Theories of vision and hearing

Theories of Vision and Hearing – Intro Notes

• Vision and hearing rely on sensory organs that detect stimuli and send signals to the brain.

• When these systems are overused, they can become fatigued.

Computer Vision Syndrome (CVS)

• Computer Vision Syndrome (CVS) happens after long periods of screen use.

• It is caused by eye strain from staring at visual displays for too long.

Common Symptoms of CVS

• Burning or itching eyes

• Sensitivity to light

• Blurred vision

• Headaches

• Eye fatigue

Why It Happens

• People blink less when using computers.

• Reduced blinking causes dry eyes.

• Glare and poor contrast increase eye strain.

Ways to Reduce CVS

• Reduce glare on the screen

• Increase contrast and brightness

• Adjust screen position

• Blink more often

Key Idea

Vision – Overview Notes

• Vision is the main sense most people use to gather information about their environment.

• Other senses (hearing, touch, smell, taste) support and complement vision.

Properties of Light – Notes

• Light is a form of energy that travels as electrical and magnetic waves.

Wave Properties

• Amplitude

  • Height of the wave

  • Affects brightness

• Wavelength

  • Distance between wave peaks

  • Affects color

• Frequency

  • Number of waves per second

  • Shorter wavelength = higher frequency

Visible Light

• Visible light is the part of the electromagnetic spectrum that humans can see.

• The visible light range is called the visible spectrum.

• The electromagnetic spectrum also includes:

  • radio waves

  • microwaves

  • X-rays

  • gamma rays

Visible Spectrum Order

• Red

  • Longest wavelength

  • Lowest frequency

• Violet

  • Shortest wavelength

  • Highest frequency

Outside Human Vision

• Ultraviolet (UV)

  • Wavelengths too short to be seen

• Infrared (IR)

  • Wavelengths too long to be seen

Key Memory Tip

Eye Structure and Function – Part 1 (Detecting Light)

• The human eyeball is globe-shaped and about 1 inch in diameter.

• Vision begins when light waves enter the eye.

Cornea

• The cornea is the clear, tough, protective layer at the front of the eye.

• About the size of a dime.

• Its main job is to bend (refract) incoming light.

• Light passes through the cornea before entering the pupil.

Pupil

• The pupil is a hole, not a structure.

• It appears as the black opening in the center of the eye.

• It controls how much light enters the eye.

Iris

• The iris is the colored part of the eye.

• Made of muscle tissue.

• It controls the size of the pupil.

Light Adjustment (Important!)

• Dark room:

  • Iris narrows

  • Pupil dilates (gets larger)

  • More light enters the eye

• Bright room:

  • Iris widens

  • Pupil constricts (gets smaller)

  • Less light enters the eye

Quick Definitions (Test-Ready)

• Cornea: Clear protective layer that bends light

• Pupil: Opening that lets light into the eye

• Iris: Colored muscle that controls pupil size

Memory Trick

Eye Structure and Function – Part 2 (Focusing Light)

Lens

• The lens is a transparent, elastic, disc-shaped structure.

• Works like a camera lens.

• Its job is to bend light and focus the image onto the retina.

• It can change shape to keep images clear.

Accommodation (Vision)

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

• Far objects:

  • Lens becomes longer and flatter

  • Helps focus light directly on the retina

• Near objects:

  • Lens becomes thicker and more curved

  • Allows close objects to stay in focus

Retina

• The retina is the layer of cells at the back of the eye.

• This is where transduction occurs.

• Light waves are converted into neural signals.

• Contains photoreceptors (rods and cones) and other neurons.

Vision Problems Related to the Lens

Myopia (Nearsightedness)

• Distant objects look blurred.

• Light focuses in front of the retina.

Hyperopia (Farsightedness)

• Close objects look blurred.

• Light focuses behind the retina.

Presbyopia

• An age-related condition.

• The lens loses elasticity.

• Makes it harder to focus on near objects.

• Common difficulty: reading up close.

