Chapter 5 – Sensation & Perception (Psychology 2e)

Sensory Systems Overview

• All incoming information about the external world (and the state of our bodies) reaches the brain through specialized sensory systems.

• Core modalities covered in this chapter:

  • Vision

  • Audition (hearing)

  • Olfaction (smell)

  • Gustation (taste)

  • Somatosensation (touch, pressure, vibration)

  • Pain (nociception)

  • Temperature (thermoception)

  • Balance (vestibular sense)

  • Body position (proprioception)

  • Movement (kinesthesia)

• Key idea: Each sense has dedicated receptors, neural transduction mechanisms, and cortical processing areas, yet they all follow the same broad path—​stimulus → receptor activation → neural impulse → brain interpretation.

Sensation

Sensory Receptors & Transduction

• Sensory receptors = specialized neurons that react to specific energies (photons, sound-pressure waves, chemical molecules, mechanical pressure, etc.).

• When stimulated, receptors convert physical energy into an electro-chemical signal (action potential).
  • This conversion is called transduction.

Absolute Threshold & Subliminal Messages

• Absolute threshold = minimum stimulus energy needed to be detected 50\% of the time.
  • Classic example: On a clear night, the most sensitive retinal cells can detect a candle flame from \sim 30\text{ miles} away.

• Subliminal messages fall below the absolute threshold of conscious awareness.
  • Stimulus reaches the receptors and fires an impulse but is not consciously perceived.

Just Noticeable Difference (JND)

• JND (difference threshold) = smallest change in stimulus intensity that can be detected.
  • Varies with baseline intensity (Weber’s Law).
  • Example: A phone’s brightness change is noticed in a dark theater but not in a bright mall; the physical luminance is constant, but the JND differs.

Perception

• Perception = the organization, identification, and conscious interpretation of sensory information.

Bottom-Up vs. Top-Down Processing

• Bottom-up: Build a percept from raw sensory data; starts with receptors and works “up.”

• Top-down: Existing knowledge, expectations, and context shape interpretation; flows “down” to influence what is perceived.

Factors Affecting Perception

1. Sensory Adaptation

• Diminished sensitivity to constant stimulation (e.g., stop hearing a ticking clock).

2. Attention & Inattentional Blindness

• Limited processing resources can make us miss visible stimuli.
  • Study: ~\tfrac{1}{3} of participants failed to see a red cross when focusing on other shapes.

3. Motivation & Signal Detection Theory (SDT)

• SDT: Detection depends on stimulus intensity and current mental/physical state (expectations, fatigue, incentives).
  • Hearing a phone when waiting for an important call illustrates motivational bias.

4. Beliefs, Values, Prejudice, Expectations

• Example: Positive attitudes toward "low-fat" labels increase reported flavor pleasantness.

5. Life / Cultural Experience

• Built/straight-line environments in Western culture heighten susceptibility to the Müller-Lyer illusion compared with round-hut cultures.

Müller-Lyer Illusion

• Identical lines appear unequal due to arrow-like end caps suggesting depth cues.

Gestalt Principles (Organizational Rules)

• "The whole is different from the sum of its parts."

  • Figure–ground

  • Proximity

  • Similarity

  • Continuity

  • Closure

• Ambiguous figures (e.g., duck/rabbit, vase/faces) demonstrate flexible figure-ground assignment.

• Implicit bias example: Research (Goff et al., 2014) shows observers overestimate Black boys’ ages by \approx 4.5\text{ years} and see them as less innocent—illustrating how social schemas affect perception with real-world legal/educational ramifications.

Physical Properties of Waves

• Amplitude (peak-to-trough height) → intensity (brightness or loudness).

• Wavelength (peak-to-peak distance) is inversely related to frequency f (waves per second): f = \tfrac{1}{\text{period}}.
  • Units: \text{hertz (Hz)}.

• Long wavelength → low frequency; short wavelength → high frequency.

Vision

Light Waves & Color

• Humans perceive a narrow slice of the electromagnetic spectrum: 380\text{–}740\,\text{nm}.
  • Long \approx reds, intermediate \approx greens, short \approx blues/violets.
  • Larger amplitudes → brighter appearance.

Anatomy of the Visual System

1. Light passes through cornea → pupil (iris controls diameter) → lens which focuses it onto the fovea (center of retina).

2. Photoreceptors (rods & cones) transduce light, synapse onto bipolar → ganglion cells; ganglion cell axons form the optic nerve.

3. Exit point = optic disc (blind spot) (no receptors).

4. Optic nerves meet at the optic chiasm (partial crossover: right visual field → left brain, & vice-versa) → lateral geniculate nucleus (LGN) → primary visual cortex (occipital lobe).

5. Beyond V1, information splits:
   • "WHAT" pathway (ventral) → object recognition/identification.
   • "WHERE/HOW" pathway (dorsal) → spatial location & action planning.

