Threshold Bridge Slides The Threshold Bridge
Psychophysics & Transduction
What is Psychophysics?
The scientific study of the relationship between physical stimuli and the sensations and perceptions they evoke.
"How much change in a stimulus is required for us to actually notice it?"
Physical World
Photons, Sound Waves
Mental World
Brightness, Pitch
Absolute Threshold
Definition
The minimum stimulus intensity that can be detected 50% of the time.
Vision: A candle flame 30 miles away on a clear night.
Audition: The tick of a watch 20 feet away in quiet conditions.
Detection Curve
50%
Stimulus Intensity →
Signal Detection Theory (SDT)
Detection isn't just about intensity; it's about psychological factors:
Expectation & Motivation
Alertness (Fatigue)
Noise (Environmental Distraction)
Stimulus Status Responded "Yes" Responded "No" Present HIT MISS Absent FALSE ALARM CORRECT REJECTION
Weber's Law
\[ \frac{\Delta I}{I} = k \]
The Core Concept:
The Just Noticeable Difference (JND) is a constant proportion of the original stimulus intensity.
Translation: If you're carrying 100 lbs, you won't notice an extra 1 lb. But if you're carrying 1 lb, you'll definitely notice an extra 1 lb.
Typical Constants (k)
Weight 0.02 (2%)
Brightness 0.08 (8%)
Pitch 0.003 (0.3%)
Transduction: The Conversion
Physical Energy
TRANSDUCTION
Neural Impulse
"Sensory receptors convert physical signals (light, sound, pressure) into electrochemical patterns that the brain can understand."
Psychophysics Lab Worksheet Psychophysics Laboratory
Investigating Thresholds & Weber's Law
NAME:
DATE:
Experiment 1: The Absolute Threshold
In this trial, you will observe a series of light flashes of varying intensities. Record whether you detected the stimulus in each trial.
Trial # Intensity (Lumens) Detected? (Y/N) Notes 1 5 2 10 3 15 4 20 5 25
1. Based on your data, estimate your Absolute Threshold. At what intensity did you transition to a 50% detection rate?
Experiment 2: Calculating Weber's Constant
Scenario
A subject is holding a 200g weight. The smallest weight added that they can perceive as a change is 4g.
A) Calculate the Weber fraction (k) for this subject's weight perception.
Show work here...
B) Predict the JND if the initial weight were increased to 500g.
Show work here...
Theoretical Application
Why does a phone screen at minimum brightness feel "blinding" in a dark room, but barely visible in direct sunlight? Use the concept of Background Intensity and Differential Thresholds to explain.
Experiment 3: Signal Detection Scenarios
Identify the result (Hit, Miss, False Alarm, Correct Rejection) for each scenario below:
1. A radiologist identifies a tumor on an X-ray, and a subsequent biopsy confirms it is indeed cancerous.
2. An air traffic controller believes they see a plane on the radar, but it turns out to be a flock of birds.
3. A parent is sleeping; the baby cries, but the parent does not wake up to hear it.
Ocular Engine Slides The Ocular Engine
Anatomy & Phototransduction
The Path of Light
1. Cornea
Light enters and is focused (refracted).
2. Pupil/Iris
Regulates the amount of light entry.
3. Lens
Accommodation: Fine-tunes focus for depth.
4. Retina
The surface where transduction occurs.
"Light projects an inverted image onto the retina."
Photoreceptor Duel
Rods
~120 Million
Scotopic (Low Light)
High Sensitivity
Peripheral Location
Low Acuity (Blurry)
Cones
~6 Million
Photopic (Daylight)
Color Vision
Concentrated in Fovea
High Acuity (Fine Detail)
Phototransduction Mechanics
"Unlike most neurons, photoreceptors are depolarized in the dark and hyperpolarize when light strikes."
1. Dark State
Na+ channels open via cGMP. Neurotransmitter (Glutamate) is released constantly.
2. Photon Strike
Rhodopsin splits into retinal and opsin. Enzyme PDE breaks down cGMP.
3. Hyperpolarization
Na+ channels close. Membrane potential drops. Glutamate release stops.
Neural Circuitry of the Retina
Photoreceptors
Rods & Cones
Bipolar Cells
The "middle management"
Ganglion Cells
Form the Optic Nerve
Horizontal and Amacrine cells allow for lateral inhibition (edge detection).
