Sensation and Perception Final Flashcards
Why Do We Perceive Colour?
Aids in perceptual organization: Helps segregate elements.
Classification and identification: Helps us recognize and distinguish objects.
Evolutionary advantage: Helps in food foraging (e.g., spotting ripe fruit).
What is Colour?
Colour = Light wavelength reflected, emitted, or transmitted by an object.
Chromatic colours (hues): Objects reflect some wavelengths more than others (selective reflectance).
Achromatic colours: White, black, and grey (reflect all wavelengths equally).
Key Colour Properties
Wavelength: Determines hue (e.g., blue, red).
Intensity: Changes perceived brightness.
Saturation: Adding white reduces saturation (e.g., red → pink).
Color and Light - Newton’s Discovery
White light = mixture of all colours.
Different wavelengths are bent differently by a prism.
Reflectance curve
% of light reflected at each wavelength.
Transmission curve
% of light transmitted through a transparent object.
Additive colour mixing
Mixing lights of different wavelengths.
Example: Blue + yellow light = white.
Subtractive colour mixing
Mixing paints/pigments (fewer wavelengths reflected).
Example: Blue + yellow paint = green.
Perceptual Dimensions of Colour
Hue: Colour type (e.g., red, green).
Saturation: Intensity/purity of colour.
Value/Lightness: Brightness of the colour.
Trichromatic Theory (Young-Helmholtz)
Three cone types in the retina:
Short-wavelength cones (S-cones): Peak at 419 nm.
Medium-wavelength cones (M-cones): Peak at 531 nm.
Long-wavelength cones (L-cones): Peak at 558 nm.
Colour perception = combination of cone responses.
Evidence: Colour-matching experiments (Maxwell) show people need at least 3 wavelengths to match any colour.
Opponent-Process Theory (Hering)
Opponent pairs:
Red ↔ Green
Blue ↔ Yellow
White ↔ Black
Opponent neurons: Found in the retina & LGN, they respond to one colour while inhibiting the opposite.
Evidence:
Colour afterimages: Seeing green after staring at red.
Simultaneous contrast: A grey square looks greenish on a red background.
How the Theories Work Together
Trichromatic theory: Explains cone responses in the retina.
Opponent-process theory: Explains neural signals processed “higher up” in the visual system (e.g., LGN, visual cortex).
Both of these are correct and expain different stages of visual processing
Metamers
Different wavelengths can produce the same colour perception if they stimulate cones in the same way.
Colour Deficiency & Vision Disorders -Monochromatism
One receptor type (rods only).
See only shades of grey.
Poor visual acuity, very sensitive to light.
Colour Deficiency & Vision Disorders - Dichromatism
Two cone types: See chromatic colours like blue and green but with limitations.
Protanopia
missing L-cones
No red-green distinction.
Deuteranopia
missing M-cones
No green-yellow distinction.
Tritanopia
missing S-cones
No blue-yellow distinction.
Colour Deficiency & Vision Disorders - Anomalous Trichromatism
Three cones, but one is abnormal.
Needs different wavelength proportions to match colours.
Colour Deficiency & Vision Disorders - Unilateral Dichromatism
One eye trichromatic, the other dichromatic.
Why Are Three Cone Types Necessary
One receptor type: No colour vision (same response to any wavelength).
Two receptor types: Limited colour vision (like dichromats).
Three receptor types: Full colour spectrum perception.
Color Constancy
Perception of colors remains relatively constant even under varying lighting conditions.
Examples of Light Sources
Sunlight: Balanced energy across visible wavelengths.
Tungsten Lighting: More energy in long wavelengths (red/yellow tones).
Fluorescent Lighting: More energy in short wavelengths (blue tones).
Chromatic Adaptation
Prolonged exposure to a particular color reduces sensitivity to that color and helps maintain color constancy
Lightness Constancy
Perception of achromatic colors (e.g., black, white, gray) remains constant even when lighting changes
Lightness Constancy
- Ratio Principle
Two areas that reflect different amounts of light look the same if their intensity ratios are consistent.
Lightness Constancy - Reflectance Edges vs. Illumination Edges
Reflectance Edges: Light changes between surfaces.
Illumination Edges: Light changes due to shadows or different lighting conditions.
Our perceptual system identifies shadows based on their contours and meaning, helping us distinguish between illumination and reflectance edges.
Penumbra
The fuzzy edge of a shadow, signaling an illumination edge.
Color as a Neural Construction
Light waves themselves are not “colored.”
Perceptions vary between species (e.g., honeybees perceive UV colors).
Perceived colors are created by the brain’s interpretation of light waves.
Functions of Motion Perception
Survival: Predators detect prey through motion; prey avoids detection by remaining still.
Perceiving Events: Movement helps organize stimuli in our environment.
Akinetopsia
Blindness to motion; inability to perceive motion as a continuous stream.
