W6: Pyschophysics

Question 1

Define psychophysics and the 4 stages of visual perception

Answer 1

direct quantification of sensory performance applying physics to psychology measuring ability to perfom:

1. Detection
2. Identification
3. Discrimination
4. Scale

Question 2

Define absolute threshold and it’s shape in practice

Answer 2

Minimum stimulus energy to elicit a detection response
Ogive shape due to visual system being noisy

Question 3

Describe the Method of Limits with example/pros/cons

Answer 3

Stimuli presented in ascending/descending order, observer reports number of times stimulus seen, repeated, averaged and plotted e.g. Snellen

Quick but subject can anticipate threshold at repeat levels

Question 4

Describe the Staircase Method with an example

Answer 4

Uses ascending/descending limits
Subject reports stimulus then direction reverses/stimulus decreased until they no longer see it.
Staircase is reversed and stimulus increased until visible again; usually deduced aver 3-4 reversals
E.g. Visual Field Testing

Question 5

Describe the Method of Constant Stimuli with pros/cons

Answer 5

Stimuli presented in random order multiple times
Subjects report seeing or not for stimulus values and intensity plotted against % of seen responses

Subjects can’t anticipate stimulus visibility but very time consuming

Question 6

Describe the Method of Adjustment with pros/cons

Answer 6

Subject adjusts the independent variable until threshold is reached

Quick but suffers from variations in subjects threshold criterion

Question 7

Describe a forced choice procedure and it’s use

Answer 7

Subject shown 2-4 choices with one correct stimulus.
% correct is plotted

Reduces subjective criteria

Question 8

Give an example of an Adaptive Method

Answer 8

Parameter by Estimated Sequential Testing
Subject sees stimulus, intensity reduced until not visible then increased by half the previous change until seen again.
Intensity reduced again by half the previous step and so on until threshold intensity is reached

Question 9

State Weber’s Law

Answer 9

JND/Lb = k

Breaks down at high luminance when photoreceptors saturate and low luminance due to internal retinal noise limiting sensitivity

Question 10

State 7 Factors that affect Visual Sensitivity

Answer 10

Light Adapation (Background Illumination)
Dark Adaptation
Eccentricity (high sensitivity in photopic, low in scotopic)
Pupil Diameter
Spatial Summation (Ricco’s Law: LA = k)
Temporal Summation (Bloch’s Law: LT = k)
Optical/Neural (Aberrations/Refractive State/Age/Disease

Question 11

State Ricco’s Law

Answer 11

LA=k (light added over a critical diameter)

Detection threshold reached when LA>k

Question 12

State Bloch’s Law

Answer 12

LT = k (temporal analogue of Ricco’s Law)

Threshold reached when LT>k

Photopic system has shorter temporal summation than scotopic (Cones break at 20ms, Rods break at 100ms)

Question 13

Define Fourier Analysis with sine wave properties

Answer 13

Analysing a visual scene in terms of differing sine wave gratings: varying properties include:

Contrast
Spatial Frequency
Spatial Orientation
Spatial Phase Shift

Question 14

Define the contrast sensitivity function and how eye diseases affect them

Answer 14

Measured in dB = 1/threshold contrast
Measure of the sensitivity of visual system to spatial frequency
adjust grating pattern contrast to determine threshold for given spatial frequency

Most Severe: Cataracts, Refractive Error, Multiple Sclerosis

Question 15

State the Ferry-Porter Law and define Critical Flicker Frequency

Answer 15

Critical Flicker Frequency =a logLuminance + b

transition point when flickering light becomes continuous
decreases away from fovea