Retinal Circuitry Worksheet Retinal Circuitry & Mapping
Structural Analysis of the Human Retina
NAME:
DATE:
1. Structural Anatomy
Define the functional role of the following structures in visual processing:
CORNEA:
LENS:
FOVEA:
OPTIC DISK:
Sketch the "Inverted Image" principle below, showing light passing through the lens to the retina.
2. Retinal Cell Hierarchy
The retina is organized in a precise vertical and horizontal structure. Sequence the information flow by numbering the cell types (1-5) from environmental stimulus to optic nerve.
Order: ___
Bipolar Cells
Order: ___
Ganglion Cells
Order: ___
Photoreceptors
Order: ___
Optic Nerve
Order: ___
LGN (Thalamus)
Critical Thinking:
Why must light pass through the ganglion and bipolar cell layers before reaching the photoreceptors? What are the evolutionary consequences of this "backward" design, and how does it relate to the blind spot?
3. The Biochemistry of Light
The "Dark Current"
Explain why photoreceptors are considered "inverted" relative to standard neuronal firing patterns.
Describe the role of cGMP and Na+ channels here...
Scotopic Vision (Rods)
Pigment: ______________
Speed of recovery: ______
Degree of convergence: ___
Photopic Vision (Cones)
Pigment: ______________
Speed of recovery: ______
Degree of convergence: ___
Spectrum Secrets Slides Spectrum Secrets
Theories of Color Perception
The Modern Consensus: Two Stages
Stage 1: Retina
Trichromatic Theory
Three types of cones (RGB) respond to different wavelengths.
Stage 2: Thalamus/Cortex
Opponent-Process
Neurons respond to pairs of colors (e.g., Red vs Green).
"Color vision is not a single process; it's a hierarchy."
Young-Helmholtz Theory
The retina contains three distinct types of cone receptors, each sensitive to a specific range of light wavelengths:
S-Cones: Short (Blue)
M-Cones: Medium (Green)
L-Cones: Long (Red)
Color Mixing Logic
The brain interprets color by comparing the ratio of activation across these three cone types.
"Yellow is perceived when both Red and Green cones are stimulated significantly, but Blue is not."
Opponent-Process Theory
Red vs. Green
Blue vs. Yellow
Black vs. White
Proposed by Ewald Hering. He noted that we never see "reddish-green" or "yellowish-blue."
Mechanism: Bipolar and Ganglion cells receive excitatory signals from one color and inhibitory signals from its pair.
Neural Fatigue & Afterimages
The Explanation:
When you stare at one color (e.g., Red) for a long time, the neurons sensitive to that color fatigue.
When you look away to a white surface, the "Opponent" color (e.g., Green) fires more strongly because the Red signal is suppressed.
"Stare at this green circle for 30 seconds, then look at the white space below it. You will see its opponent: Red."
Color Logic Challenge Worksheet Color Logic Challenge
Analyzing Dual-Process Theory
NAME:
DATE:
1. Theoretical Frameworks
Trichromatic Theory
Location of process: ____________________
Explain why this theory fails to account for negative afterimages.
Opponent-Process Theory
Location of process: ____________________
Explain why this theory cannot fully explain why we can see three primary colors.
2. The Afterimage Prediction Matrix
For each stimulus color below, predict the color of the resulting negative afterimage based on Opponent-Process Theory.
RED
Afterimage: _______
YELLOW
Afterimage: _______
BLUE
Afterimage: _______
BLACK
Afterimage: _______
3. Case Study: Daltonism (Color Blindness)
"A patient reports that they are unable to distinguish between red and green. When looking at a field of poppies, the flowers look yellowish-gray."
Question A:
Which theory of color vision (Trichromatic or Opponent-Process) is primarily responsible for this defect at the retinal level?
Identify theory...
Question B:
Specifically, which biological structure is most likely malfunctioning or missing in this patient?
Identify structure...
4. Synthesis: How does the "Dual Process" model resolve the conflict between the two historical theories?
Explain the transition from retinal cones to ganglion cells and the LGN...
Sonic Waves Slides Sonic Waves
Mechanics of Hearing & Pitch
Sound: The Vibration
Frequency
Measured in Hertz (Hz). Corresponds to our perception of Pitch.
Amplitude
Measured in Decibels (dB). Corresponds to our perception of Loudness.
The Mechanical Chain
Outer Ear
Pinna & Canal
Acts as a funnel to concentrate pressure waves.
Middle Ear
Ossicles
Hammer, Anvil, Stirrup. Amplifies sound 20x.
Inner Ear
Cochlea
Snail-shaped organ where transduction occurs.
The Oval Window is the gateway to the fluid-filled cochlea.