Quick Definitions (Test-Ready)

• Lens: Changes shape to focus images on the retina

• Retina: Converts light into neural signals

• Accommodation: Lens changing shape to focus

• Presbyopia: Loss of near-focus with aging

Memory Trick

Eye Structure and Function: Part 3 (Retina & Neural Pathway)

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1. Retina (Overview)

• The retina is the light-sensitive layer at the back of the eye.

• It contains multiple layers of neurons that process light before signals ever reach the brain.

• Its main job: convert light into neural signals and begin visual processing.

2. Photoreceptors

These are the first cells to detect light.

Rods

• Respond to low levels of light

• Important for night vision and peripheral vision

• Do not detect color

• Very sensitive, but low detail

Cones

• Responsible for color vision

• Work best in bright light

• Provide sharp detail and visual acuity

• Three types (red, green, blue wavelengths)

3. Transduction

• Transduction = conversion of light energy into electrochemical nerve impulses

• Occurs in rods and cones

• This is the moment light stops being light and becomes information

4. Bipolar Cells

• Located between photoreceptors and ganglion cells

• Receive signals from rods and cones

• Act as relays, passing information forward

• Do minimal processing compared to later stages

5. Ganglion Cells

• Receive input from bipolar cells

• Their axons bundle together to form the optic nerve

• These are the final output cells of the retina

⚠ Exam note:
Ganglion cells are the only retinal neurons that fire action potentials

6. Optic Nerve

• Formed by the axons of ganglion cells

• Carries visual information from the retina to the brain

• Travels to the thalamus (LGN) and then to the visual cortex

Visual Signal Flow (MEMORIZE THIS)

Light → Rods/Cones → Bipolar Cells → Ganglion Cells → Optic Nerve → Brain

Fovea, Rods, and Cones (Visual Acuity & Light Conditions)

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Fovea

• The fovea is a small depression near the center of the retina.

• When you look directly at an object, its image is focused on the fovea.

• It is densely packed with cones and contains very few rods.

• The fovea provides the sharpest and most detailed vision (highest visual acuity).

Cones

• Specialized photoreceptors responsible for:

  • Color vision

  • Fine detail

  • High visual acuity

• Function best in bright light

• Do not work well in low-light or dark conditions

• Each eye contains over 6 million cones

• Highly concentrated in the fovea

Rods

• Photoreceptors sensitive to low levels of light

• Responsible for:

  • Black, white, and gray vision

  • Night vision

  • Peripheral (side) vision

• More sensitive to light than cones, but less detailed

• Each eye contains over 120 million rods

• Most abundant in the peripheral retina, not the fovea

Light Adaptation

• In bright environments → cones dominate

• In dark environments → rods take over

• This switch explains why color and sharp detail fade in darkness

Key Comparison (Exam Gold ⭐)

Dark Adaptation and Light Adaptation

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Dark Adaptation

• Dark adaptation occurs when moving from a bright environment into a dark one.

• The iris dilates, enlarging the pupil to let in more light.

• Cones shut down because they require bright light to function.

• Rods take over, allowing vision in low-light conditions.

• Because rods do not detect color, objects appear black, white, or gray.

• Color vision is lost temporarily (e.g., red appears black).

• Full dark adaptation takes 20–30 minutes.

📌 Result:
Better night and peripheral vision, but no color and low detail.

Light Adaptation

• Light adaptation occurs when moving from a dark environment into bright light.

• The iris contracts, shrinking the pupil to reduce incoming light.

• Rods shut off because they are overwhelmed by bright light.

• Cones activate, restoring:

  • Color vision

  • Sharp detail

• The sudden brightness can feel temporarily blinding.

📌 Result:
Clear, colorful, high-acuity vision in bright conditions.

Movie Theater Example (Concept Application)

1. Enter dark theater → Dark adaptation

   • Iris dilates

   • Rods active

   • Cones inactive

   • No color vision

2. Exit into sunlight → Light adaptation

   • Iris contracts

   • Rods inactive

   • Cones active

   • Color and detail restored

Quick Comparison (Perfect for MCQs ⭐)

From the Eye to the Brain: How Vision Works

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1. Light Enters the Eye

A vertical object reflects light into the eye, passing through these structures in order:

1. Cornea

   • Transparent outer covering

   • Begins bending (refracting) incoming light

2. Iris

   • Colored ring of muscle

   • Controls pupil size to regulate light entry

3. Lens

   • Flexible, transparent structure

   • Fine-tunes focus so the image lands on the retina

4. Retina

   • Light-sensitive inner surface

   • Converts light into neural signals

2. Retina Processing

• The retina does significant preprocessing before signals ever reach the brain.

