Unit 1.6: Sensation

Remember: Sensation = Detection (Bottom-Up) | Perception = Interpretation (Top-Down)

What is Transduction?

Transduction is the process of converting physical energy from environmental stimuli into neural signals (action potentials) that the brain can process.

Examples of Transduction by Sense:

• Vision: Light waves → electrical signals in retinal photoreceptors (rods and cones)
• Hearing: Sound waves → vibrations → electrical signals in cochlear hair cells
• Touch: Pressure → electrical signals in mechanoreceptors
• Taste: Chemical molecules → electrical signals in taste receptor cells
• Smell: Airborne chemicals → electrical signals in olfactory receptor neurons

Key Point: Without transduction, sensory information could not reach the brain because neurons only communicate through electrical and chemical signals.

Absolute Threshold

The absolute threshold is the minimum stimulus intensity required for detection 50% of the time.

Why 50%? Because detection isn't perfect—it varies due to neural noise, attention, and individual differences. The 50% cutoff provides a consistent, measurable point.

Difference Threshold (Just Noticeable Difference - JND)

The difference threshold or JND is the smallest detectable change in stimulus intensity that can be noticed 50% of the time.

Example: The smallest increase in volume you can detect on your phone, or the minimum weight difference you can feel when holding two objects.

The difference threshold changes depending on stimulus intensity — it's easier to notice a 1-pound difference between 1 lb and 2 lbs than between 50 lbs and 51 lbs.

Weber's Law

Weber's Law states that the JND is a constant proportion of the original stimulus intensity, not a fixed amount.

Memory Tip: Weber's Law means "the bigger the stimulus, the bigger the change needed to notice it".

Signal Detection Theory

Signal detection theory proposes that stimulus detection depends on both the stimulus intensity and the individual's physical and psychological state (attention, expectation, motivation).

Key Concepts:

• Sensitivity: Your ability to detect the stimulus when present
• Response bias: Your tendency to say "yes" or "no" regardless of whether the stimulus is present
• Context matters: You'll notice a dim light better in a dark room when you're alert and expecting it than in a bright room when you're distracted

Example: A radiologist looking for cancer in an X-ray — detection depends on image quality AND the doctor's experience, fatigue, and expectations.

What is Sensory Adaptation?

Sensory adaptation is the process by which our sensory receptors become less sensitive to constant, unchanging stimuli over time.

Purpose: Adaptation helps the brain focus on new or changing stimuli by tuning out constant background information that isn't immediately important.

⚠️ Vision Does NOT Adapt

Vision is the only sense that does NOT experience sensory adaptation.

Why? If vision adapted, the things you're looking at would slowly disappear, which would be dangerous.

Solution: The eyes make constant tiny movements called saccadic movements (saccades) that ensure nothing in the visual field remains constant, preventing adaptation.

Eye Anatomy and Light Path

Light Path Through the Eye:

1. Cornea: Clear outer covering that protects the eye and bends (refracts) light
2. Pupil: Opening that controls how much light enters the eye
3. Iris: Colored muscle that adjusts pupil size (dilates in darkness, constricts in brightness)
4. Lens: Flexible structure that focuses light onto the retina through accommodation (changing shape)[web:92]
5. Retina: Light-sensitive layer at the back of the eye containing photoreceptors[web:92]
6. Optic nerve: Transmits visual information from retina to the brain

Accommodation

Accommodation is the process by which the lens changes shape to focus images clearly on the retina.

• Near objects: Lens becomes thicker (rounder) to bend light more
• Far objects: Lens becomes thinner (flatter) to bend light less

The Retina: Where Transduction Occurs

The retina is the light-sensitive layer at the back of the eye that contains photoreceptors (rods and cones) that transduce light into neural signals.

Key Retinal Structures:

• Fovea: Central area of the retina with the highest concentration of cones; provides sharpest, most detailed vision
• Blind spot (optic disc): Where the optic nerve exits the eye — no photoreceptors here, so there's a gap in vision
• Brain fills in the blind spot using surrounding visual information, so we don't normally notice it

Photoreceptors: Rods and Cones

Memory Tip: "Rods for the Road" (peripheral vision for safety), "Cones for Color" (center of vision).

Theories of Color Vision

Two complementary theories explain how we perceive color at different stages of visual processing.

Both theories are correct: Trichromatic explains cones, opponent-process explains neural processing afterward.

Properties of Sound Waves

Ear Anatomy and Sound Path

Sound Path Through the Ear:

1. Outer ear (pinna): Collects and funnels sound waves into the ear canal
2. Eardrum (tympanic membrane): Vibrates when sound waves hit it
3. Middle ear (ossicles): Three tiny bones (hammer, anvil, stirrup) amplify vibrations
4. Oval window: Membrane that transfers vibrations to the inner ear
5. Cochlea: Fluid-filled, snail-shaped structure in the inner ear where transduction occurs
6. Basilar membrane: Vibrates in response to sound, bending hair cells
7. Hair cells: Receptor cells that transduce mechanical vibrations into neural signals
8. Auditory nerve: Carries signals to the brain

Theories of Pitch Perception

Three theories explain how we perceive different pitches; each works best for different frequency ranges.