How do we hear Pitch?
Place Theory
Different frequencies vibrate different places on the basilar membrane.
Best for High Frequencies.
Frequency Theory
The whole membrane vibrates at the same frequency as the sound wave.
Best for Low Frequencies.
The Volley Principle
"For intermediate pitches, neurons fire in rapid succession (like a volley of cannon fire) to match high-frequency waves."
The Biological Micro-Engine
TRANSDUCTION
Hair Cells (Cilia)
Fluid ripples in the cochlea bend the hair cells on the basilar membrane. This mechanical bending pulls open ion channels, triggering neural impulses in the Auditory Nerve.
Warning: Once hair cells are destroyed (by loud noise), they do not regenerate.
Auditory Architecture Worksheet Auditory Architecture
Mapping the Cochlear Pathway
NAME:
DATE:
1. The Mechanical Chain Reaction
Arrange the following structures in the correct order of vibration, starting from the environmental sound wave.
Ossicles
Auditory Canal
Basilar Membrane
Tympanic Membrane (Eardrum)
Oval Window
Pinna
Wave
Sequence them here...
The Ossicles
Explain the functional significance of having three bones in the middle ear rather than a direct connection to the cochlea.
Transduction
Where specifically does the conversion from mechanical energy to electrical energy occur?
2. Place vs. Frequency Theory
Feature Place Theory Frequency Theory Primary Mechanism Location on membrane Frequency Range Low frequencies (< 1000 Hz) Major Limitation
3. Auditory Scenarios
Conductive Hearing Loss:
A patient has a buildup of fluid in the middle ear. Explain which part of the auditory pathway is interrupted and why hearing aids are effective in this case.
Sensorineural Hearing Loss:
A construction worker has worked without earplugs for 20 years. Explain why they can hear low-pitched thunder perfectly but struggle to hear high-pitched human speech.
Spatial Sound Slides Spatial Sound
Localization & Scene Analysis
Two Ears, One World: Binaural Cues
Interaural Time Difference (ITD)
The sound reaches one ear slightly sooner than the other. (Microseconds difference!)
Interaural Intensity Difference (IID)
The head acts as a "sound shadow," making the sound slightly louder in the closer ear.
"Your brain calculates the difference between ears to pinpoint the source."
The Cone of Confusion
What if the sound is directly in front, behind, or above you?
In these cases, ITD and IID are zero. The brain cannot localize the sound using horizontal cues alone.
Solution:
We tilt our heads to create a difference, or use the Pinnae (outer ear) to filter sound based on vertical angle.
Ambiguous Data Point
Auditory Scene Analysis
"How do we segregate a cocktail party into individual voices?"
Grouping Principles
Spatial Location: Sounds from the same place group together.
Temporal Onset: Sounds starting at the same time group together.
Pitch Similarity: Sounds with similar timber group together.
Signal from Noise
The brain uses Gestalt-like principles to organize the auditory field.
Top-Down Processing
Expectations and language knowledge help us fill in missing sounds.
The Virtual Haircut
Binaural recordings place microphones inside a mannequin's ears to capture exact Interaural Differences.
When played back through headphones, the brain is completely fooled into "placing" the sound in 3D space around the listener.
Scene Analysis Summary Worksheet Scene Analysis Summary
The Capstone Challenge
NAME:
DATE:
1. Localization Logic
"You are walking in the woods and hear a twig snap to your immediate left."
A) Describe the Interaural Time Difference (ITD) for this sound.
Explain the timing...
B) Describe the Interaural Intensity Difference (IID) for this sound.
Explain the loudness difference...
The Vertical Challenge
Why is localization significantly worse for a sound coming from directly above your head compared to a sound from your left? What specific strategy does the human body use to solve this?
Analyze the vertical localization problem...
2. Segregating the Stream
At a crowded party, you can follow one conversation while ignoring the music and other guests. Identify which auditory grouping principle is being applied in each scenario:
1. You group a cello's notes together because they share the same deep timber, even when a flute plays simultaneously.
Principle: ________________
2. You realize two separate noises are part of the same door slam because they occur at exactly the same moment.
Principle: ________________
3. You hear a voice drifting from the kitchen and can keep it distinct from the voice sitting right next to you.
Principle: ________________
3. The Final Pathway: From Wave to Meaning
Synthesize everything you have learned. Briefly map the transformation of a 1000 Hz sound wave from the Pinna to the Auditory Cortex (A1). Be sure to include the role of the thalamus and the basilar membrane.
Write your sensory map here...