• Light is converted into electrical signals by rods and cones.

• Signals travel through:

  • Bipolar cells

  • Ganglion cells

• The axons of ganglion cells bundle together to form the optic nerve.

• Each optic nerve contains about one million ganglion cell fibers.

3. Blind Spot

• The blind spot is the area of the retina where:

  • Blood vessels enter and exit the eye

  • The optic nerve leaves the eyeball

• No rods or cones are present here.

• This creates a small gap in vision, usually unnoticed because:

  • The other eye compensates

  • The brain fills in missing information

Blind spot definition:
An area in which vision is absent because no photoreceptor cells are located there.

4. Optic Chiasma

• The optic nerves from both eyes meet at the optic chiasma.

• At this junction:

  • Nerve fibers cross over to the opposite side of the brain

• This crossover allows:

  • Both hemispheres to receive information from both eyes

  • Depth perception and three-dimensional vision

5. Thalamus

• From the optic chiasma, visual signals travel to the thalamus.

• The thalamus acts as a sensory relay station.

• It directs visual information to the correct area of the cerebral cortex.

Thalamus definition:
A brain structure that relays sensory information to the cerebral cortex.

6. Primary Visual Cortex (Occipital Lobe)

• Located in the occipital lobe at the back of the brain.

• Responsible for processing visual information, especially from the fovea.

• Contains specialized neurons called feature detectors.

7. Feature Detectors

• Neurons that respond to specific visual features, such as:

  • Edges and angles

  • Movement

  • Light and dark

  • Texture

  • Color

• These cells help the brain assemble raw signals into recognizable objects.

Full Visual Pathway (MEMORIZE THIS ⭐)

Cornea → Iris → Lens → Retina → Ganglion Cells → Optic Nerve → Optic Chiasma → Thalamus → Primary Visual Cortex (Occipital Lobe)

Color Vision

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Color as a Psychological Experience

• Color does not exist outside the brain.

• Objects reflect light of different wavelengths.

• Color perception occurs when the visual system interprets wavelength information processed by specialized retinal cells.

• Thus, color is a psychological experience, not a physical property of objects.

Theory 1: Trichromatic (Young–Helmholtz) Theory

⚠ Important correction: this is NOT related to the trigeminal nerve.

• Proposed by Thomas Young and Hermann von Helmholtz.

• States that the retina contains three types of cones.

• Each cone type is maximally sensitive to a different wavelength range.

Three Cone Types

• S cones (short wavelengths) → blue

• M cones (medium wavelengths) → green

• L cones (long wavelengths) → red

• All colors are perceived through different combinations of activation of these three cone types.

• Explains:

  • Color mixing

  • Color blindness due to missing or malfunctioning cones

📌 Example:
Yellow light stimulates both red (L) and green (M) cones.

Theory 2: Opponent-Process Theory (Hering)

• Proposed by Ewald Hering.

• Suggests color perception is based on opposing color pairs.

• Each neuron is sensitive to two opposite colors.

Opponent Color Pairs

• Red ↔ Green

• Blue ↔ Yellow

• Black ↔ White

• When a neuron is activated by one color, it is inhibited for its opposite.

• Explains:

  • Why you cannot see “reddish-green” or “bluish-yellow”

  • Afterimages (staring at red → seeing green afterward)

📌 Example:
If blue–yellow cells are strongly activated by yellow, they are simultaneously suppressed for blue.

How the Two Theories Work Together

This is key for exams:

• Trichromatic theory explains color detection at the retinal cone level.

• Opponent-process theory explains color processing at later neural stages (ganglion cells, thalamus, visual cortex).

• Modern neuroscience supports both theories simultaneously.

