Module 7,8 & 9 Flashcards
(43 cards)
State Weber’s Law and define the Weber fraction.
Weber’s Law holds that the ratio of the DL (ΔS) to the standard stimulus (S) is constant (K):
Δ
𝑆
𝑆
=
𝐾
S
ΔS
=K
The Weber fraction K is this constant ratio (e.g., 0.02 for weight discrimination).
What is Stevens’s Power Law and its formula?
It describes perceived magnitude (P) as a power function of stimulus intensity (S):
𝑃 =𝐾*𝑆^𝑛
where K is a constant and n is the exponent.
What does the exponent (n) signify in Stevens’s Power Law?
n < 1: Compresses perception (e.g., brightness)
n ≈ 1: Linear scaling (e.g., electric shock)
n > 1: Expands perception (e.g., perceived heaviness)
What is a Receiver Operating Characteristic (ROC) curve?
It’s a plot of hit rate versus false‐alarm rate across different decision criteria, showing the trade‐off between sensitivity and specificity.
How does the shape of an ROC curve reflect sensitivity, and what does it mean if two observers’ points lie on the same curve?
The curve’s shape encodes an observer’s sensitivity independently of their criterion. If two observers’ hit/FA points all lie on the same ROC curve, they have equal sensitivity.
How do criterion shifts affect the ROC curve and an observer’s sensitivity?
Shifting the decision criterion (e.g., via payoffs or instructions) moves the operating point along the ROC curve.
Sensitivity (area/shape of the curve) does not change with criterion shifts.
What does sensitivity (d′) represent in Signal Detection Theory, and how does it affect the ROC curve?
d′ is the distance between the peaks of the noise (N) and signal+noise (S + N) distributions.
Larger d′: Distributions are farther apart → ROC curve bows sharply toward the upper‐left (high sensitivity).
Smaller d′: More overlap → ROC curve lies closer to the diagonal (low sensitivity).
How does the steepness of the psychometric function slope relate to the JND?
The psychometric function is a curve that relates a measure of perceptual experience (e.g., whether a person detects a stimulus) to the intensity of a physical stimulus.
Steeper slope → smaller JND (higher discriminability)
Shallower slope → larger JND (lower discriminability)
What effect does bias (criterion shifts) have on ROC performance?
Bias moves the operating point along the existing ROC curve, changing hit and false‐alarm rates but not sensitivity.
Opponent Processing
After the initial cone-based encoding, visual signals are fed into antagonistic neural channels that compare cone outputs in pairs.
- These opponent channels typically include:
- A red–green channel, which contrasts signals from “long-wavelength” cones against “medium-wavelength” cones
- A blue–yellow channel, which pits “short-wavelength” cone signals against the combined output of the other two cone types
- A luminance channel, summing inputs to encode brightness rather than hue
What role do retinal ganglion cells (RGCs) play?
They act as opponent neurons, combining excitatory (+) and inhibitory (–) cone inputs.
Which circuit is “+S – (M + L)”?
The blue–yellow opponent channel excited by S, inhibited by M+L.
Which circuit is “+(M + L) – S”?
The yellow–blue opponent channel excited by M+L, inhibited by S.
What perceptual pair does “+L – M” correspond to?
Red–green (excited by L/yellow-reddish, inhibited by M/greenish).
What perceptual pair does “+M – L” correspond to?
Green–red (excited by M/greenish-yellowish, inhibited by L/yellowish-reddish).
What does “uniform center–surround receptive field” mean for these RGCs?
They respond best when their entire RF sees a uniform patch of preferred (or opponent) color.
A +L–M RGC sees a uniform green field—what happens?
It is suppressed (firing rate drops), because M (green) inhibits that cell.
What is a single-opponent neuron in V1? What stimulus best drives single-opponent neurons?
A cell with a center–surround RF where the center is driven by one cone type and the surround by its opponent. A uniform patch of their preferred hue (average color over the RF).Encode the average hue within a region (detect uniform color fields).
What makes a neuron “double-opponent”? What stimulus best drives double-opponent neurons?
Both its center and its surround compare two cone inputs with opposite signs (e.g. center +L/–M, surround +M/–L).A sharp border between two opponent hues (chromatic edge).Detect local hue transitions (chromatic contours) even without luminance contrast.
What is “discounting the illuminant”?
The brain estimates and removes the light source’s effect to recover the object’s true color.
What is chromatic adaptation?
Cones adjust sensitivity after exposure to dominant light, helping maintain stable color perception.
Dichromacy (Partial Color Blindness)
Mechanism: Missing exactly one cone type → two remaining receptor classes.
- Types & Frequencies:
- Protanopia (no L-cones): ~1% of males, 0.02% of females.
- Deuteranopia (no M-cones): ~1% of males, 0.01% of females.
- Tritanopia (no S-cones): ~0.002% of population.
What are oculomotor depth cues?
Convergence (eye rotation) and accommodation (lens shape changes).
Retinal Image Cues What are some key visual cues the brain uses to perceive depth from 2D retinal images?
Binocular disparity (differences between eyes),
partial occlusion (closer objects block others),
motion parallax (nearby objects move faster across the retina),
visual angle (smaller angle = farther object),
familiar size (known objects help judge distance), and
linear perspective (parallel lines appear to converge with distance).