Real Motion
Actual physical movement of an object.
Illusory Motion
Perceived motion without physical movement (e.g., movies, animated signs).
Induced Motion
Motion of one object causes the perception of motion in another object.
Motion Aftereffect
Observing stationary objects moving in the opposite direction after prolonged viewing of motion (e.g., waterfall illusion).
Ecological Approach (Gibson)
Information about motion is directly available in the environment.
Ecological Approach (Gibson) - Local Disturbance in Optic Array
Movement relative to a background indicates motion.
Ecological Approach (Gibson) - Global Optic Flow
Observer’s movement causes the entire scene to appear to move.
Corollary Discharge Theory
Movement perception depends on three signals:
Image Displacement Signal (IDS): Movement of an image across the retina.
Motor Signal (MS): Signal sent to the eye muscles to move the eyes.
Corollary Discharge Signal (CDS): A copy of the motor signal sent to other parts of the brain.
Movement Perception: When the comparator receives only one of the above signals.
Corollary Discharge Theory - Image Displacement Signal (IDS)
Movement of an image across the retina.
Corollary Discharge Theory - Motor Signal (MS)
Signal sent to the eye muscles to move the eyes.
Corollary Discharge Theory- Corollary Discharge Signal (CDS)
A copy of the motor signal sent to other parts of the brain.
Corollary Discharge Theory - Movement Perception
When the comparator receives only one of the above signals.
Reichardt Detectors
Neurons that respond to movement in one direction.
Medial Superior Temporal Area (MST)
Damage to this area can cause the perception of environmental motion during eye movements.
Aperture Problem
Viewing a moving object through a small opening can lead to incorrect conclusions about the direction of motion.
What do neurons in the MT Cortex respond to
Respond to specific directions of motion.
Microstimulation Experiments
Stimulating motion-detecting neurons alters motion perception.
Biological Motion
Perception of movement produced by living organisms.
Biological Motion - Point-Light Walker Stimulus
Visual display of lights at specific points on a person’s body to detect biological motion.
Biological Motion
- Processing Regions
Superior Temporal Sulcus (STS) and Fusiform Face Area (FFA).
Representational Momentum
Perceiving implied motion in still images.
Example: A photograph of a running person may be perceived as “moving” in the viewer’s mind.
Cue Approach to Depth Perception
Relies on information from the retinal image that correlates with depth in the scene.
Occlusion
When one object partially blocks another, indicating depth.
Automatic Processing
We learn these cues through repeated exposure until they become automatic.
Oculomotor Cues
Based on sensing eye position and muscle tension.
Oculomotor Cues - Convergence
Inward movement of eyes when focusing on nearby objects.
Oculomotor Cues
- Accommodation
Lens shape changes when focusing on objects at various distances.
Monocular Cues (Using One Eye) - Pictorial Cues
Found in 2D images like pictures.
Monocular Cues (Using One Eye) - Relative Height
Objects below the horizon higher in the field of vision are more distant.
Monocular Cues (Using One Eye) - Relative Size
When objects are the same size, the closer one appears larger.
Monocular Cues (Using One Eye) - Familiar Size
Knowing an object’s size helps determine distance.
Monocular Cues (Using One Eye) - Atmospheric Perspective
Distant objects appear fuzzy and bluish.
Monocular Cues (Using One Eye) - Texture Gradient
Objects packed closer together as distance increases.
Monocular Cues (Using One Eye) - Shadows
Provide information about object placement and enhance 3D appearance.
Movement-Based Cues - Motion Parallax
Closer objects move quickly across our visual field compared to distant objects.
Movement-Based Cues - Deletion & Accretion
Objects are covered or uncovered as we move relative to them.
Binocular Disparity
The difference in images between the two eyes due to their different positions.
Horopter
Imaginary sphere that passes through the point of focus, where objects fall on corresponding points on the two retinas.
Absolute Disparity
Angle difference between points not on the horopter.
Relative Disparity
Difference in absolute disparity between two objects.
Stereopsis
Depth information derived from binocular disparity, demonstrated by stereoscopes and random-dot stereograms.
Neural Processing and Binocular Disparity
Some neurons respond best to binocular disparity, allowing depth perception.
Size-Distance Relationship
Our perception of an object’s size depends on our perception of its distance.
Size Constancy
We perceive an object’s size as constant even when its image on the retina changes size (due to distance).
Size-Distance Scaling Equation
S = K (R × D)
S = perceived size
K = constant
R = retinal image size
D = perceived distance.
Müller-Lyer Illusion
Lines with inward or outward fins appear different in length despite being the same length.
Emmert’s Law
Perceived size of an afterimage changes depending on the distance of projection.
Ponzo Illusion
Objects placed over converging lines (e.g., railroad tracks) appear different in size despite being the same.
Explanation: Misapplied size-constancy scaling.