Quick Comparison (Exam Gold ⭐)

⚠ Important Correction

Trigeminal nerve:

• Is the largest cranial nerve

• Controls facial sensation and jaw movement

• Has NO role in vision or color perception

If you want, I can:

Opponent-Process Theory and Afterimages

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Afterimage Explanation (Opponent-Process Theory)

• When you stare at a colored image for a prolonged time, specific opponent cells become fatigued.

• In your example:

  • Green/red opponent cells adapted to green

  • Black/white opponent cells adapted to black

• Fatigue means these cells respond less strongly than usual.

When you then look at a white surface:

• White light stimulates all color systems equally

• However, the fatigued cells cannot respond fully

• As a result:

  • The red side of the red/green cells fires more strongly than green

  • The white side of the black/white cells fires more strongly than black

📌 This imbalance produces an afterimage in the opponent colors.

Duplicity (Two-Stage) Theory of Color Vision

Modern research shows that both classic theories are correct, but they operate at different stages.

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Stage 1: Trichromatic Processing (Retina)

• Occurs in the cones of the retina

• There are three cone types:

  • S cones → short wavelengths (blue)

  • M cones → medium wavelengths (green)

  • L cones → long wavelengths (red)

• Color is initially coded by patterns of cone activation

• Matches the theory proposed by Young and Helmholtz

Stage 2: Opponent Processing (Retina & Visual Cortex)

• Occurs in:

  • Ganglion cells

  • Thalamus

  • Visual cortex

• Neurons respond to opposing color pairs:

  • Red ↔ Green

  • Blue ↔ Yellow

  • Black ↔ White

• Increased activation for one color inhibits its opposite

• Explains:

  • Afterimages

  • Color contrast

  • Why certain color combinations cannot be perceived simultaneously

Why This Matters (Big Picture)

• Color perception is not a single step

• It is a two-stage biological process:

  1. Cone-based detection (trichromatic)

  2. Neural comparison and contrast (opponent-process)

• This combined model is called the duplicity (two-stage) theory of color vision

One-Sentence Exam Answer ⭐

Sound Waves — Core Notes

What is Sound?

• Sound is the movement of air molecules traveling in a wave pattern.

• It is produced when a vibrating object creates rapid changes in air pressure.

• Sound waves move through compressions (high pressure) and rarefactions (low pressure).

Psychological Characteristics of Sound

1. Pitch (Frequency)

• Frequency = number of sound wave cycles per second.

• Measured in Hertz (Hz).

• Determines how high or low a sound is.

  • High frequency → high pitch (e.g., whistle)

  • Low frequency → low pitch (e.g., drum)

🧠 Brain interprets frequency as pitch.

2. Loudness (Amplitude)

• Amplitude = height of the sound wave.

• Determines how loud or soft a sound is.

• Measured in decibels (dB).

  • Large amplitude → loud sound

  • Small amplitude → soft sound

🧠 Brain interprets amplitude as loudness.

3. Timbre (Complexity)

• Timbre = quality or texture of sound.

• Caused by a mixture of different wavelengths.

• Explains why two instruments playing the same note sound different.

  • Example: violin vs. piano playing the same pitch

🧠 Brain interprets complexity as sound quality.

Quick Memory Table

Pitch, Loudness, and Timbre — Sound Wave Notes

Pitch → Frequency

• Frequency = number of complete changes in air pressure per unit of time.

• Measured in hertz (Hz).

  • 1 Hz = 1 cycle per second

• Frequency determines pitch, which we perceive as high or low.

• The faster an object vibrates, the higher the pitch.

  • Example: whistle (high frequency) vs. bass drum (low frequency)

Loudness → Amplitude

• Amplitude = energy or height of a sound wave.

• Determines loudness (also called intensity).

• Measured in decibels (dB).

  • Normal conversation ≈ 60 dB

  • Sounds above 120 dB (e.g., jet engine) are painful and can cause damage

• Larger amplitude → louder sound

• Smaller amplitude → softer sound

Timbre → Complexity

• Timbre = quality or texture of a sound.

• Determined by the specific mixture of frequencies and amplitudes in a sound wave.

