth Perception

Monocular Cues (one eye)

  • Linear perspective: Parallel lines converge in distance.

  • Texture gradient: More detail = closer.

  • Shading: Indicates curvature.

  • Occlusion: Objects blocking others appear closer.

  • Relative size: Smaller retinal image = farther.

  • Motion parallax: Closer objects move faster across the visual field.

Binocular Cue

  • Retinal disparity: Differences between each eye’s image → depth information.

    • Basis of 3D movies.

Common Illusions Explained

  • Ames Room: Distorted room tricks size perception.

  • Müller-Lyer: Arrow tails affect distance judgments.

  • Ponzo illusion: Converging lines trick us into thinking one line is longer.

Sensation & Perception – Vision, Audition, Somatosensation, Chemical Senses

5-2d — Developmental & Individual Differences in Vision

Infant Vision

  • Infants see the same things as adults but with less detail and need more contrast.

  • Prefer large, high-contrast patterns.

  • By 4 months: normal colour vision & binocular disparity.

  • By 2–7 months: develop cues like occlusion and relative size.

  • Newborns prefer mother's face over a stranger’s.

Aging & Vision

  • Mid-adulthood: slower lens accommodation → trouble focusing on near objects.

  • Slower adaptation to brightness changes (dark to light).

  • Pupil muscles weaken → smaller pupils.

  • Lens yellows → colour perception changes.

Individual Differences

  • Nearsightedness: elongated eyeball → trouble seeing far.

  • Farsightedness: short eyeball → trouble seeing near.

  • Astigmatism: uneven cornea.

  • Strabismus: eyes misaligned; untreated leads to amblyopia (“lazy eye”).

  • Adults with strabismus → double vision; children usually suppress one image.

Key Visual System Structures

  • Cornea: bends light

  • Pupil/Iris: controls incoming light

  • Lens: focuses

  • Retina: rods & cones

  • Fovea: detailed vision

  • Thalamus: relay

  • Primary Visual Cortex: first stage analysis

5-3 — How Do We Hear? (Audition)

5-3a Auditory Stimulus

  • Sound = vibrations of molecules (air, water, solids).

  • Cannot occur in a vacuum (no molecules).

  • Frequency (Hz) = pitch

  • Amplitude (dB) = loudness

  • Ultrasound > 20,000 Hz

  • Infrasound < 20 Hz

    • Can induce fear, chills, nausea (thought to cause “haunted” feelings).

5-3b Biology of Audition

Outer Ear

  • Pinna: collects sound + helps vertical localization.

  • Auditory canal → Tympanic membrane (eardrum).

Middle Ear

  • Three tiny bones transfer sound to inner ear fluid:
    Malleus → Incus → Stapes

  • Amplify sound to compensate for energy loss from air → fluid.

Inner Ear

  • Cochlea (snail-shaped).

  • Contains:

    • Basilar membrane

    • Organ of Corti (hair cells → 15,500 per ear).

  • Hair cell movement → neurotransmitter release → auditory nerve.

Pathway

Cochlea → Auditory nerve → Medulla → Midbrain (sound localization) → Thalamus → Primary Auditory Cortex (temporal lobe).

5-3c Auditory Perception & Cognition

Pitch Perception

  • Place Theory (best for > 4000 Hz):

    • High frequencies → base of cochlea

    • Low frequencies → apex

  • Temporal Theory:

    • Neural firing matches sound frequency (< 4000 Hz).

Loudness

  • Loudness grows slower than actual intensity.

  • Most sensitive to frequencies 80–10,000 Hz (speech range).

Localization

  • Vertical: pinna

  • Horizontal:

    • Interaural time difference

    • Interaural intensity difference

  • Vision interacts with hearing (e.g., movie sound appears to come from actors’ lips).

Auditory Illusions

  • McGurk effect: conflicting lip movements and sounds → different perceived speech.

5-3d Development & Individual Differences

  • Newborns recognize mother’s voice; prefer native language.

  • 6 months: turn toward sounds.

  • Hearing thresholds near adult level.