Visual Illusions
- Ames Room
A distorted room making people appear different in size due to altered shape and distance cues.
Explanation: Size-distance scaling and relative size comparison.
Visual Illusions
- Moon Illusion
The moon appears larger near the horizon than when higher in the sky.
Explanation: Apparent-distance theory and angular size-contrast theory.
Physical definition of Sound
Sound is created by vibrations in a medium (such as air or water) that produce changes in air pressure.
Perceptual definition of Sound
Sound is what we hear when these vibrations reach our ears and are processed by the brain.
How Sound Waves Work
Sound waves are produced by objects that vibrate.
These vibrations cause alternating areas of condensation (high-pressure regions) and rarefaction (low-pressure regions) in the surrounding air.
Types of Sound Waves - Pure Tones
Consist of a single frequency (e.g., a tuning fork).
Defined by two key characteristics:
- Amplitude
- Frequency
Amplitude
How strong the vibrations are, determining loudness. Measured in decibels (dB).
Frequency
The number of vibrations per second, determining pitch. Measured in Hertz (Hz).
Complex Tones
Contain multiple frequencies.
The fundamental frequency is the lowest and most dominant frequency.
Harmonics are additional frequencies that shape the tone’s quality.
Loudness
Depends on amplitude—higher amplitude = louder sound.
Measured in decibels (dB) (e.g., whisper: ~30 dB, rock concert: ~120 dB).
Pitch
Determined by frequency (Hz)—higher frequency = higher pitch.
Humans hear frequencies from 20 Hz to 20,000 Hz.
Lower frequencies = deep sounds (bass), higher frequencies = high-pitched sounds (squeaks).
Timbre (Sound Quality)
Why a piano and a violin sound different even when playing the same note.
Affected by harmonics, attack (start of a sound), and decay (end of a sound).
Ear Structure Parts
Outer Ear (Captures Sound):
- Pinna
- Auditory Canal
Middle Ear (Amplifies Sound):
- Tympanic Membrane (Eardrum)
- Ossicles
Inner Ear (Processes Sound into Neural Signals):
- Cochlea
- Basilar Membrane
- Organ of Corti
Outer Ear - Pinna
The visible part of the ear, helps with sound localization (knowing where a sound is coming from).
Outer Ear - Auditory Canal
Protects the inner structures and amplifies certain frequencies to enhance hearing.
Middle Ear - Tympanic Membrane (Eardrum)
Vibrates in response to sound waves.
Middle Ear - Ossicles
Three small bones (Malleus, Incus, Stapes) that transmit and amplify vibrations from the eardrum to the inner ear.
Inner Ear (Processes Sound into Neural Signals) - Cochlea
A spiral, fluid-filled structure that contains
Basilar Membrane
Organ of Corti
Inner Ear (Processes Sound into Neural Signals) - Basilar Membrane
Moves in response to vibrations.
Inner Ear (Processes Sound into Neural Signals) - Organ of Corti
Contains hair cells that convert sound vibrations into electrical signals.
How Hair Cells Work
Hair cells inside the cochlea bend when vibrations pass through the basilar membrane.
This bending generates electrical signals sent to the auditory nerve, which then transmits them to the brain.
Place Coding
Different parts of the basilar membrane respond to different frequencies:
High frequencies: Activate the base of the cochlea.
Low frequencies: Activate the apex (tip) of the cochlea.
Temporal Coding
Neurons fire in sync with the frequency of the sound (phase locking).
Volley principle: Multiple neurons work together to capture higher frequencies.
Pathway to the Brain
Cochlea →
Auditory Nerve → Brainstem →
Auditory Cortex (in the temporal lobe).
Presbycusis
Age-Related Hearing Loss
Gradual loss of high-frequency sounds, more common in men.
Noise-Induced Hearing Loss
Caused by exposure to loud sounds (e.g., concerts, machinery, headphones).
Damage to hair cells is permanent.
Interaural Time Difference (ITD)
If a sound comes from the right, it reaches the right ear slightly earlier than the left.
The brain measures this tiny delay to determine direction.
Interaural Level Difference (ILD)
The head blocks some sound waves, making them quieter in the far ear.
This cue is stronger for high-frequency sounds.
Spectral Cues
The shape of the pinna changes how sound waves reflect, helping determine if a sound is coming from above, below, or behind.
Cone of Confusion
Some sounds give the same ITD and ILD, making it hard to tell where they are coming from.
Moving the head helps resolve this confusion.
Cues for grouping sounds
Sounds from the same location tend to be grouped together.
Pitch and timbre similarity help distinguish different voices or instruments.
Vision and hearing interact
Seeing a speaker’s lips move helps process speech (McGurk effect).
Hearing in Infants
Infants have higher hearing thresholds (need louder sounds to hear).
Can recognize their mother’s voice soon after birth.
Early exposure to speech sounds is crucial for language development.