• Explains why two sounds with the same pitch and loudness still sound different.

  • Example: violin vs. piano playing the same note

Quick Exam Summary

Structure and Function of the Ear — Part 1 (Outer Ear)

Major Divisions of the Ear

• The ear has three main parts:

  1. Outer ear

  2. Middle ear

  3. Inner ear

(This section focuses on the outer ear.)

Outer Ear: Structures & Functions

1. Pinna

• The visible part of the ear on the side of the head.

• Function:

  • Gathers sound waves from the environment.

  • Funnels sound into the auditory canal.

2. Auditory Canal (Ear Canal)

• About 1 inch long.

• Tube-like structure leading inward.

• Lined with hairs (cilia).

• Function:

  • Directs sound waves toward the eardrum.

  • Helps amplify certain sound frequencies.

3. Eardrum (Tympanic Membrane)

• Located at the end of the auditory canal.

• Thin, flexible membrane about ⅓ inch in diameter.

• Function:

  • Vibrates when struck by sound waves.

  • Converts sound waves into mechanical vibrations.

  • These vibrations activate the middle ear.

Key Flow of Sound (Outer Ear)

Sound waves → Pinna → Auditory canal → Eardrum vibrates → Middle ear activated

Quick Exam Reminders

• Outer ear = collects and funnels sound

• Eardrum vibration = start of hearing process

• Tympanic membrane = eardrum (same thing)

Structure and Function of the Ear — Part 2 (Middle Ear)

Middle Ear — Overview

• Located between the eardrum and the inner ear.

• Contains three tiny bones that transmit and amplify sound vibrations.

• These bones are collectively called the ossicles.

The Ossicles (Middle Ear Bones)

1. Malleus (Hammer)

• Attached directly to the eardrum.

• Receives vibrations from the vibrating eardrum.

2. Incus (Anvil)

• Located between the malleus and stapes.

• Transfers vibrations from the malleus to the stapes.

3. Stapes (Stirrup)

• Smallest bone in the human body.

• Connects to the oval window of the inner ear.

• Presses on the oval window, causing it to vibrate.

Oval Window

• A membrane-covered opening to the inner ear.

• Receives vibrations from the stapes.

• Converts mechanical vibrations into fluid movement in the inner ear.

Function of the Middle Ear

• Amplifies sound vibrations from the outer ear.

• Transfers vibrations efficiently from air (outer ear) to fluid (inner ear).

Sound Pathway So Far

Sound waves → Eardrum → Malleus → Incus → Stapes → Oval window → Inner ear

Quick Exam Tips

• Ossicles = malleus, incus, stapes

• Stapes always connects to the oval window

• Middle ear’s main job = amplification

Structure and Function of the Ear — Part 3 (Inner Ear)

Cochlea

• A bony, snail-shaped chamber in the inner ear.

• Filled with fluid.

• Movement of the oval window creates waves in this fluid.

Basilar Membrane

• A flexible membrane that lines the cochlea.

• Fluid waves cause it to move up and down.

• Different regions respond to different sound frequencies.

Hair Cells (Cilia)

• Hearing receptors attached to the basilar membrane.

• Bend as fluid waves move through the cochlea.

• Their bending is the key trigger for hearing.

Transduction

• Transduction = process by which physical sound energy is converted into electrochemical neural impulses.

• Occurs when hair cells bend.

• The resulting neural signals travel via the auditory nerve.

Neural Pathway of Sound

Oval window vibration → Cochlear fluid waves → Basilar membrane movement → Hair cells bend → Transduction → Auditory nerve → Thalamus → Auditory cortex (temporal lobe)

Key Exam Reminders

• Cochlea = fluid-filled (not air-filled)

• Hair cells bending = start of neural signal

• Transduction = sound → neural impulse

• Auditory cortex is located in the temporal lobe

Theories of Hearing

Two Major Theories

Hearing theories explain how we perceive pitch. There are two main approaches:

1. Place theory

2. Frequency theory

Place Theory

• Pitch is determined by where the basilar membrane vibrates.

• Different locations (places) along the basilar membrane respond to different pitches.