Age-related Hearing Loss

  • After 30: lose high frequencies.

  • After 70: difficulties above 6000 Hz → harder to understand speech.

Perfect Pitch

  • Linked to larger left hemisphere structures + early musical training.

5-3e Sociocultural Influences

Sine-Wave Speech

  • Without context → sounds like birds.

  • With context → perceived as speech (top-down processing).

Language Experience

  • Japanese speakers struggle with /r/ vs /l/.

  • English speakers struggle with Hindi /p/ contrasts.

Deaf Culture

  • ASL = full language with its own grammar (not just signed English).

Indigenous Language Learning

  • Ultrasound tongue videos help learners master non-English speech sounds.

5-4 — Somatosensation (Touch, Body Position, Pain)

5-4b The Biology of Somatosenses

Vestibular System (Balance)

  • Located in inner ear near cochlea.

  • Detects head position, linear acceleration, rotation.

  • Coordinates with visual system to keep stable vision.

Touch

  • Skin = largest sensory organ.

  • Specialized receptors detect:

    • Pressure

    • Vibration

    • Stretch

    • Pain

    • Temperature

  • Touch signals → spinal cord → thalamus → primary somatosensory cortex.

Sensory Homunculus

  • Distorted brain map of body based on sensitivity.

  • Hands, lips heavily represented.

  • No pain receptors in brain itself.

Plasticity

  • Touch cortex reorganizes with:

    • Loss of body parts (phantom limb)

    • Increased use (musicians, Braille readers)

    • Technology: prosthetics with touch feedback

Pain

Pain Receptors

  • Free nerve endings responding to:

    • Mechanical damage

    • Temperature

    • Chemicals (e.g., capsaicin)

Fast vs Slow Pain Fibres

  • Fast myelinated → sharp pain

  • Slow unmyelinated → dull ache

Pathway

Body → Spinal cord → Thalamus →

  • Anterior cingulate cortex (emotion)

  • Insula (emotion)

  • Somatosensory cortex (location, intensity)

Gate Control Theory (Melzack & Wall)

Pain must pass through a spinal “gate”:

  • Gate opens: stress, focus on pain

  • Gate closes: rubbing area, distraction, arousal, emotional context

Explains why both psychological and physical factors influence pain.

Placebo Effects

  • Even when knowingly taking a placebo, people can experience pain relief if they expect it.

Sociocultural Influences

  • Rituals: e.g., hook-swinging in India → low pain due to cultural meaning.

  • Prepared childbirth classes decrease pain.

  • Athletes tolerate more pain than nonathletes.

  • Holding a loved one’s hand reduces pain-related brain activity.

5-5 — Chemical Senses (Smell & Taste)

5-5b Biology

Olfaction (Smell)

  • Airborne molecules → olfactory epithelium → olfactory nerve → olfactory bulbs.

  • Only sensory system that does not go to thalamus first.

  • Strong links with amygdala → emotional reactions to smells.

Gustation (Taste)

  • Basic tastes: sweet, sour, salty, bitter, umami.

  • Taste buds in papillae; each bud = 50–150 receptor cells.

  • Taste → medulla → thalamus → insula + orbitofrontal cortex (pleasantness).

Perception & Cognition

  • Humans can distinguish 1 trillion odours.

  • Labels influence perceived pleasantness.

  • Flavour = taste + smell (orbitofrontal cortex combines them).

Development & Individual Differences

  • Newborns recognize sweet, sour, bitter.

  • Older age: fewer taste buds + smell decline → reduced appetite.

  • Supertasters: high density of papillae.

  • Females more sensitive to smell.

Smell & PTSD

  • Trauma-related odours can trigger flashbacks due to amygdala activation.

Sociocultural Influences

  • Cultural differences in odour preferences.

  • Experience shapes taste preferences from prenatal exposure.

  • Foods considered delicacies vary culturally.

End of Chapter Summary

  • Perception = interaction of bottom-up sensory input and top-down expectations.

  • Culture, development, and experience deeply shape how we perceive the world.

  • All senses contribute to survival and social functioning, not just vision.