Types of Hearing Loss

Taste (Gustation)

Gustation is the chemical sense that detects dissolved substances in the mouth through taste receptor cells in taste buds.

Basic Taste Qualities:

• Sweet — sugars and some proteins
• Sour — acids
• Salty — sodium and minerals
• Bitter — potentially toxic compounds
• Umami — savory, protein-rich foods (glutamate)

Note: Most of what we call "taste" is actually flavor, which combines taste + smell + texture + temperature.

Smell (Olfaction)

Olfaction detects airborne chemical molecules through olfactory receptor neurons in the nasal epithelium.

Unique Features:

• Only sense that bypasses the thalamus — goes directly to olfactory bulb, then to limbic system
• Direct connections to emotion and memory centers (amygdala, hippocampus)
• Explains why smells trigger powerful emotional memories

Sensory Interaction: Taste and smell work together to create flavor perception through retronasal olfaction (smell from inside the mouth).

Touch and Pain (Somatosensation)

The skin contains various specialized receptors that detect pressure, temperature, and pain.

Types of Touch Receptors:

• Mechanoreceptors: Detect pressure and vibration
• Thermoreceptors: Detect temperature (hot and cold)
• Nociceptors: Detect pain

Kinesthesis (Body Position Sense)

Kinesthesis is the sense that provides information about body position and movement through receptors in muscles, tendons, and joints.

Enables coordinated movement without looking — typing, walking, catching a ball all rely on kinesthetic feedback.

Vestibular Sense (Balance)

The vestibular system monitors head position and movement, providing information about balance and spatial orientation.

Key Structures in the Inner Ear:

• Semicircular canals: Three fluid-filled tubes that detect rotational head movements
• Otolith organs (utricle and saccule): Detect linear acceleration and head tilt

Function: Works with vision and kinesthesis to maintain balance and coordinate movement.

Senses Work Together

Sensory interaction occurs when different senses combine to create a unified perceptual experience.

Examples:

• Taste + Smell = Flavor: Holding your nose makes food taste bland
• Vision + Hearing: Watching lips move helps understand speech (McGurk effect)
• Vision + Vestibular + Kinesthesis: Work together to maintain balance

Synesthesia

Synesthesia is a rare condition where stimulation of one sense automatically triggers another sensory experience.

Examples: Seeing colors when hearing music, tasting shapes, associating numbers with colors. It's a neurological cross-activation between brain areas.

Multiple Choice Tips

• Know the difference between sensation (detection) and perception (interpretation)
• Master thresholds: absolute threshold, JND, Weber's Law formula
• Understand transduction — how each sense converts stimuli to neural signals
• Vision: Rods vs. cones, trichromatic vs. opponent-process theories, myopia vs. hyperopia
• Hearing: Place, frequency, and volley theories; conduction vs. sensorineural deafness
• Know which sense bypasses the thalamus (olfaction)

Free Response Question (FRQ) Tips

• Use precise terminology: transduction, photoreceptors, basilar membrane, olfactory bulb
• Explain pathways: trace the path of stimuli through sensory organs to the brain
• Apply Weber's Law: calculate or explain JND in context
• Compare theories: trichromatic vs. opponent-process, place vs. frequency
• Link to behavior: explain how sensory deficits affect daily functioning

🔑 The Big Picture

Sensation is the detection of physical stimuli through sensory receptors and conversion into neural signals (transduction). Sensory thresholds define detection limits (absolute threshold) and noticeable differences (JND/Weber's Law). Vision uses rods (peripheral, low light) and cones (fovea, color, detail) explained by trichromatic and opponent-process theories. Hearing processes sound waves through the cochlea with place, frequency, and volley theories explaining pitch. Other senses include taste, smell (bypasses thalamus), touch/pain, kinesthesis, and vestibular sense. Sensory interaction combines multiple senses for unified perception.

💡 Essential Concepts

• Sensation vs. perception
• Transduction
• Absolute threshold
• Difference threshold (JND)
• Weber's Law: \(\frac{\Delta I}{I} = k\)
• Signal detection theory
• Sensory adaptation
• Rods and cones
• Fovea and blind spot
• Accommodation
• Myopia vs. hyperopia
• Trichromatic theory
• Opponent-process theory
• Frequency and amplitude
• Cochlea and hair cells
• Place theory
• Frequency theory
• Volley theory
• Conduction vs. sensorineural deafness
• Five basic tastes
• Olfaction bypasses thalamus
• Gate control theory
• Kinesthesis
• Vestibular sense
• Synesthesia