• When a specific area vibrates, it stimulates specific hair cells.

• Best explains high-frequency (high-pitch) sounds.

🧠 Brain interprets location of vibration as pitch.

Frequency Theory

• Pitch is determined by the rate of vibration of the entire basilar membrane.

• The brain detects pitch based on the firing rate of the auditory nerve.

• Faster firing rate → higher pitch

• Slower firing rate → lower pitch

• Best explains low-frequency (low-pitch) sounds.

🧠 Brain interprets neural firing rate as pitch.

Dual (Combined) Theories

• Modern researchers combine both theories.

• Frequency theory explains perception of low frequencies.

• Place theory explains perception of higher frequencies.

• Together, they provide a more complete explanation of hearing.

Quick Comparison Table

Exam Traps to Watch

• Place theory ≠ firing rate

• Frequency theory ≠ location

• Basilar membrane involved in both

Routing of Sensory Information in the Brain

Big Idea

• Sensory processing is distributed, not localized to one single brain area.

• Some brain structures play major routing roles, especially the thalamus.

The Thalamus: Sensory Relay Station

• Routes most sensory information to the cerebral cortex.

• Involved senses:

  • Vision

  • Hearing

  • Taste

  • Touch

• Also sends signals to the hindbrain, linking sensation with emotion.

🚨 Exception: Smell does not go through the thalamus first.

Senses That Bypass the Thalamus

• Olfaction (smell)

• Balance / Vestibular sense

• These go directly to:

  • Hindbrain

  • Cerebral cortex

Sensory Processing in the Cerebral Cortex

Vision

• Processed in the occipital lobes

• Located at the back of the brain

• Primary visual cortex

Hearing

• Processed in the auditory cortex

• Located in the temporal lobes

Smell & Taste

• Processed mainly in the temporal lobes

• Strong connections to the limbic system

• Explains why smells trigger strong emotions and memories

Touch

• Processed in the somatosensory (sensory motor) cortex

• Runs across the top of both hemispheres

• Body areas with more sensitivity have larger cortical representation

Balance & Vestibular Sense

• Processed in:

  • Sensory motor cortex

  • Cerebellum (major role in coordination and balance)

Summary of Sensory Mechanisms (AP Focus)

Vision (Eyes)

• Stimulus: Light waves (electromagnetic spectrum)

• Key structures:

  • Cornea, pupil, iris, lens

  • Retina: rods (light/dark), cones (color)

• Theories:

  • Duplicity theory

  • Trichromatic theory

  • Opponent-process theory

Audition (Ears)

• Stimulus: Sound waves (air pressure changes)

• Key structures:

  • Outer ear: pinna, auditory canal, eardrum

  • Middle ear: malleus, incus, stapes

  • Inner ear: oval window, cochlea, cilia, organ of Corti

• Theories:

  • Place theory

  • Frequency theory

Olfaction (Nose)

• Stimulus: Chemical molecules in the air

• Key structures:

  • Olfactory epithelium

  • Olfactory bulb

• Theory:

  • Lock-and-key theory

• ⚠ Bypasses the thalamus

Gustation (Taste)

• Stimulus: Chemical molecules (sweet, salty, sour, etc.)

• Key structures:

  • Papillae (taste buds)

• Theory:

  • Lock-and-key theory

Skin Senses (Touch, Temperature, Pain)

• Stimulus: Pressure, temperature, pain

• Key structures:

  • Receptors in the skin

• Notes:

  • Different receptors for different sensations

  • Most dense in fingertips, lips

• Pain theory:

  • Neuromatrix theory

Kinesthesia

• Function: Body posture and movement

• Receptors:

  • Muscles, joints, tendons

Vestibular Sense (Balance)

• Stimulus: Gravity and spatial orientation

• Key structures:

  • Semicircular canals

  • Vestibular sacs (cilia)

• Strong link to the cerebellum

One-Line AP Memory Hooks

• Thalamus = sensory relay (except smell)

• Occipital = vision

• Temporal = hearing, smell, taste

• Somatosensory cortex = touch & balance

• Cerebellum = balance + coordination