Photoreceptors: Rods vs. Cones

• Cones
  • Photopic (day) vision, high acuity, color, concentrated in fovea.

• Rods
  • Scotopic (night) vision, very light-sensitive, low acuity, motion detection, peripheral retina.

Theories of Color Vision

• Trichromatic Theory (Young-Helmholtz): 3 cone types (R, G, B) combine to make all colors; operates at receptor level (retina).

• Opponent-Process Theory: Post-receptor cells code antagonistic pairs (Black–White, Red–Green, Blue–Yellow).
  • Explains negative afterimages and why certain combos (e.g., "reddish-green") are impossible.

• Both are valid at different stages (retinal vs. post-retinal processing).

Depth Perception

• Ability to perceive 3-D spatial relations in a 2-D retinal image.

• Binocular cues (require both eyes)
  • Binocular disparity (stereopsis).

• Monocular cues (single eye)
  • Linear perspective (converging lines)
  • Interposition (occlusion)
  • Relative size, texture gradient, etc.

Color Blindness

• Results from missing or altered cone photopigments; affects specific opponent pairs.

Audition (Hearing)

Sound Waves

• Frequency → pitch
  • Audible to humans: 20\text{–}20{,}000\,\text{Hz}.

• Amplitude → loudness in decibels (dB):
  • Conversation \approx 60\,\text{dB}
  • Rock concert \approx 120\,\text{dB}
  • Hearing risk zone 80\text{–}130\,\text{dB}; pain threshold \approx 130\,\text{dB}.

Anatomy of the Ear

• Outer ear: pinna → auditory canal → tympanic membrane.

• Middle ear: ossicles (malleus, incus, stapes).

• Inner ear: oval window → cochlea (fluid-filled) with basilar membrane & hair cells.

Auditory Transduction Pathway

1. Sound waves vibrate tympanic membrane.

2. Ossicles amplify & press stapes on oval window.

3. Cochlear fluid moves → basilar membrane oscillates → hair-cell stereocilia bend.

4. Hair cells depolarize, releasing neurotransmitter → auditory nerve (cranial VIII).

5. Brainstem → inferior colliculus → medial geniculate nucleus (MGN) of thalamus → auditory cortex (temporal lobe).

Pitch Perception

• Temporal (frequency) theory: firing rate of neurons codes pitch (works up to \sim 4000\,\text{Hz}).

• Place theory: specific place on basilar membrane corresponds to a particular frequency (base = high f; apex = low f).

• Combined model explains entire audible range.

Sound Localization

• Monaural cue: spectral shaping by single ear.

• Binaural cues:
  • Interaural level difference (ILD)—louder at nearer ear.
  • Interaural timing difference (ITD)—arrival-time gap µs-ms scale.

Hearing Loss

• Congenital deafness: present at birth.

• Conductive loss: mechanical transmission failure (eardrum/ossicles); hearing aids often help.

• Sensorineural loss: cochlear or nerve damage; causes include Menière’s disease, infections, autoimmune issues, & environmental noise (e.g., rock musicians, construction workers).

Chemical Senses

Taste (Gustation)

• Six proposed basic tastes: Sweet, Salty, Sour, Bitter, Umami, (Fatty? emerging evidence).

• Taste buds (10-14 day turnover) contain receptor cells whose microvilli project into the taste pore.

• Transduction: Tastant molecules bind receptors → cellular depolarization → cranial nerves VII, IX, X → thalamus → gustatory cortex (insula/frontal operculum).

Smell (Olfaction)

• Olfactory receptor neurons in mucosa possess cilia with odorant receptors.

• Transduction: Odorant binds → depolarization → axons form olfactory nerve → olfactory bulb (no thalamic relay) → primary olfactory cortex & limbic structures (emotion/memory link).

• Pheromones: conspecific chemical signals, often reproductive; strong in many species, debated in humans.

Somatosensation

Touch Receptors

• Meissner’s corpuscles: light pressure, low-frequency vibration.

• Pacinian corpuscles: deep pressure, high-frequency vibration.

• Merkel’s disks: sustained light touch.

• Ruffini endings: skin stretch.

• Free nerve endings contribute to thermoception & nociception.

Thermoception & Nociception

• Temperature and pain signals ascend via spinal cord → medulla → thalamus → somatosensory cortex.

Pain Types

• Inflammatory pain: tissue damage signal.

• Neuropathic pain: nerve pathway damage.

• Congenital insensitivity to pain: genetic; feel temperature/pressure but not pain → injury risk & shortened lifespan.

Body Senses

Vestibular Sense

• Organs (semicircular canals, utricle, saccule) near cochlea detect head motion & gravity via hair-cell deflection in fluid.

• Maculae sense linear acceleration (otolithic membrane inertia bends stereocilia).

Proprioception & Kinesthesia

• Proprioceptors in muscles, tendons, joints inform about limb position; kinesthetic receptors track movement.

• Integrated with vestibular data to coordinate posture, reflexes, and voluntary motion.